SMPS Trainer - Specjalistyczne Testery, Przyrządy i ... Nvis Technologies Pvt. Ltd. 3 Introduction NV7002 SMPS trainer is a very adaptable kit has been designed to explain a very

SMPS TrainerNV7002

Learning MaterialVer 1.1

Designed & Manufactured by:

141-B, Electronic Complex, Pardesipura, Indore- 452 010 India, Tel.: 91-731- 4211500,

Telefax: 91-731-4202959, Toll free: 1800-103-5050, E-mail: [email protected]

Website: www.nvistech.com

www.hik-consulting.pl

mailto:[email protected]

NV7002

Nvis Technologies Pvt. Ltd. 2

SMPS TrainerNV7002

Table of Contents1. Introduction 3

2. Features 4

3. Technical Specifications 5

4. Theory 6

5. EMI (Electromagnetic Interference Filter) 11

Experiment 1 15Study of Primary rectifier and Filter section

6. Switching Transformer 16

Experiment 2 19Study of Switching Transformer

7. MOSFET Switching Device 21

Experiment 3 27Study of PWM switching device

8. Optocoupler 28

Experiment 4 30Study of Optocoupler

9. Regulation Section 31

Experiment 5 34Study of Regulation

Experiment 6 35Study of SMPS with Variac input (Variable AC)-Regulation Test

Experiment 7 36Study of various faults and Procedure of their trouble shooting

10. Warranty 37

11. List of Accessories 37


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Introduction

NV7002 SMPS trainer is a very adaptable kit has been designed to explain a veryremarkable and frequently used switching based power supply-The SMPS (SwitchedMode Power Supply).

The kit is designed keeping in mind that a student can comprehend each block ofSMPS in a very easy way. Different test points have been provided so that one canobserve the inputs and outputs of each block contained. Being different from aconventional block diagram internal structures of different blocks are also shown.Switching Transformer and Chopper (The Heart of SMPS) are presented in such away that a student can readily understand their functioning and pin configuration.

Since SMPS is different from a traditional power supplies because it can be used fordifferent voltage inputs (from 80V to 300V AC).

Figure 1


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Features

A Low cost trainer kit demonstrating all basic concepts of SMPS

In Depth elucidation of Switching Transformer, that is one of the mostimportant components of SMPS

Variac can be connected with the kit

Fault identification feature enabled

Easy illustration of each block

Designed with considering all safety standards

Learning Materialwww.hik-consulting.pl

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Technical Specifications

Mains : 80 to 230V AC, 50/60Hz.

Switching Transformer specifications

Input of Transformer : 320V switching DC at 132 kHz.

Output of Transformer : 30V AC

DC Outputs of kit : +12V DC regulated

-12V DC regulated

+5V DC regulated.

Current rating 500mA

Dimensions : 355×260×125mm

Fuse : 500mA


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Theory

SMPS compared with linear “Power Supply Unit”.

There are two main types of regulated power supplies available: SMPS and Linear.The reasons for choosing one type or the other can be summarized as follows.

Size and weight : Linear power supplies use a transformer operating at theMains frequency of 50/60Hz. This component is larger and heavier by severaltimes than the corresponding smaller transformer in an SMPS, which runs at ahigher frequency (usually above the highest audible frequency, around 50kHz to400kHz)

Efficiency : Linear power supplies regulate their output by using a highervoltage in the initial stages and then expending some of it as heat to improve thepower quality. This power loss is necessary to the circuit, and can be reducedbut never eliminated by improving the design, even in theory. SMPSs drawcurrent at full voltage based on a variable duty cycle, and can increase ordecrease their power consumption to regulate the load as required.Consequently, a well designed SMPS will be more efficient.

