LCD TV SMPS

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LCD TV SMPSIC CHIP FAIL HANDBOOK

For Professional Technicians and Advanced Students

Brought to you by Damon Morrow

http://www.PlasmaTVRepairGuide.com

LCD TV Repair Secrets Guide

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The reader is expressly warned to consider and adopt all safety precaution that might be indicated by the activities herein and to avoid all potential hazards. This E-book is for informational purposes only and the author do not accept any responsibilities or liabilities resulting from the use of this information. While every attempt has been made to verify the information provided here, the author cannot assume any responsibility for any loss, injury, errors, inaccuracies, omissions or inconvenience sustained by anyone resulting from this information. Most of the tips and secrets given should only be carried out by suitably qualified electronics engineers/technicians. Please be careful as all electrical equipment is potentially dangerous when dismantled. Any perceived slights of policy, specific people or organizations are unintentional.

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Limit of Liability / Disclaimer of Warranty:The authors and publisher of this book and the accompanying materials have used their best efforts in preparing this program. The authors and publisher make no representation or warranties with respect to the accuracy, applicability, fitness, or completeness of the contents of this program. They disclaim any warranties (expressed or implied), merchantability, or fitness for any particular purpose. The reader is expressly warned to consider and adapt all safety precautions that might be indicated by the activities here in and to avoid all potential hazards. By following the instructions contained herein, the reader willingly assumes all risks in connection with such instructions. The authors and publisher shall in no event be held liable for any loss or other damages, including but not limited to special, incidental, consequential, or other damages. As always, the advice of a competent legal, tax, accounting or other professional should be sought.

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Dedication

This book is dedicated to: Jestine Yong, Sunny, David Maltz, Teonna Flags, and Michael Danish. I would like to give special thanks to Jestine for being a great teacher to me and a great friend and always inspiring me to study harder to become an Engineer of electronics. Also special thanks to David for being my big brother and keeping my spirits up and always encouraging me to stay fit and healthy and to go further and to never give up. Thank you

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Content1. Introduction…………………………………….………….….…..7

2. List Of 3 SMPS IC Chips with Known Failures..….….…...7

3. What Technicians Should Get Used To….……………....…9

4. SMPS and Linear…………………………….………..……10

5. LCD TV SMPS: Referred to as DC-to-DC Convertors...15

6. Active-PFC for LCD TV SMPS:……….………….……..17

7. Power Good Signaling………………………....…..….…....21

8. What Is Power Good……………………..……………...….22

9. What Is Faking Power-Good Signaling?...….……………..23

10. Active Power Factor Correction IC….…….………………28

11. SMD sensory network circuits for APFC IC…………....32

12. Why LCD TV Engineers Are Forced To Implement Active PFC In SMPS Design…………….………….……33

13. Testing IC Chips…….………………………....…..….…...34

14. In Or Out Of Circuit IC Testing ……..……………...……35

15. The APFC IC Pin Names and Their Basic Function.…..41

16. Error Amplifier…………………………...….……………..42

17. PWM IC……………………………….…….………………43

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18. Current Mode-Control IC……………………………..…50

19. Conclusion………………….…………….………….….....57

20. Recommendation…….………………………....…..….…...58

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Introduction

This handbook is quick reference material in relation to 3 known (recorded) faulty IC chip semiconductors on LCD TV power boards. This manual assumes that the readers are professional electronics technicians who have full knowledge of circuit fundamentals and a robust education on using various test equipment. This manual does not teach about electronic circuits or how to use test equipment. This manual is intended for professional technicians getting familiar with testing various IC’s on LCD TV power boards and the basic troubleshooting methods used to help confirm a faulty IC.

Information here is general (broad range) and does not pertain to one specific IC part-number, power board revision type, or its topology (SMPS or Linear). These are the IC’s that have caused well-known problems on LCD TV SMPS power boards (not linear), the IC's are listed below. Some basic testing methods are also provided. The IC chip fault symptoms vary according to function and its intended use in a power circuit. Basic IC chip testing methods also vary according to the IC’s design implementation, functional topology, and its intended use (category of operation) in a SMPS circuit.

List Of 3 SMPS IC Chips With Known Failures: LCD TV Power Boards

PFC IC: dead, or power-on & immediate shut-off

PWM IC: dead, or main power momentarily cycles off then on again

Mode Control IC: sudden shutdown from main power to 5V-STB only

This handbook is compiled under the assumption that all other 'thorough-hole' components on the power board have been tested and confirmed to be in good-standing having only the IC's left to test (also testing SMD components tied to the IC pins). This handbook is geared towards identifying various IC chip semiconductors located on LCD TV power boards in a range of 3 functional categories, and a list of case history of failure symptom’s. The handbook covers a popular range of power board IC

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failures and fault symptoms such as the failure list mentioned above' and how the chip failed according to service records.

The test equipment used to service these IC’s are standard requirements such as a digital oscilloscope, and a good DM meter. Voltage test are a standard and waveform measurements using digital oscilloscopes are also a standard to observe various high-speed drive waveforms such as PWM, Inverter Drive Signals, Gate Drive Signals, Logic Signals, Oscillator Signals….and other critical I/O waveform data. This handbook is a servicing reference compilation collected over the last 7 years of component level troubleshooting on LCD TV power boards. The information compiled here are about various power board IC’s that were found to serve critical functions in specific areas of troubleshooting on the power board.

The troubleshooting data listed for LCD power board IC’s are from many different LCD TV manufacturers and various LCD TV models serviced between 2003-2010. The original intent of compiling this handbook data was to be used as an in-house reference source for fast troubleshooting diagnoses and repairs. Now this secretive information has been made available to help other professional TV technicians. Professional TV technicians who use this handbook can develop an idea as to what some of these IC do, how they have failed, and their placement category.

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What Technicians should get used to:

When servicing LCD TV switching-mode power supplies (SMPS) there are IC's that control the switching of current which determines the DC voltage level in various area's of the power board. These IC's also control other functions such as the timing of linear-current-switching, logic commands, and logic levels between inverter board(s) and the main digital board. These IC's on LCD TV SMPS boards have different functions that serve a few purposes, and that is to supply and control current/voltage conditions when the SMPS is turned fully on (main power) and at the same time be energy efficient along with reducing EMI conditions as well as monitoring and feedback control.

However, many times the location of these IC's will be found physically mounted on the foil side (solder side) of the power circuit board. This means technicians will have to get used to physically orientating (turning over) the power circuit board to access the pins if these IC's for troubleshooting tests. The physical placement and mounting structure of many LCD TV power boards make turning the board over (for testing) difficult and frustrating as many times the wire latches that physically connect the power board too the main-board are not long enough to stay connected while turning the board over. In cases where the wire latches are to short to flip the board over for testing, the technician will have no choice but to cut and extend the wires, which is required in many cases (this is part of the frustrating nature when testing IC's mounted on the foil side of power boards).

Wire latches for the inverter boards are usually long enough for board flipping, and sometimes their not. NOTE: Its very important for safety reasons to use a vari-ac and resistive insulation material between the power board and its mounting area when the board is on its foil side and being tested. The bottom portion of most power board mounting areas are the metal casing for the CCFL/EEFL lamps, and its important to insulate this area and any other exposed metal near or around the SMPS when testing voltage on the foil side. Also, testing the SMD components that are tied to the critical I/O pins of these IC's are very important to become familiar with as well, and not to passively overlook . In-house service records reveal faulty SMD components such as shorted signal-diodes, open switching NPN's, and open load resistors have caused integrated circuits like high dc-voltage Power-Switch IC's to fail at start-up.

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Fig1A illustrates a basic example of placing the insulation material in-and-around the bottom portion of the SMPS mounting area.

SMPS & Linear

Large screen LCD TV's exclusively use a power supply technology called 'Switching Mode' (SMPS) which is also known as a DC/DC converter board. SMPS and Linear are the two most popular families of regulated power supply technology used in today’s electronics. However, SMPS is by-far the number one preferred technology over linear for large LCD TV screen sizes (37-55 inches). Linear power supply boards are usually found on smaller LCD TV's, for example 15 inch-to-27 inch screen sizes, look at Fig2A below for a basic physical idea between a SMPS board and a Linear power supply board, i.e. LCD TV's.

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As illustrated in Fig2A the voltage input differences are basic, as the SMPS takes in the AC main voltage at the input and uses a filter circuit and rectifier circuit to convert the AC to DC (high-low voltage) at the output. The Linear does basically the same (aside from a few switching characteristics) and all the AC filter and rectifier circuits are housed in the casing for the transformer (which is the square block that plugs into the ac-wall outlet). The DC output from the linear power supply is regulated at around 18Vdc +/- and this 18Vdc output is fed to the input of the linear circuit board. The 5 IC chips in this handbook have special emphasis on SMPS technology (due to the IC's usage in the SMPS circuit) because shop records indicate that SMPS's are the type of power circuits these IC's were pulled from. However, to add some basic contrast to the overall subject between the two power technologies, we will do a quick briefing on SMPS and Linear Power Supply (LPS). After which, we will start covering the main subject which are the 3 chips found on LCD TV SMPS boards that are known to fail, with notes on how they failed, the fault symptom they caused, and basic testing methods.

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SMPS & LinearQuick Briefing

Let's start with linear power boards:

Linear power supplies operate by taking in 120VAC (or 220VAC outside U.S.A.) from the AC-mains outlet and decreases the AC to a lower value by use of a step-down transformer. The lower voltage is still AC, from this point a set of diodes are use for rectification which transforms the low voltage AC into a low pulsating AC-voltage. An electrolytic capacitor is used for filtering the pulsating low-AC into semi-DC (still not 100% DC) therefor the DC voltage received after the filter capacitor still has small amounts of oscillation. The small amounts of oscillation are what engineers call ripple', in which case voltage regulation is required at this point. To achieve 100% DC-voltage regulation (eliminate ripple) engineers use a voltage regulator IC chip at the regulation stage or some engineers use a Zener diode. The voltage output from this point is genuine DC voltage, take a look at Fig3A. Linear topology is very straight forward.

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Fig3A illustrates a basic functional standard of linear power supplies, and as mentioned previously linear power supplies are practical for low power applications such as very small LCD TV's. When high power is required for large LCD TV sizes (37-55+ inches) linear power supplies are extremely impractical, as they would physically be to large for the task. First off, the transformers physical size and the capacitance (size) of the electrolytic filter capacitor are inversely proportional to the frequency of the AC input voltage. The smaller the AC voltage frequency the larger the size of those components will be physically, and the larger the AC voltage frequency the smaller those components will be physically. 60Hz is low AC voltage frequency from the AC outlet (U.S.A.) so if large screen LCD TV's used linear power supplies the transformer and capacitor would be extremely large in physical size (impractical).

This is conversely related to wattage as well because the greater the current consumption required by the load circuit (i.e. power) the larger the transformer will be in physical size. Considering a linear power supply for large screen LCD TV's is highly impractical if not insane simply because it would be very large in physical size and very heavy in physical weight. Of-course the solution engineers have used is a high-frequency switching concept (SMPS). Some SMPS have a loop circuit, meaning the circuit that switches the 'switching-transistor' is fed back from the power supply outputs. This system raises or lowers the duty-cycle of the applied voltage going to the transformer depending on the TV's consumption, and this is known as Pulse Width Modulation (PWM).

This means the SMPS looped power supply will readjust its self (via PWM control) in accordance to the current consumption of the load connected to it. So when the LCD TV is consuming a lot of power, the power supply readjusts its self to produce less current, causing the transformer and all associated components to run cooler by dissipating less power resulting in far less heat generation. A linear power supply in comparison is always set to produce its maximum power output regardless of the load circuit not needing maximum power 'at a given time. Even though the load circuit of a linear supply may only require a little bit of current (at a given time), the linear power supply still produces maximum power.

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As an inevitable result, all the components of the linear supply are functioning at their full operational capacity even when its totally unnecessary. As an end result large amounts of heat are being generated unnecessarily, and the intended operational life expectancy of the components are continuously hampered by these hazardous thermal conditions.

Let's move on with SMPS power boards:

Fig4A illustrates a very basic diagram of a SMPS using an Active Power Factor Correction IC and a PWM IC, NOTE The PWM control IC can be integrated into another IC such as a PFC IC, or Mode Control IC,, it doesn't have to be a separate semi-conductor chip (as illustrated in Fig4A). However for learning purposes we will keep the PWM IC as an external function (separate IC) for now.

Some SMPS manufactures don't incorporate PFC chips with in the design of the SMPS circuitry and these are usually very cheap, low-end SMPS of

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minimal quality and usually have a 110v/220v power switch and voltage doublers (consisting of two large electrolytic caps and extra power rectifiers).. PFC IC chips (and other chips) are used on better quality higher-end SMPS board designs found inside many LCD TV's. In Fig4A the block diagram is very basic, and there are other circuits such as the feedback monitoring circuit, short protection circuit, start-up circuit, stand-by circuit, etc, which have been left out to try and keep the concept of critical function & failure as simple as possible to understand.

LCD TV SMPS: referred to as DC-to-DC convertors

Perhaps the term (dc/dc-smps) is referenced to a SMPS using the PWM IC's loop circuit (FB) to perform the voltage regulation at the secondary side. (Fig4A). At the primary the voltage input rectification is accomplished before going through the switching-mode transistors, after which the switching transistors output is squarewave, so the input to the transformer is a square-waveform. So, therefore the output of the transformer is square-wave AC as opposed to a sine-wave AC, and a square-wave at the transformers output make the task of full DC voltage conversion (and further regulation) a very easy task. So after the rectification process and from the output of the transformer (and through clipping diodes) the resulting output voltage is now DC (hence the term dc/dc-smps).

The feedback loop circuitry that's used to address the PWM control IC is what makes all the necessary DC regulations. If an erroneous output voltage occurs, the PWM control IC alters the duty cycle (waveform) of the transistors applied-signal so that the PWM IC can correct the transistors output according to the error (duty-cycle refers to the on/off switching time of the transistors). The percentage of the duty-cycle (full-on or full-off) is in accordance to the LCD TV's power consumption, meaning when the TV's power consumption is elevated the voltage output declines and when the TV's power consumption declines the voltage output is elevated. Many 'Senior Engineering Technicians' also refer to TV SMPS as 'AC/DC inverters' hence inverting 120VAC to high-voltage DC hot-side, then the hv-DC get's stepped down to lower DC values i.e. 12V, 24V, 5V, 3.3V, cold-side (hence DC/DC converter).

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Basic Notation:

110v/220v switches will not be found on SMPS that have Active PFC IC chips, in which case they don't require a voltage doubler circuit. North America uses 110VAC-120VAC from the power grid (AC outlet), Asia and the United Kingdom use 220VAC from the power grid (AC outlet). On LCD TV SMPS usually 2 or 3 power MOSFET's transistors are used to form the switching circuit. There are various mathematical design methods that engineers can use to configure FET switching. After the transient filtering stage, the waveform applied to the transformer is a squarewave (at switching-mode Q1 Fig4A), so therefore the transformers output is not sinewave, instead it is squarewave (makes for simple DC conversion).

The PWM control IC is always isolated from the primary using a smaller sized transformer, but instead of a smaller transformer at this stage most engineers use an opto-isolator IC (aka opto-coupler). The PWM control IC utilizes the outputs of the power supply to control the way in-which it will drive the switching function of the transistors (correcting the transistors output when an error occurs). Most Active-PFC chips have a self-shutdown or 'suspend' feature if a sudden voltage-error extends beyond a predetermined frequency threshold.