Heat output or power dissipation : A linear supply will regulate the voltage orcurrent by wasting excess voltage or current as heat, which is very inefficient. Aregulated SMPS will regulate using Pulse Width Modulation or, at power ratingsbelow 30W, ‘On/Off’ control. In all SMPS topologies, the transistors are alwaysfully on or fully off. Thus, an "ideal" SMPS will be 100% efficient. The onlyheat generated is because ideal components do not exist. Switching losses in themain switching transistors, non-zero resistance in the "on" state, and rectifiervoltage drop will produce a fair amount of heat. However, by optimizing SMPSdesign, the amount of heat produced can be minimized. A good design can havean efficiency of more than 95%.

Complexity : A linear regulator ultimately consists of a power transistor,voltage regulating IC and a noise filtering capacitor. An SMPS typicallycontains PWM controller, one or several power transistors and diodes as well aspower transformer, inductor and filter capacitors. Multiple voltages can begenerated by one transformer core. For this an SMPS has to use pulse widthmodulation on the primary winding and "post-regulating" such as phase controlon the secondary windings, while the linear PSU normally uses independentvoltage regulators for the auxiliary outputs. Both need a careful design for theirtransformers, which therefore are often produced in series and available in stock.Due to the high frequencies in SMPS the inductances and capacitances of thetraces become important.

Radio frequency interference : The currents in a SMPS are switched at a highfrequency. This high-frequency currents can generate undesirableelectromagnetic interference. EMI filters and RF shielding are needed to reducethe disruptive interference. Linear PSUs, however, generally do not produceinterference.


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Electronic noise at the output terminals : Inexpensive linear PSUs with poorregulation may experience a small AC voltage "riding on" the DC output attwice Mains frequency (100/120Hz). These "ripples" are usually on the order ofmillivolts, and can be suppressed with larger filter capacitors or better voltageregulators. This small AC voltage can cause problems in some circuits. Qualitylinear PSUs will suppress ripples much better. SMPS usually do not exhibitripple at the power-line frequency, but do have generally noisier outputs thanlinear PSUs; the noise may be correlated with the SMPS switching frequency orit may also be more broad-band.

Acoustic noise : Linear PSUs typically give off a faint, low frequency hum atMains frequency, but this is seldom audible. (The transformer is responsible.)SMPSs, with their much higher operating frequencies, are not usually audible tohumans (unless they have a fan, in the case of most computer SMPSs). Amalfunctioning SMPS may generate high-pitched sounds, since they do in factgenerate acoustic noise at the oscillator frequency.

Power factor : The current drawn by simple SMPS is non-sinusoidal and do notfollow the supply's input voltage waveform, so the early SMPS designs have amediocre power factor of about 0.6, and their use in personal computers andcompact fluorescent lamps presented a growing problem for power distribution.Power factor correction (PFC) circuits can reduce this problem, and are requiredin some countries (European in particular) by regulation. Linear PSUs also donot have unity power factors, but are not as problematic as SMPSs.

Electronic noise at the input terminals : In a similar fashion, very low costSMPS may couple electrical noise back onto the Mains power line; Linear PSUsrarely do this.

How an SMPS works?

Figure 2

Block diagram of a Mains operated AC-DC SMPS with output voltageregulation.


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Input rectifier stage :

Figure 3AC, half-wave and full wave rectified signals :

If the SMPS has an AC input, then its first job is to convert the input to DC. This iscalled rectification. The rectifier circuit can be configured as a voltage doubler by theaddition of a switch operated either manually or automatically. This is a feature oflarger supplies to permit operation from nominally 120volt or 240volt supplies. Therectifier produces an unregulated DC voltage which is then sent to a large filtercapacitor. The current drawn from the Mains supply by this rectifier circuit occurs inshort pulses around the AC voltage peaks. These pulses have significant highfrequency energy which reduces the power factor. Special control techniques can beemployed by the following SMPS to force the average input current to follow thesinusoidal shape of the AC input voltage thus the designer should try correcting thepower factor. A SMPS with a DC input does not require this stage. A SMPS designedfor AC input can often be run from a DC supply, as the DC passes through therectifier stage unchanged. (The user should check the manual before trying this,though most supplies are quite capable of such operation even though no clue isprovided in the manual!)

If an input range switch is used, the rectifier stage is usually configured to operate as avoltage doubler when operating on the low voltage (~120VAC) range and as a straightrectifier when operating on the high voltage (~240VAC) range. If an input rangeswitch is not used, then a full-wave rectifier is usually used and the downstreaminverter stage is simply designed to be flexible enough to accept the wide range of dcvoltages that will be produced by the rectifier stage. In higher-power SMPSs, someform of automatic range switching may be used.


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Inverter stage :

The inverter stage converts DC, whether directly from the input or from the rectifierstage described above, to AC by running it through a power oscillator, whose outputtransformer is very small with few windings at a frequency of tens or hundreds ofkilohertz (kHz). The frequency is usually chosen to be above 20kHz, to make itinaudible to humans. The output voltage is optically coupled to the input and thusvery tightly controlled. The switching is implemented as a multistage (to achieve highgain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistanceand a high current-handling capacity. This section refers to the block marked"Chopper" in the block diagram.

Voltage converter and output rectifier :

If the output is required to be isolated from the input, as is usually the case in Mainspower supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required outputlevel on its secondary winding. The output transformer in the block diagram servesthis purpose.

If a DC output is required, the AC output from the transformer is rectified. For outputvoltages above ten volts or so, ordinary silicon diodes are commonly used. For lowervoltages, Schottky diodes are commonly used as the rectifier elements; they have theadvantages of faster recovery times than silicon diodes (allowing low-loss operationat higher frequencies) and a lower voltage drop when conducting. For even loweroutput voltages, MOSFET transistors may be used as synchronous rectifiers;compared to Schottky diodes, these have even lower "on"-state voltage drops.

The rectified output is then smoothed by a filter consisting of inductors andcapacitors. For higher switching frequencies, components with lower capacitance andinductance are needed.

Simpler, non-isolated power supplies contain an inductor instead of a transformer.This type includes boost converters, buck converters, and the so called buck-boostconverters. These belong to the simplest class of single input, single output converterswhich utilise one inductor and one active switch (MOSFET). The buck converterreduces the input voltage, in direct proportion, to the ratio of the active switch "on"time to the total switching period, called the Duty Ratio. For example an ideal buckconverter with a 10V input operating at a duty ratio of 50% will produce an averageoutput voltage of 5V. A feedback control loop is employed to maintain (regulate) theoutput voltage by varying the duty ratio to compensate for variations in input voltage.The output voltage of a boost converter is always greater than the input voltage andthe buck-boost output voltage is inverted but can be greater than, equal to, or less thanthe magnitude of its input voltage. There are many variations and extensions to thisclass of converters but these three form the basis of almost all isolated and non-isolated DC to DC converters. By adding a second inductor the Ćuk and SEPICconverters can be implemented or by adding additional active switches various bridgeconverters can be realized.


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Other types of SMPS use a capacitor-diode voltage multiplier instead of inductors andtransformers. These are mostly used for generating high voltages at low currents. Thelow voltage variant is called charge pump.

Regulation :

A feedback circuit monitors the output voltage and compares it with a referencevoltage, which is set manually or electronically to the desired output. If there is anerror in the output voltage, the feedback circuit compensates by adjusting the timingwith which the MOSFETs are switched on and off. This part of the power supply iscalled the switching regulator. The "Chopper controller" shown in the block diagramserves this purpose. Depending on design/safety requirements, the controller may ormay not contain an isolation mechanism (such as opto-couplers) to isolate it from theDC output. Switching supplies in computers, TVs and VCRs have these opto-couplersto tightly control the output voltage.

Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding aconstant voltage to the input of the transformer or inductor, and assume that theoutput will be correct. Regulated designs work against the parasitic capacity of thetransformer or coil, monopolar designs also against the magnetic hysteresis of thecore.