Our emphasis through out this handbook are LCD TV SMPS that use Active PFC

On LCD TV SMPS that use Active-PFC will have one filter capacitor as SMPS without Active-PFC will have two large filter capacitors (voltage doubler). Its very common to see LCD TV SMPS containing three transformers between a couple of metal heat sinks, Fig5A illustrates a very basic example of the transformer layout (there are dozens of configurations). T1 is the main transformer and usually the largest of the three, T3 generates the DC bias 5Vstb-3.3V output, and the PWM chip uses T2 (isolator) to isolate the primary from the secondary.

Many SMPS for LCD TV's incorporate opto-isolators as an option to using coiled components such as T2. SMPS using opto-isolators tend to have only one or two transformers on board. As you can see one heat sink is dedicated to the primary side of the board as the other heat sink is dedicated to the secondary side. The switching transistors, the Active-PFC transistors, and 3-pin diodes will be screwed into the primary heat sink if PFC is used. In some

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design cases there will be two heat sinks dedicated to the primary side. This is because some engineers decide to use a separate heat sink for the PFC transistors and 3-pin power diodes.

Often on the secondary heat sink there will be several 3-pin power rectifiers each of which contain two internal power diodes, along with smaller electrolytic capacitors and energy coils in the low-DC filtering stage. The DC filtering stage filters the DC output voltages for DC bias i.e. 24V, 5V, 3.3V, 12V, etc.

Now let us quickly focus on Active-PFC for LCD TV SMPS:

PFC IC's on LCD TV SMPS can have up to 4 power MOSFET transistors, usually there will be 2 as illustrated in Fig6A for example. These transistors are screwed in to a metal heat sink located on the primary area of the SMPS board. The PFC power diode is also screwed on to the same heat sink, and

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this diode can be in a 3-pin or 2-pin configuration, and the PFC inductor coil is usually the larges coil on the SMPS board. The resistor illustrated in Fig6A is a NTC thermistor, and this is a resistor that alters its resistance in accordance to temperature conditions. The resistor-thermistor configures thermal (hot) conditions on the SMPS after a few hours of operational use, it is a Negative Temperature Coefficient (NTC) device.

NOTE: the PFC control IC can also have an internal PWM control circuit built inside, in which case is known as a PFC/PWM combo IC semi-conductor. Fig7A illustrates the power components that are screwed into the metal heat sink located on the primary side of the SMPS, the PFC components consist of two power MOSFET's, one power diode, and the switching transistors are on the same heat sink. LCD TV SMPS that use two switching transistors are either two of the following SMPS technology, and that is: Forward based (2 power diodes), or Push-Pull based (no power diodes). T1's primary end is connected to the switching transistors (PWM IC corrects the transistors on/off duty-cycle).

The secondary end of T1 is connected to the rectifier diodes and filtering capacitor-circuits producing SMPS low voltage DC outputs (3.3V, 24V, 12V, -5V, +5V). T2 provides the +5V stand-by voltage output, a stand-alone circuit generates this voltage better known as 'stand-by power'. The stand-alone 5VSTB circuit is always 'On' (even when main power is off) and this voltage is used by the main board. When the power key is pressed the main board will instruct the SMPS to turn-fully-on 'main-power'. T3 is used as an isolator transformer joining the PWM IC to the switching transistors, but many times T3 will be substituted by an opto-isolator IC.

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We will discuss what Power-Good Signaling is, shortly.

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SMPS engineers have a couple of design options for rectification at the secondary stage, one of the most popular designs among low-quality SMPS is implementing a +3.3V regulator to the +5V output rails, sometimes this approach will include a positive Buck-Regulator IC. Also many low-quality SMPS's use a coupling-configuration where the 3.3V is developed through the +5V line and both +3.3V & +5V outputs share the same rectifier. The most common design choice for high-quality SMPS is by implementing a solid rectification and filtering circuit for the +3.3V-line while simultaneously using the same transformer occupied by the +5V rectifying circuit. Separate rectifiers are used for the +3.3V & +5V outputs on high-quality SMPS, and the 12V/24V output ports have their own individual rectifier as well.

The small electrolytic capacitor-circuits coupled to the power rectification didoes (on secondary side of high-end SMPS) are what provide further low-DC filtering. High-end or high-quality SMPS boards will usually have a Power Good Signal IC (or circuit), this 'power-good' function can be integrated into a PFC IC, PWM IC, or Current Mode Control IC chip. When PG is integrated into an IC (like a PFC chip for example) its better known as a PG time-out feature labeled at one of the pins of the PFC chip. Power-Good (PG) which is also known as a Power-OK circuit can comprise of discrete components (and not an IC) as I will explain further because this is important to know when troubleshooting an LCD TV SMPS with PFC. Equally important to know is that cheap low-end SMPS will usually have artificial (fake) power-good signaling which I will also explain.

Power Good Signaling

High-end LCD TV's will always have an authentic power-good circuit or IC on the power board. There are failure cases with these kinds of LCD TV's where at the time the power key is pressed the start-up LED indicator go's from steady-on, to flashing once momentarily (0.5sec) then immediately defaults to stand-by only. Even though this kind of fault symptom can be very similar to a PFC chip failure, in most cases if its the PFC failing critically the 5Vstb circuit will be disabled. This leaves a totally dead power situation after a start-up attempt, requiring a hard-reset (unplugging/plugging AC) which will simply repeat the fault over again. Usually a power-good failure will default from start-up attempt, then immediately returns back into

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stand-by mode keeping the independent 5VSTB circuit on. (independent meaning the 5VSTB circuit is separate from the 5V system voltage, better known as 5VDC bias i.e. 5V-SYS voltage).

When this quick start-up flash happens (half second) on the LED indicator, its usually caused by the absence of a power-good signal from the SMPS. When the power key is pressed (or turn on by remote control) the main board actuates a logic signal (called PWR-On) which is addressed to the SMPS instructing it to turn on full power (enable main converter). As the Soft-Start (SS) boot-up process begins, the start-up voltages used to initialize boot-up are closely monitored by feedback circuitry on the main board which is tied to the power-good line of the SMPS.

If all voltages and current conditions are in good standing, the power-good line will alter its logic state to an Active low or high (0 or 1) depending on logic command operands. So in a fault situation described above where the SMPS is attempting to start-up, the likely cause rooting the symptom is: no power-good signal being issued at start-up. NOTE: The previous 'above' assumes all other possible causes of a 'start-up/shutdown symptom' have been confirmed and cleared.

What is Power Good?

Power-Good is simply a circuit or an IC on a SMPS that prevents the SMPS from starting-up prematurely, meaning without a PG signal the main board would be too quick in producing instruction sets (at start-up), and the SMPS too slow in developing stable voltages for the main board to instruct properly. When main power on the SMPS is engaged (via power key) time is required for the components (especially linear circuits) on the SMPS to stabilize and reach nominal electrical operating conditions. A window of time is necessary 'electrically' for a SMPS to do a power-good self-check, then properly supply the main board with accurate and stable DC voltages required to start-up its other circuits (A/V switching).

A high-end LCD TV turning on without power-good signaling will simply not happen under normal operating conditions and this is by intentional design (time in 'mS' is required for authentic PG). This is a large reason why it takes a few seconds for an LCD TV to turn on under normal conditions..