The feedback circuit needs power to run before it can generate power, so an additionalnon-switching power-supply for stand-by is added.

Power factor :

Early switched mode power supplies incorporated a simple full wave rectifierconnected to a large energy storing capacitor. Such SMPS draws current from the ACline in short pulses when the Mains instantaneous voltage exceeds the voltage acrossthis capacitor. During the remaining portion of the AC cycle the capacitor providesenergy to the power supply. As the result, input current of such basic switched modepower supplies has high harmonics content and relatively low power factor. Thiscreates extra load on utility lines, increases heating of the utility transformers, andmay cause stability problems in some applications such as in emergency generatorsystems or aircraft generators. In 2001 the European Union put into effect thestandard IEC/EN61000-3-2 to set limits on the harmonics of the AC input current upto the 40th harmonic for equipment above 75W. The standard defines four classes ofequipment depending on its type and current waveform. The most rigorous limits(class D) are established for personal computers, computer monitors, and TVreceivers. In order to comply with these requirements modern switched-mode powersupplies normally include an additional power factor correction (PFC) stage.

Applications :

Switched-mode PSUs in domestic products such as personal computers often haveuniversal inputs, meaning that they can accept power from most Mains suppliesthroughout the world, with rated frequencies from 50Hz to 60Hz and voltages from100V to 240V (although a manual voltage "range" switch may be required). Inpractice they will operate from a much wider frequency range and often from a DC


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supply as well. In 2006, Intel proposed the use of a single 12V supply inside PCs, dueto the high efficiency of switch mode supplies directly on the PCB. Most moderndesktop and laptop computers already have a DC-DC converter on the motherboard,to step down the voltage from the PSU or the battery to the CPU core voltage --typically 1.8V as of 1998. Most laptop computers also have a DC-AC inverter to stepup the voltage from the battery to drive the backlight, typically around 1000 Vrms.

Cars, trucks, telecom lines, and production plants, but not planes, supply DC to avoidhum and ease the integration of capacitors and batteries used to buffer the voltage.

In the case of TV sets, for example, one can test the excellent regulation of the powersupply by using a variac. For example, in some models made by Philips, the powersupply starts when the voltage reaches around 90volts. From there, one can changethe voltage with the variac, and go as low as 40volts and as high as 260, and theimage will show absolutely no alterations.

Electromagnetic Interference

What is EMI?

Electromagnetic interference (also called EMI, radio frequency interference, or RFI)is an unwanted disturbance caused in a radio receiver or other electrical circuit byelectromagnetic radiation emitted from an external source. The disturbance mayinterrupt, obstruct, or otherwise degrade or limit the effective performance of thecircuit. The source may be any Objective, artificial or natural, that carries rapidlychanging electrical currents, such as an electrical circuit, the Sun or the NorthernLights. EMI can be induced intentionally, as in some forms of electronic warfare, orunintentionally, as a result of spurious emissions and responses, intermodulationproducts, and the like. It frequently affects the reception of AM radio in urban areas.It can also affect FM radio and television reception, although to a lesser extent.

The most important means of reducing EMI are the use of bypass or "decoupling"capacitors on each active device (connected across the power supply, as close to thedevice as possible), rise time control of high-speed signals using series resistors, andVCC filtering. Shielding is usually a last resort after other techniques have failedbecause of the added expense of RF gaskets and the like.

The efficiency of the radiation depends on the height above the ground or power plane(at RF one is as good as the other) and the length of the conductor in relation to thewavelength of the signal component (fundamental, harmonic or transient (overshoot,undershoot or ringing)). At lower frequencies, such as 133MHz, radiation is almostexclusively via I/O cables; RF noise gets onto the power planes and is coupled to theline drivers via the VCC and ground pins. The RF is then coupled to the cable throughthe line driver as common-mode noise. Since the noise is common-mode, shieldinghas very little effect, even with differential pairs. The RF energy is capacitive coupledfrom the signal pair to the shield and the shield itself does the radiating. One cure forthis is to use a braid-breaker to reduce the common-mode signal.