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Engineering an LCD TV to turn on without power-good will certainly result in weird problems suddenly arising (at the start-up phase). .and here's why: If an LCD TV were designed to turn on without the time required for power-good signaling, unstable power will be present and improper voltages will be addressed to the main board. A SMPS can take a full second to reach electrical stability, and a full second to a micro-processor is the equivalent to 1000 years. Modern A/V processors and micro-controllers can process and cycle a half billion instruction sets in milliseconds. So, in order to prevent the LCD TV from starting up and turning on prematurely, the SMPS has a power-good circuit or IC ad-dressing a PG signal to the main board. When the SMPS boots-up it will first run a 'self-test' to confirm all of its circuits are functional (meaning circuits are Good or OK, hence power-good or power-OK).

Once the SMPS finishes its 'self-test' it will address a power-good signal to the main board, informing the main board that the power is good and ready to be occupied. The main board will not start-up the TV without the power-good signal. The SMPS (under popular design) will disable the power-good circuit (or IC) under a critical fault condition such as a power MOSFET suddenly shorting out, thus forcing the SMPS to malfunction (shutdown). Once the MOSFET is replaced the SMPS will enable the PG signal again which resets the TV at a fresh start-up profile (an industry term meaning a new power-up attempt after a part replacement or servicing). Power-Good features are a standard with LCD TV's, more and more mid-grade LCD TV's have power-good signaling, and operating spec's will post the approximate time when the PG sequence is asserted (power-up time). Its the low-grade or low-end LCD TV's that have been well known to use fake power-good signaling, and very few these low-end LCD TV's actually use authentic power-good signaling circuitry.

What is Faking Power-Good Signaling?

When LCD TV manufactures are engineering and designing a new main board for a new LCD TV to be produced, the processing chips they purchase to be used on the main board will only work with power-good signaling for safety reasons (these processors have a PG receive pin, to be received from the SMPS PG circuit or IC). The intended purposes of Power-Good signaling is to deter any potential electrical hazards and improper system

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start-up. Power-Good Signaling is an industry design standard regulated by major standards-committees like the IEEE and UL. +5V is the normal operating voltage of the power-good signal line. Low-end LCD TV manufactures (that fake PG) will simply connect the processors +5V Power-Good pin to a +5V line of the SMPS. Connecting the Power-Good pin to a +5V line of the SMPS is NOT an authentic working power-good signaling system at all because there's no self-check system, no timer, and no monitoring logic system in place to confirm that power is 'functional and ready' (no Active high/low PG logic signals (timing) between the main board and SMPS).

Connecting the +5V power-good pin of the processor to a +5V line is simply a dummy PG line, and it falsely triggers the processor on the main board to start-up. This is bad, because this forces the main board to attempt system start-up before the SMPS has any reasonable time to stabilize output power. This is another contributor as to why low-end LCD TV's have such a short functional life span, often failing with-in already limited warranty claims.

Simply put!Power Good

PG's purpose on a SMPS is to inform the main board that all is great with the SMPS and the main board can turn on its core voltages and begin system boot-up. If the PG signal is not available at the start-up sequence, then the main board will automatically be stationed in 'reset mode' (won't boot-up core voltages). If PG is suddenly unavailable during operation, the core processor of the main board will shutdown promptly. The PG signal simply deters the TV from functioning with inappropriate voltages and erroneous timing sequencing resulting in self damage to the main board. Power Good is a +5V signal produced from the SMPS board only after the SMPS has confirmed and passed its own self-test and its output supply voltages have stabilized.

The SMPS's approximate time between its self-test and sending a PG signal is usually around 100mS to 500mS after its turned fully-on and the way to test a PG signal is using a scope and measuring its time period in milliseconds (mS) and faulty time values are (under 125mS or over 950mS) a good PS-ON output signal (from the main board) is usually between 10mS

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and 100mS. After the SMPS's 500mS second self-check the PG signal is then addressed to the main boards core processor which has a PG receive pin and internal timers that control the reset line to the processor. This means the internal timing feature of the core processor controls the timing of the high/low state change of the reset line which is tied to PS_ON & POWER GOOD terminals.

In Fig9A you can see a basic schematic illustration of the components that create a power-good circuit on the SMPS board. They are SMD components on the foil side of the board (excluding the MOSFET Q2 which is through hole). If a power-good component fails such as Q3 being shorted at the collector this will not only stop a PG output signal but also pull the system 5volts (5V_SYS) down which results a 'No power-up' fault symptom. The reason there's no power-up is because the main boards core processor is not reviving a power-good signal at its PG receive pin from the SMPS board.

Therefore the internal timing feature of the core processor keeps the processor in constant 'reset mode' which stops the LCD TV from operating its self using hazardous and unstable voltages. When Q3 is replaced with a new NPN transistor, the power-good signal is now produced and then sent to the main boards core processor. When the core processor receives the PG signal (at its PG receive pin), the processors internal timer will cancel the resetting of the processor (which wakes it up) and from this point the processor on the main board immediately starts a binary-address code for booting-up its core voltages for ROM, RAM, and A/V-switching.

So, keep in mind when the PG signal from the SMPS is suddenly unavailable, the processor on the main board is immediately held in rest mode (will not boot). If a power surge were to happen, the PG signal is canceled immediately (by the SMPS) and the TV go's into reset mode, also known as protect mode. So by canceling the PG signal and shutting the TV down (in the event of a surge) the surge will not make it into the TV because the operating power of the TV is quickly stopped by removing the PG signal. Again this action prevents the TV from operating under unstable voltages and inadequate power levels which cause errors in timing sequences and other parity faults. The low-end SMPS that use fake-PG only connect the PG output line to 5 volts, so when a surge or fault occurs the processor on the main board has no way of resetting....that’s why fake-PG wiring is such a

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bad practice, in fact some PG designs don't monitor the +12V/-12V lines and still issue a PG signal anyway.

Fig10A will provide the basic timing sequences between the input of the PS_ON signal, the DC stabilizing time, and the PG output signal. Exact timing values are optional to board design, signaling speed, and switching topology used for the SMPS design. PG output signal is almost always 500mS as this is a popular time window for PG signaling on most quality LCD TV's.

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This concludes SMPS & Linear Briefing

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Lets get started on faulty SMPS IC's

Active Power Factor Correction ICPFC -IC is situated on the HOT side of SMPS's

The first IC noted is the 8 pin Active Power Factor Correction IC, and there are two kinds of PFC IC's one is Active-PFC the other is Passive PFC. The two chips cater to two different PFC topologies, APFC is configured for active components such as transistors, op-amps, and logic-gates. PPFC is configured for passive components such as filters, capacitors, resistors, and inductors. Passive-PFC uses filters to cancel harmonic currents and phase-shifting inductors or capacitors to adjust the current into phase with the voltage.

LCD TV manufactures don't use Passive-PFC topology on their SMPS's because this design approach is physically big and heavy in weight. PPFC involves using a huge low-pass filter at the AC-mains to counteract inefficient loading, not to mention very expensive (however, some Passive techniques are applied to stand-by power efficiency). APFC IC's are now an increasingly required feature for new SMPS design architectures. APFC circuits have dual feedback control loop circuits, one loop functions fast by utilizing the voltage input as a reference source to control the current input (current-mode control).

This particular control loop circuit is quick and makes the current input proportional to the voltage input in reference to a resistive load. The second control loop circuit functions at a slower pace and it controls the continuum of proportionality, to make the average current correct, this is to keep the voltage output from the PFC buck/boost circuit constant and steady 'regulated' (called a BUS voltage).

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How APFC works:The purpose of using APFC on LCD TV SMPS is to make the current input seem like voltage input on a time-by-time platform, the same function as would be together with a resistive load. Fig13A illustrates a basic example of the line current and the line voltage together with APFC, and with no APFC.

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Active PFC is an IC chip semiconductor that controls the input current of the load so that the waveform of the current is proportional to the AC mains voltage waveform i.e. I/V in-phase (making the current follow the voltage, thus making the load on the AC line appear resistive). By having the AC line appear resistive is what basically keeps the current proportional to the voltage with PFC (the same fundamental principle of a standard resistor in a circuit keeps the current proportional to the voltage). Active PFC is referenced to the reduction of distorted harmonic content and the phase angle alignment of an incoming current. Active PFC is necessary in the reduction of disturbance on the AC mains line and maximize the real power drawn by the SMPS from the AC mains line.

The two FET's back in Fig7A create a buck/boost converter circuit and they switch on & off simultaneously (also consisting of some passive components such as a PFC-inductor, diode, and capacitor) which simply produce a BUS-voltage from the rectified AC input. Above Fig11A in previous pages two loop circuits were mentioned, one fast (current-mode control) and the other slow (regulate the output BUS voltage). This is done by initializing a fast response to the AC input voltage (instantaneous rectification) and a slower complex response to regulating the BUS voltage. So this means even the though there’s still resistance sensed by the AC line, the value of the resistance is slowly altered as required to maintain the BUS voltage as close as possible to its intended value.

The point in having the power-diode is to regulate the average current passing through it so that the current remains proportional to the instantaneous rectification of the AC input (any shorts/opens at the PFC diode will cancel the PFC function, resulting in a high crest factor then board shutdown). Basically, the function of an active-PFC control IC is sensing (detecting) the shape of the AC input waveform and modulating the duty cycle of the buck/boost converter FET's to mimic the behavior a resistor. Meaning there’s no current drawn at the zero crossing and maximum current drawn at the AC peaks.

The duty cycle control of the buck/boost FET's are handled by the PWM IC or in many cases the PWM will be an added feature in the PFC IC. In reference to Fig7A the APFC control IC turns the MOSFET's fully-on which raises the current through the PFC inductor (largest inductor on the SMPS board) and turning the MOSFET's fully-off allows current to pour into the load (which reduces the current). In a basic sense, the APFC control IC

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determines the on/off period of the FET's to control the current through the PFC inductor (it also regulates the voltage output).

SMD sensory network circuits for APFC IC

The SMD sensing circuits (on foil side of an LCD TV SMPS) for an APFC control IC are critical because the waveshape sensing circuits simply provide a reference input signal to the APFC control IC. The reference signaling is usually in the form of current that references the AC waveshape after the bridge rectifier. If the APFC IC has a PWM feature built-inside, it will use these PWM pins to control the duty cycle of the FET's on the buck/boost converter circuit. If the PWM IC is a separate external IC (rare) it will control the PFC IC's duty output to the buck/boost FET's.

SMD sensory circuits for an APFC IC are as follows:

Sensory circuit for AC input waveshape sensing,Sensory circuit for DC output, (BUS-voltage)Sensory circuit for MOSFET current. (Current-mode control)

Some key benefits to SMPS Active -PFC technology:

1) Optimizes and promotes circuit performance2) Minimizes power costs3) AC mains peak current is decreased sufficiently4) Natural harmonic content is sharply reduced5) Reduces transformer size and weight6) Improves regulation of DC/DC converters (down stream)7) Meets Green Energy requirements

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Why LCD TV Engineers Are ForcedTo Implement Active-PFC in SMPS design

Designing and engineering SMPS for LCD TV's have always a monumental challenge to even the most experienced engineers. Even though engineers have worked very hard to overcome traditional problems in SMPS design, growing and emerging regulatory industry standards that govern efficiency levels are starting the cycle of design standards all over again. The first stage of this regulatory cycle is already up and running and is totally focused on improving stand-by power consumption levels, but this is for passive mode so no real worries here for LCD TV SMPS engineers.

But, the second stage of this regulatory movement confronts a tougher challenge of improving active-mode efficiency levels. Governmental agencies around the globe who are influenced by the Institute Of Electrical & Electronics Engineering (IEEE) US Environmental Protection Agency (EPA) and their Energy Star program along with the China Certification Center Standard, are all announcing new and innovative performance standards for active mode efficiency for LCD TV SMPS (and other electronics).

This means LCD TV service technicians need to familiarize themselves with Active-PFC SMPS technology, and other IC's such as Mode Control and PWM which we will cover. The same as engineers are now having new design headaches and challenges (because of new regulatory standards), servicing technicians will be challenged as well, as they are now forced to develop more intuitive troubleshooting techniques. The new standards are intense and very aggressive (here in 2013), and this will require joint efforts of manufactures and their suppliers and this especially includes semiconductor manufactures to produce solutions that meet these new design challenges which are now standard among SMPS design architectures. To go any further about PFC would be beyond the fundamental scope of this handbook, below are the failure case history on 8-pin APFC IC's on LCD TV SMPS. There are various 8-pin APFC chips that may have an extra feature or two but they all have the same critical pins (in respect to 8-pin APFC IC's).

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Testing IC Chips:

Before we continue on with the APFC IC and its case history of critical failures (along with other SMPS IC's), lets go over some basic methods of testing IC's in a general sense. Many times junior-technicians (resent university graduates) who enter the field of industrial video servicing will ask a professional TV technician in which way should they test a particular IC chip.

Often the question asked will be:'is the IC tested in-circuit or out of circuit?'

Well, the answer is 'both' because it depends on the IC being tested and the nature of the failure that’s rooting the fault symptom. Some experienced technicians will go ahead and change a chip simply because their testing methods have lead them to suspect that particular chip (usually under strong suspicion). In many servicing cases their suspicion will be correct (which means replacing the IC cured the fault), and sometimes their suspicion is incorrect.

An incorrect diagnoses of an IC is known immediately when the suspected faulty IC is replaced and the fault symptom of the circuit-board still remains. Professional technicians who replace IC's without testing them usually do this because of three reasons. The first reason is because they are usually correct in their suspicion (i.e. professional) the second reason is usually to save troubleshooting time, and 3rd because the IC under suspicion is so cheap ($1.50 gate IC) that its not a large financial loss if its a misdiagnoses. The 3rd reason would not be a loss in troubleshooting time, because logic-gate IC's are generally easy and quick to test with a scope or logic probe.

However, there are IC's that are not so cheap and not so quick and easy to test, usually costing $35.00+ dollars such as a particular type of driver IC, or a special-function quasi-IC (some micro-controllers/processors costing $85.00+). A misdiagnoses with these chips can easily cut into profits and can make the difference between a profitable LCD TV repair or a financial loss on that customers LCD TV. Bare in mind that some of the most experienced technicians in this industry make an occasional misdiagnoses with IC chips, but this is how knowledge is earned (trial, error, and headaches) and is well worth the effort.

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In Or Out Of Circuit IC Testing:

The method of testing an IC also determines if the IC test should be conducted in-circuit or out of circuit. For example if a resistance test is being conducted on an IC (such as an audio IC) then its better to remove the IC out of circuit for accurate resistance readings. Conducting resistance readings on an audio IC usually means the technician already knows what the proper ohm values should be between specific pins. The technician will usually know these resistance values either from a schematic of the audio IC or recording the values of a new audio IC and comparing those values against a suspected audio IC of the same part number. Some technicians may decide to troubleshoot the same audio IC by testing for an I/O waveform-error between the pins, and this is an in-circuit test of the audio IC.

But some IC's are tested for voltage out of circuit, such as an Op-amp IC that’s used in circuit as a step-up DC converter. The op-amp is taken out of circuit and a voltage is applied to its input, and the resulting output determines if the Op-amp is functioning properly. For example if the step-up Op-amp is suppose to take in +5VDC at its input and step it up to +11VDC at its output, all the technician does is take it out of circuit and apply +5VDC input and measure the DC output (which should be +11VDC). If the +11VDC is not there, then its confirmed that the Op-amp is not functioning. Performing voltage and/or waveform tests on IC's are usually the best testing method testing opposed to a resistance test (which can prove inaccurate especially with modern Smart IC's). Putting the IC under a voltage really reveals the IC's true performance characteristics and the effect its having (or not having) on the circuit under test. Chips like Op-amps, and FET IC's can be tested out of circuit using a working voltage to determine their output function or a resistance test for a short condition of zero ohm's.