At higher frequencies, usually above 500MHz, traces get electrically longer andhigher above the plane. Two techniques are used at these frequencies: wave shaping


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with series resistors and embedding the traces between the two planes. If all thesemeasures still leave too much EMI, shielding such as RF gaskets and copper tape canbe used. Most digital equipment is designed with metal, or coated plastic, cases.

Switching power supplies can be a source of EMI, but have become less of a problemas design techniques have improved.

Most countries have legal requirements that electronic and electrical hardware muststill work correctly when subjected to certain amounts of EMI, and should not emitEMI which could interfere with other equipment (such as radios).

EMI filters are used to keep stray signals from polluting your design. Commonlyknown as "feed through", the basic EMI filter is a low-pass filter, and uses acombination of shunt capacitance and series inductance to prevent EM signals fromentering your housing our enclosure. EMI is electromagnetic interference and is calledconducted emissions or radiated emissions or radiated emissions. Conductedemissions mean any unwanted signal or noise on the wiring or copper. The reason forthe reference to power cabling is that EMI filters are the part of the power wiring andare designed to remove these unwanted properties from the copper wiring when anycurrent flow creates an associated magnetic field. You cannot have one without theother. Therefore, this high frequency unwanted signal creates a magnetic field that caninterfere with surrounding equipment. It is the filter’s function to remove this currentso that its associated magnetic field will not interfere. This noise can originate eitherfrom the line or from the associated equipment that the filter is built into. From theequipment side, the noise could be coming from computer clock frequencies, parasiticoscillations in the switcher power supply inductors or transformers, power supplydiode noise, harmonics of the line frequencies due to the high peak current chargingthe power supply storage capacitor, and many other sources. From the line, the noisecould be due to flattening of the sine wave voltage caused by the high peak currentsslightly ahead of 90 and 270 degrees due to the total of the power supplies fed fromthe line without power factor correction circuitry. This generates odd harmonics thatfeed the EMI filter. Other sources of noise from the line are other equipment withoutany filtering and heavy surges of equipment being turned on and off. Lighting andEMPs (Electromagnetic pulses, possibly from nuclear explosions) create other lineproblems for the filter.

To review, EMI is any unwanted signal from either the power line or the equipmentand must be removed to prevent a magnetic field from interfering with closelyassociated equipment or to stop a malfunction of the equipment containing the filter.


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EMI waveformsFigure 4

All electronic devices give off electromagnetic emissions. This is radiation that is abyproduct of electrical or magnetic activity. Unfortunately, the emissions from onedevice can interfere with other devices, causing potential problems. Interference canlead to data loss, picture quality degradation on monitors, and other problems withyour PC, or problems with other devices such as television sets and radios. These aregenerally categorized as electromagnetic interference or EMI problems.

There are actually two different issues here: EMI emissions by the PC and EMIemissions received by the PC. PCs generally do not cause very much interferencewith other devices; they are required by the Federal Communications Commission(FCC) to be certified as Class B devices. This certification is used to show that the PCconforms to standards that limit the amount of EMI that a PC can produce. The onlycatch to this is that you have to keep the cover on the PC. This is one reason why thecover is always made from metal. (Keeping the cover on is also an important part ofensuring proper ventilation).

PCs can be affected by electromagnetic interference from other devices, in two majorways. One is direct effects through proximity with other devices; another is electricalinterference over the power lines. Most PCs generally do not have many problemswith EMI, but those that do can cause incredible frustrating to their owners. There areseveral things that you can do to avoid or at least reduce EMI if you think it isaffecting your PCs :


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Physical Isolation : Devices that emit electromagnetic radiation should be kepta reasonable distance from your PC, peripherals and media. This includestelevision sets, radios, lights, kitchen appliances, and stereo speakers (the onesdesigned for use with PCs are generally shielded and are much less of an issue).

Use Dedicated Circuits : Many office buildings especially, have separatepower circuits that are intended for use by PCs. Keeping your PC on a circuitthat is separate from the circuit running your refrigerator and air conditioningunit means that there will be much less interference passing to the computerfrom the other devices (and this will also improve the quality of the power beingsent to your machine in general).

Power Conditioning : The use of a line conditioner or uninterruptible powersupply can filter out interference caused by other devices that share a line withyour PC. www.hik-consulting.pl

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Experiment 1

Objective :Study of Primary Rectifier and Filter section.

Equipments Needed :

1. Oscilloscope

2. DMM (Digital Multimeter)

3. Attenuator Probe (1:10)

Procedure:

1. Before performing experiment first select ‘AC Mains/From Variac’ switch ofthe Trainer kit at ‘AC Mains’ position and all Fault switches at ‘Off’ position.

2. Now connect the Mains cord into the Trainer kit and switch ‘On’ the powersupply.

3. Connect Oscilloscope at TP1 with (1:10) attenuator probe at ×10 position, whichis nothing but a sine wave; record its frequency and amplitude in Observationtable.

4. Now connect a DMM (at higher DC voltmeter range) across TP4 and Ground(TP21) and check the voltage. It will be a pulsating DC high voltage.

From this experiment you can observe following parameters :

Parameter Value

Voltage at TP1 ---------------Vpp

--------------Vrms

Frequency at TP1 ---------------Hz


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Switching Transformer

How it works?

Figure 5

Unlike Mains transformers and audio transformers, a Switching Transformer isdesigned not just to transfer energy, but also to store it for a significant fraction of theswitching period. This is achieved by winding the coils on a ferrite core with an airgap. The air gap increases the reluctance of the magnetic circuit and therefore itscapacity to store energy.

The primary winding of the Switching Transformer is driven by a relatively lowvoltage saw tooth wave, which is ramped up (and sweeping the beam across thescreen to draw a line) and then abruptly switched off (and causing the beam to quicklyfly back from the right to the left of the display) by the horizontal output stage. This isa ramped and pulsed waveform that repeats at the horizontal (line) frequency of thedisplay. The fly back (vertical portion of the saw tooth wave) is extremely useful tothe fly back transformer: the faster a magnetic field collapses, the greater the inducedvoltage. Furthermore, the high frequency reduces the size of the transformer. Intelevision sets, this high frequency is about 15 kilohertz (15,750Hz for NTSC), andvibrations from the related circuitry can often be heard as a high-pitched whine. Inmodern computer displays the frequency can vary over a wide range, from about30kHz to 150kHz.

The alternating current coming from the fly back transformer is converted to directcurrent by a high-voltage rectifier. If the output voltage of the Switching Transformeris not high enough by itself, the rectifier is replaced by a voltage multiplier.Conversely, early color television sets used a regulator to control the high voltage.The rectified voltage is then used to charge the anode of the cathode ray tube. Thereare often auxiliary secondary windings that produce lower voltages for driving otherparts of the display's circuitry — often the CRT's filament will be driven from the flyback. In tube sets, a two-turn filament winding is located on the opposite side of thecore as the HV secondary, used to drive the rectifier tube.


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Practical considerations :

In modern displays, the Switching Transformer, voltage multiplier and rectifier areoften integrated into a single package on the main circuit board. There is usually athick wire from the Switching Transformer to the anode terminal (covered by a rubbercap) on the side of the picture tube. The thickness of this wire is mostly due to thethickness of the plastic insulation, the copper conductor inside being much thinner asit carries only a small current.

One advantage of operating the transformer at the fly back frequency is that it can bemuch smaller and lighter than a comparable transformer operating at Mains (line)frequency. Another advantage is that it provides a failsafe mechanism — should thehorizontal deflection circuitry fails; the fly back transformer will cease operating andshut down the rest of the display, preventing the screen burn that would otherwiseresult from a stationary electron beam.