However, for smart-semiconductor IC's such as those on an LCD TV SMPS are tested in-circuit because IC's like AFPC, Current Mode-Control, PWM, and Supervisory IC's on SMPS are monolithic (multifunction) and require external circuits to operate and therefore are tested in-circuit to determine their area of failure (meaning which I/O pin(s) on the IC are faulty). Some IC's are tested in-circuit by suspending a pin or two and observing its resulting output, this testing method requires adequate knowledge of the IC's functional influence on the circuit under test. Smart IC's for SMPS require the function of their external circuits (discrete through-hole and SMD

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components) for them to operate properly and perform their intended circuit function. So, out of circuit testing with Smart IC's on LCD TV SMPS is not recommended, however some engineering technicians know enough about circuit design calculations and use resistors to simulate the load of external circuits connected to the pins of smart-IC's. This approach of using resistors to simulate an IC's external load connection are normally used by circuit engineers for design-and-test purposes and this requires plenty of mathematical algorithms to determine impedance and resistive load values. Needless to say this an engineer's method of testing smart-IC's and is generally beyond the interest and purpose of servicing technicians for obvious reasons.

More and more SMPS for LCD TV's are incorporating hybrid IC's on-board to improve operating conditions as well as reduce the number of discrete components. This is all in a design effort to keep the SMPS small, and as low profile as possible. Some professionals refer to hybrid integrated circuit boards as Multifunction Chip Modules simply because one hybrid board can comprise of a few monolithic IC's performing various processes and instructions.

The figure below illustrates a very basic example of the physical difference between an IC chip and a Hybrid Integrated Circuit (HIC)-board. Often hybrid circuits will be found molded in a white, brown, black, orange, or gray epoxy coating which makes repairing the hybrid board close to impossible as this epoxy hardens and bonds to the IC's and SMD components mounted on the hybrid board. The epoxy coating is applied for various reasons such as thermal protection, physical handling during assembly, and to resist against moisture build up.

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This concludes testing IC chips.

Back to the APFC IC:

Fig14A reveals a typical internal architecture of an 8-pin APFC, the pin setup normally includes (but not limited too) #1 invert pin, #2 compensation pin, #3 multiplier pin, #4 current sense pin, #5 zero current pin, #6 ground pin, #7 gate drive pin, #8 Vcc pin. The case history of failure happens at pin #2, pin# 4, pin #7 and pin #8, failure between these pins have caused a totally dead TV (no stand-by) or have caused an immediate shutdown condition after full power-up. There are also some discrete components tied to these pins that create different circuits such as compensation circuits and a couple of other external circuits that will be mentioned.

These discrete components that are tied to the APFC pins formulate external circuits, and they too have failed and need to be tested as well. The 8-pin APFC IC is commonly located on the foil side of the SMPS, and its on the hot-end of the board usually mounted underneath the primary transformer but on the flip-side of the board. Sometimes the APFC IC is found burned at

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the CS pin(s). Fig15A demonstrates a typical APFC IC schematic setup in an LCD TV SMPS circuit, and testing an APFC IC on an LCD TV SMPS usually starts with testing the SMD components that create the start-up circuit for the APFC chip. The discrete components that create the APFC start-up circuit comprise of a bridge rectifier (AC mains) and a charge pump circuit made up of 2-4 capacitors, 1-2 resistors, and1-2 diodes. Another part of the APFC IC's start-up circuit is a charge-pump circuit, which normally comprises of 2 capacitors, 1 resistor, and 1 diode. These start-up components should be checked for shorts or opens before testing the APFC IC for electrical faults.

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In Fig16A the green out line are the components for start-up and charge pump circuits, and they are connected to the Vcc pin #8. Many times the start-up resistors (+/-220Kohm) have been found to be opened and in which case the SMPS will not engage full power-up without APFC start-up. In many servicing cases if the bridge rectifier shorts out it will also take out the APFC IC along with some of the components that create the start-up circuitry. This is the reason why in many servicing cases the technician will replace the shorted bridge rectifier with a new one and immediately after plugging the ac cord in the bridge will short again along with a blown fuse. Professional TV technicians will normally replace the APFC IC if the bridge rectifier is shorted as a precautionary measure.

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In Fig17A Pin #7 of the APFC IC is the gate-drive pin (GD) and this pin controls the buck/boost FET to switch it on & off by a resistor so that the average input current waveform is proportional to the ac mains voltage waveform (sinewave). This FET is part of the buck/boost circuit back in

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Fig7A even though two FET's are shown in Fig7A many times there will be only one huge IGBT(along with power diode).

The APFC IC pin names and their basic function Fig14A :

Pin #1 INV: Inverting input of the error amplifier. The data that rides the output voltage of the APFC per-regulator is applied to this pin through an external resistor-divider network mainly comprising of SMD resistors or a resistor network IC chip.

Pin #2 COMP: This is the output of the error amplifier. There's a compensation network (comprising of SMD components) that's placed between this pin and the INV (pin #1) so that the voltage control loop is stabilized and guarantee high power factor and very low total-harmonic-distortion (THD)

Pin #3 MULT: This is the main input to the multiplier function. This pin is tied to the rectified ac mains voltage by a resistor divider and produces the sinewave reference to the current loop.

Pin #4 CS: This is the input to the APFC IC's internal PWM comparator. The current flowing into the buck/boost FET is sensed through a resistor. The final voltage is applied to this pin and then compared with an internal sinusoidal shaped reference signal generated by the APFC IC's internal multiplier function. This is to determine the buck/boost FET's turn-off time. As stated in previous pages most APFC IC's have an internal PWM function.

Pin #5 ZCD: This is the buck/boost inductors demagnetization sensing input for a transition-mode function (not all PFC IC's have a transition-mode feature). A negative current edge triggers the buck/boost FET turn-on time.

Pin #6 GND: This is the ground pin. Current return path for the signaling function of the IC and for its gate driver, any shorts to ground will hamper the APFC IC from starting-up and turning on.

Pin #7 GD: This is the gate-driver output pin. The output stage can drive a FET or an IGBT (usually a very big IGBT) with peak currents of

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approximately 500mA source and 700mA sink. The high level of voltage on this pin is usually clamped at approximately 12V to deter excessive gate voltages in the event that this pin is supplied with a high input Vcc.

Pin #8 Vcc: This is the input supply pin for the APFC IC, which supplies the signal portion of the chip and its gate drive portion. Some APFC IC's have a high input limit (24V) to allow more room for supply voltage changes.

ERROR Amplifier:EA is an amplifier that compares the differences (error) between two signals, one signal being the input and the other being a reference. The main characteristic of the EA is the gain factor, because the larger the gain, the smaller the error it can compare (bandwidth is another of its characteristics). EA is commonly used in negative feedback circuits to monitor the input voltage difference between positive and negative voltage inputs such as a dc/dc converter or voltage-to-current converter or an LDO. An EA is used in bi-directional voltage control circuits in which the circuit under control has a sampled output voltage that is fed back and compared to a stabilized reference voltage. The differences between the two voltages produces a compensating error-voltage (external compensation circuit) which pushes the output voltage towards the target loop circuit in respect to APFC.