Construction :

Around a ferrite rod the primary is wound first and around this the secondary. This isto reduce the leakage inductance of the primary. And around this a ferrite framecloses the magnetic field lines. The gap between the rod and the frame is air gap withreduces the remanence. The secondary is wound layer by layer with enameled wirewith Mylar foil between the layers. In this way parts of the wire with higher voltagebetween them have more dielectric material between them. The outside of thewinding is on the highest voltage so insulation and screening may be needed toprotect the surrounding. In a variant, to avoid some stray capacity, every layer of thewindings is connected by a rectifying diode to the next layer. Windings go up the rodand the diodes go down. In this way the AC voltage increases along the rod (axial)and the DC voltage increases radial from inside to outside. When applied to tapewound coils this would mean each coil goes from inside to outside and the diode goesback to the inside.

Failure :

Fly back transformers are a frequent source of failure for television sets; the highvoltage present in the many turns of wire with the somewhat thin insulation requiredfor the transformer to be of reasonable size is likely to eventually result in leakage atone point or another; as the leakage heats the insulation it carbonizes and conductsmore, which leads to even more heat and carbonization, until the leaked current ishigh enough for the high voltage to cease to function. As a result, replacement flyback transformers for almost every set on the market are available through dealers inelectronic parts, typically for a few tens of dollars. The problem is exacerbated by thetendency of the fly back to accumulate a coating of dust due to electrostatic attraction,which serves as a path to ground for leaks which might otherwise not be of sufficientmagnitude to initiate the chain of events leading to destructive failure, as described.As a result, occasional cleaning of the accumulated dust from the high voltagecircuitry inside a television can be beneficial, provided the proper precautions aretaken, as below.


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A fly back transformer and its associated circuitry operate at very high voltages at lowcurrents (4-15ma), far beyond Mains voltage. While most fly backs do not supplyenough power to kill directly, the voltage they employ can cause violent musclespasms if touched; and such spasms usually cause injury. Therefore, only trainedpersons should touch or modify these devices, after first ensuring that the transformeris switched off and any stored energy has been safely discharged. The CRT has aninherent capacitance which can hold a high voltage charge for a period up to severaldays after the power is switched off. Often, a high-resistance bleeder resistor isconnected in parallel to ensure the charge is safely grounded when not in use, but it isnot wise to assume that this is the case.

In many recent televisions, after replacing the fly back transformer, the controlfirmware must be recalibrated to account for slight differences in performancebetween transformers in order to maintain accurate color reproduction.www.hik-consulting.pl

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Experiment 2

Objective :Study of Switching Transformer and its working.

Equipments Needed :

1. Oscilloscope



Procedure :



3. Connect Oscilloscope with (1:10) attenuator probe at ×10 position, at TP4 andobserve the waveform with respect to Ground (TP21), which is nothing but aPWM waveform, this signal is generated by Tiny switch IC TNY268P whichgenerates the PWM signal.

Note : To observe this waveform triggered on Oscilloscope, use Normaltriggering with Level adjustment at Time base position of 5-10 μs and verticalsensitivity of 10 V/div.

Figure 6

4. Observe the average frequency of this waveform on Oscilloscope which isapproximately 132kHz.

5. Now to measure the output of transformer, for this connect DMM (at DCvoltmeter range) at TP7 with respect to TP6, which is approximately 30V DC.


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6. Similarly you can measure the voltages at other points of transformer like TP8,TP9, TP10 with respect to TP6.

From this experiment you can observe following parameters :

Parameter Value

Frequency of this PWM signal ---------------- kHz

Output voltages of transformer at

TP7 with respect to TP6




------------------V DC

------------------V DC

------------------V DC

------------------V DC


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MOSFET Switching Device

This IC has been used to switch the input waveform of transformer since it is in DCform. This switching is done at very high frequency (about 132kHz), which behaveslike approximate AC.

Tiny Switch’s Features :

Fully integrated auto-restart for short circuit and open loop fault protection–saves external component costs.