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This concludes 8-pin APFC IC

PWM ICPWM -IC is isolated between the Cold/Hot side of SMPS's

As mentioned in previous pages, the PWM function is available as an extra embedded feature in other special-function chips, so engineers rarely use a single-separate PWM chip anymore, because of attractive savings on production costs. Again, for learning purposes we will focus on a stand-alone (single chip) PWM IC to gain a basic understanding of its principle operation. PWM is a digital technique used for controlling analog circuits such as switching circuits, conversion circuits, and power-control circuits in a digital format. PWM is vital in analog circuits because the output of an analog circuit is linearly proportional to its input. There are a number of contributing factors as to why PWM is a standard in SMPS design and other circuits as well. Analog circuits tend to drift over time and prove very difficult to tune, even though precision circuits address this problem the drawback is that they are physically huge in size and also quite heavy (economically inefficient). Another drawback with analog circuits (among others) is that they tend to get very hot because the power dissipated

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(thermal factor) is directly proportional to the voltage across the active components which is then multiplied by the current traveling through them. Analog signals have infinite resolution and any noise on an analog signal will absolutely change its intended current value.

PWM Control:

Pulsed Width Modulation is digital control of analog signals using digital-PWM as the signal controller. Most special function IC's today already include an on-chip PWM function as an added feature which makes design implementation easy (from an engineering perspective). PWM is a popular method of digitizing analog signal levels, and PWM is widely adopted because there’s a large reduction in system costs and overall power consumption. PWM uses high-resolution counters so that the duty cycle of a squarewave is modulated to encode a specific analog signal, at a specific level. The PWM signal stays digital because the total DC supply is either fully-on or fully-off, and the source of the voltage/current is supplied to the analog load by on-and-off pulses repeating over and over again in series formation. The on-time is the period at which the DC supply is applied to the load, the off time is the exact opposite, meaning the off-time is the period at which that same voltage/current supply is switched off.

PWM can encode any analog signal with proper bandwidth, another benefit to using PWM is that the signal output stays in its digital format all the way to the input of its analog load, meaning there's no need for digital-to-analog conversion. Keeping the signal in a digital format means the noise effects are kept to a bare minimum, and the only way noise can hamper a digital signal is if its large enough to change its logic state (logic-1 to a logic-0 or logic-0 to a logic-1) When the PWM signal reaches its analog load there are RC or LC circuits used to remove the modulating high frequency squarewave and reformat the signal back to analog.

PWM output pulses are measured in whats called Duty-Cycle (requires a scope) and the duty cycle is simply the ratio (or percentage) of the switch on-time and switch off-time of a MOSFET for example. In electronics servicing duty cycle is normally measured in percentage, for example lets take a time period of 1 second, and say that a FET is switched-on 10% and switched-off 90% of that 1 second period, the duty cycle would be 10/90 percentage across a window of 1 second (usually micro-sec). This means the

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FET is operational (switched on) 10% of a 1 second time period and is non-operational (switched-off) 90% of that same 1 second time period. So this PWM output interprets a specific analog value, meaning it encodes an analog signal at 10% of its full strength level (in respect to the inverse modulating squarewave signal). By varying or modulating the time period that the voltage/current is supplied (which is the width of the positive pulse) the average voltage can now be altered, that's why its called pulse-width-modulation. Fig19A is a very basic illustration of a PWM duty cycle expression on a square-wave output.

PWM Control Continued.....

As mentioned in previous pages more and more SMPS manufactures are using digital processing (hybrids) mounted directly on the SMPS board. These hybrids usually incorporate a micro-controller IC with PWM as an added feature, in fact some micro-controllers have two PWM control features. The only problem TV technicians may face with hybrid SMPS is when the PWM feature inside micro-controller suddenly has a flawed output, the technician not only has to change the micro-controller but also

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has to program the PWM parameters with in the micro-controller. Programming the micro-controllers PWM feature is required because the software of the micro-controller demands the following parameters to be set. This programming requirement is standard for most special function IC's that have an internal PWM control feature. The companies that manufacture these special function IC's sometimes offer the programming software and interface module for the chip.

Required Programming Parameters for S.F. IC's With PWM:

In order to provide the time period of the modulating squarewave, the micro-controllers internal PWM counter/timer needs to be edited and set.

The 'on-time' in the PWM control register needs to be edited and set.

The direction of the PWM output signal needs to be edited and set. (standard I/O pins).

The start-timer needs to be edited and set.

The PWM controller needs to be edited and set.

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Fig20A is a straight forward example of a PWM IC that’s also a DC step-down converter, by changing the values of the 2 resistors and inductor that's connected to pins 5,6, and 1, will alter the voltage output (2.5V, 3.3V, 5V). This chip set has failed at pin #1, pin #2, and pins #5,#6, in which have caused a totally dead TV symptom or TV will suddenly shutdown then restart its self, or set into stand-by power only. The failure of most PWM IC's (or PWM controller feature) is usually its switching output pins 5, and 6, or failure of discrete components that create the feedback loop to pin 1.

The smallest upset at the feedback loop (from a flawed resistor or cap high in esr) will directly affect the IC's internal reference voltage function thus hampering the modulated switching output. Pin 2 is the turn-off & enable pin (enables step-down feature), any flawed resistor or defective cap connected to this pin will cause a logic low condition at pin 2 and all functions inside the PWM IC are stopped immediately. This results in SMPS shutdown or cycle off intermittently depending on topology and nature of the failure (the cycling off symptom is what engineering technicians call a hiccup). In Fiig21A at pin #3 the current limit threshold is set by an external resistor (SMD or through-hole) which is connected from the Vcc supply line to OCSET. The voltage is set by an internal 100uA sink-current flowing through the external resistor.

The purpose of setting the OCSET threshold is for over-current protection (OCP), because when the PWM voltage suddenly becomes less than the OCSET voltage an over-current condition is triggered (so check all diodes tied to the output pins #5,#6 for shorts or leakage symptoms). These are the failure case histories with this kind of PWM IC architecture and its surrounding external components, in which these flaws can also affect 3.3V & 5V regulated lines. In Fig22A is another known PWM IC architecture that has its Current Limiting function as one of its paramount features, the same as the PWM-Stepdown DC/DC Converter IC above. Pins #1, #3 are the servicing cases with this kind of PWM IC, where at pin #1 has no modulating pulse to drive the switching MOSFET such as the two FET's back in Fig7A (Fig22A is driving one switching FET).

No switching signal pulse from the PWM IC will stop main start-up of the LCD TV's SMPS and stays in stand-by mode, the buck/boost converter

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(which is the primary SMPS converter) is disabled. Pin #3 (red line)) sets the core voltage (via resistor divider) (3.3V) and shorted external diodes and flawed SMD resistors have caused upsets to this pin resulting in no main power-up, stuck in stand-by only. PWM IC's (or PWM function) are generally easy to diagnose as it is normally the output modulating pins and external circuits connected to the PWM IC pins that have caused OCP and internal failures with-in the PWM control function.

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This concludes 8-pin PWM IC

Current Mode-Control ICCMC -IC is situated on the Hot side of SMPS's

Current-mode control is the basis of most SMPS design in modern times, current-mode control is a hot new topic in the SMPS engineering community. Yet, CMC has been around for a while but traditionally used for analog control schemes and now CMC is readopted for load-line regulation of advanced SMPS boards. CMC is also used for cycle-by-cycle current-limiting, and protection, CMC IC's already have a built-in PWM feature so there's no need for an external dedicated PWM IC as mentioned previously. SMPS's must have consistency and control, and the output of a SMPS is kept constant with the assistance of closed-loop control. The real value of the voltage output is compared with a reference voltage (nominal voltage). The differences between real and nominal values are the boost-FET's control source (duty-cycle drive). The functional purpose of the control-loop is take the variations of the ac mains and the changing output current and regulate them (regulate is another word for control). This action is known as line-regulation and load-regulation. There are two kinds of regulation topologies with LCD TV SMPS's and that is voltage-mode control and current-mode control. Current-mode is the latest and most popular regulation method used

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on LCD TV SMPS boards. The CMC IC will have (or should have) a heat sink attached to it (this IC runs hot). Voltage-mode control regulates the duty cycle (requiring peripheral circuitry) and current-mode regulates the inductor current directly (requiring one CMC chip). An internal regulatory function with-in the CMC IC regulates the inductor current and this current feeds an output capacitor and load resistance (discrete components) and create a first order system that's extremely responsive and efficient.

Voltage-mode requires a second order system which simply means more headaches for engineers to add all the differential elements between 1st and 2nd order systems that voltage-mode topology would require. This means higher costs in production and yet still none of the benefits that current-mode offers such as superior control response, in fact that is why most IC manufactures only make current-mode IC's instead of voltage-control IC's because of obvious industry demand.

Typical CMC Function:

In Fig23A the output voltage (Vout) is compared to a reference voltage (Vref) by a voltage divider circuit comprising of R1/R2 then amplified by the regulator. The voltage output from the regulator (V2) is compared with the ramp voltage that's flowing through a current-measuring resistor (R5). When the voltage through R5 rises above V2, the output comparator resets a logic flip-flop thus turning the boost-FET off.. The flip-flop is set in advanced by the clock. The clock turns the boost-FET on and off when the ramp voltage (inductor current) gets to a certain value, and this how the inductor current is directly controlled by the regulator.

The closed-loop functions as such:

If the voltage output is too low, this means it will be lower than the reference voltage, thus leading to the voltage output from the regulator to increase. The V2 voltage and the ramp voltage are compared by the comparator going through R5. This action means V2 determines the value to which the voltage through R5 keeps rising until the boost-FET is turned off. If the voltage output (Vout) is below the reference voltage (Vref) this will increase V2, resulting in the inductor current steadily rising until the voltage output (Vout) is precisely equal to the reference voltage (Vref).

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Fig24A below illustrates a popular example of a CMC IC, it has a newly designed fully integrated STB-Power concept that's implemented into the chip so that design engineers have an easier experience designing applications for its use (such as SMPS's for LCD TV's). This IC uses an opto-coupler to control the modulating PWM output as well as to control the power conversion after the Active-Burst Mode enters stand-by mode.

This CMC IC (and most) uses a metal heat sink because it has an incredible host of internal features for current-limiting, PWM control, and current control which create high thermal conditions for the chip. However this particular IC has failed at pin #2, pin #3, pin #4, and pin #5, bear in mind that not all of these pins failed at one time, the faults are separate servicing cases (the PFC and PWM IC are held off of this point as well). Failure at these pins have caused sudden shutdown from main power into a 5V stand-by-only condition. Or has caused a totally dead condition depending on the nature of the fault related to the CMC IC, the topology of the SMPS design, and its implementation. A certain make of LCD TV has a well known problem of having an amber colored logo and when the main power is turned on the amber colored logo turns white. After which, the logo go's from white (indicating main power) then within a few seconds its back to

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amber colored (indicating stand-by). This has been a problem with the feedback pin of the CMC IC. HINT.

The following below reveals the function behind the pin names:

Soft-Start Pin #1:This pin couples the soft-start function during the start-up phase and the error detection for Auto restart Mode. FB-Feedback Pin #2Regulatory data is provided from the FB pin to an internal protection feature and the internal PWM-comparator for duty-cycle control.

CS-Current Sense Pin #3This pin senses the voltage formed through the series resistor inserted into the source of the CMC IC's internal FET. If the CS suddenly peaks the internal (or programmed) threshold of the current-limit comparator then the output of the driver is promptly switched off. In order for the PWM-comparator depends on the current-limit data in order for it to recognize the Current Mode. This is a high voltage line up to 650V.

Drain Pin #4This pin is tied to the drain of the CMC IC's internal FET which is part of the internal start-up cell function. This is a high voltage line up to 650V.

Drain Pin #5This pin is tied to the drain of the CMC IC's internal FET which is part of the internal start-up cell function. This is a high voltage line up to 650V.

N.C. No Connection Pin #6This pin has no electrical connection

Vcc (collector voltage) Pin #7This pin is used to supply the CMC IC with a DC input supply range of approximately 10VDC-25VDC

GND CMC IC Ground Pin #8This pin is the ground return path for the CMC IC.

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This particular CMC IC has failed at the internal start-up cell and Soft-Start pin as there is no source voltage-control at pin #3 and the CMC IC has gotten extremely hot, in fact so hot it was felt through its heat sink..

In Fig25A check all the components tied to these critical pins of the CMC IC, the smallest flaw in a resistor, diode, and high frequency capacitors will easily upset sensing data and trigger the CMC IC into self-protect mode and suspend all internal functions except the Active Burst mode.

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This concludes 8-pin CMC IC

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There are other advanced IC semiconductors for SMPS being used by LCD TV engineers, such as driver-MOSFET's. These IC's have a drive circuit and MOSFET's all built into the same chip, and they deliver more power and efficiency on high switching frequencies. Its important to keep updated on the latest technological advancements with new intelligent semiconductor IC's used in LCD TV's and Plasma TV's also.

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Conclusion

This manual is dedicated to the greatest technicians David Maltz, Jestine Yong, Sunny, Teonna Flags and Micheal B Danish. For any questions or concerns with LCD TV SMPS repair you can email me at [email protected]

All material and illustrations in this manual are original, written, compiled, and illustrated by Damon Diode at TechSociety in USA.

To your success,

Damon C Morrow Author of “LCD TV Repair Secrets”AndPlasma TV Repair Guide

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http://www.plasmatvrepairguide.com/

http://www.plasmatvrepairguide.com/lcdtvrepairsecrets

mailto:[email protected]



Recommended Resources

List Of Electronics Spare Part Suppliershttp://www.jestineyong.com/?cat=12 Electronics Repair Forum

1) http://forum.eserviceinfo.com 2) www.Repairworld.com 3) http://www.tv-forums.com/ 4) http://www.edaboard.com/ 5) http://www.tv.quuq.org/

Electronics Repair Websites1) www.ElectronicRepairGuide.com 2) www.Anatekcorp.com 3) www.Epanorama.net/links/repair.html

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Electronics Repair Ebooks1) www.PowerSupplyRepairGuide.com 2) www.LCDMonitorCaseHistories.com 3) www.LCD-Monitor-Repair.com 4) www.TestingElectronicComponents.com 5) www.FindBurntResistorValue.com 6) www.ElectronicsRepairArticles.com 7) www.electronicrepairguide.com/crt-tv-repair-ebook.html 8) www.electronicrepairguide.com/lcd-television-repair-ebook.html 9) www.electronicrepairguide.com/lcd-television-repair-case-

histories.html10) http://www.electronicrepairguide.com/learn-basic-electronics.html11) LCD TV Repair Volume 112) LCD TV Repair Volume 2

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http://www.electronicrepairguide.com/lcd-television-repair-ebook.html

http://www.electronicrepairguide.com/crt-tv-repair-ebook.html

http://www.ElectronicsRepairArticles.com/

http://www.FindBurntResistorValue.com/

http://www.TestingElectronicComponents.com/

http://www.LCD-Monitor-Repair.com/

http://www.PowerSupplyRepairGuide.com/

http://www.ElectronicRepairGuide.com/Recommend/LCDTelevisionRepair.htm

http://www.ElectronicRepairGuide.com/Recommend/LCDTelevisionRepair.htm

http://www.ElectronicRepairGuide.com/Recommend/PlasmaTelevisionRepair.htm

http://www.ElectronicRepairGuide.com/Recommend/PlasmaTelevisionRepair.htm

http://www.Epanorama.net/links/repair.html

http://www.Anatekcorp.com/

http://www.ElectronicRepairGuide.com/

http://www.tv.quuq.org/

http://www.edaboard.com/

http://www.tv-forums.com/

http://www.Repairworld.com/

http://forum.eserviceinfo.com/

http://www.jestineyong.com/?cat=12

Documents

LCD TV SMPS