Built-in circuitry practically eliminates audible noise with ordinary varnishedtransformer.

Programmable line under-voltage detect feature prevents power on/off glitches–saves external components.

Frequency jittering dramatically reduces EMI (~10dB) –minimizes EMI filtercomponent costs.

132 kHz operation reduces transformer size–allows use of EF12.6 or EE13 coresfor low cost and small size.

Very tight tolerances and negligible temperature variation on key parameterseases design and lowers cost.

Lowest component count switcher solution.

Description:

Tiny Switch-II maintains the simplicity of the Tiny Switch topology, while providinga number of new enhancements to further reduce system cost and component count,and to practically eliminate audible noise. Like Tiny Switch, a 700V power MOSFET,oscillator, high voltage switched current source, current limit and thermal shutdowncircuitry are integrated onto a monolithic device. The start-up and operating power arederived directly from the voltage on the DRAIN pin, eliminating the need for a biaswinding and associated circuitry. In addition, the PI-2684-101700 Wide-Range HVDC Input D, S EN/UV, BP, +, - , +, -, DC Output.

Pin Functional Description:

Figure 7


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DRAIN (D) Pin:

Power MOSFET drain connection. Provides internal operating current for both start-up and steady-state operation.

BYPASS (BP) Pin:

Connection point for a 0.1μF external bypass capacitor for the internally generated5.8V supply.

Enable/Under-Voltage (EN/UV) Pin:

This pin has dual functions: enable input and line under-voltage sense. During normaloperation, switching of the power MOSFET is controlled by this pin. MOSFETswitching is terminated when a current greater than 240μA is drawn from this pin.This pin also senses line under-voltage conditions through an external resistorconnected to the DC line voltage. If there is no external resistor connected to this pin,Tiny Switch-II detects its absence and disables the line under voltage function.

Source (S) Pin:

Control circuit common, internally connected to output MOSFET source.

Source (HV RTN) Pin:

Output MOSFET source connection for high voltage return.

Functional Description :

Tiny Switch-II combines a high voltage power MOSFET switch with a power supplycontroller in one device. Unlike conventional PWM (Pulse Width Modulator)controllers, Tiny Switch-II uses a simple ‘On/Off’ control to regulate the outputvoltage.

The Tiny Switch-II controller consists of an Oscillator, Enable Circuit (Sense andLogic), Current Limit State Machine, 5.8V Regulator, Bypass pin Under-VoltageCircuit, over

Temperature Protection, Current Limit Circuit, Leading Edge Blanking and a 700Vpower MOSFET. Tiny Switch-II incorporates additional circuitry for Line Under-Voltage Sense, Auto-Restart and Frequency Jitter. Figure 2 shows the functionalblock diagram with the most important features.

Oscillator:

The typical oscillator frequency is internally set to an average of 132 kHz. Twosignals are generated from the oscillator: the Maximum Duty Cycle signal (DCMAX)and the Clock signal that indicates the beginning of each cycle. The Tiny Switch-IIoscillator incorporates circuitry that introduces a small amount of frequency jitter,typically 8 kHz peak-to-peak, to minimize EMI emission. The modulation rate of thefrequency jitter is set to 1 kHz to optimize EMI reduction for both average and quasi-peak emissions. The frequency jitter should be measured with the oscilloscopetriggered at the falling edge of the DRAIN waveform. The waveform in Figure4illustrates the frequency jitter of the Tiny Switch-II.


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Enable Input and Current Limit State Machine:

The enable input circuit at the EN/UV pin consists of a low impedance sourcefollower output set at 1.0V. The current through the source follower is limited to240μA. When the current out of this pin exceeds 240μA, a low logic level (disable) isgenerated at the output of the enable circuit. This enable circuit output is sampled atthe beginning of each cycle on the rising edge of the clock signal. If high, the powerMOSFET is turned on for that cycle (enabled). If low, the power MOSFET remainsoff (disabled). Since the sampling is done only at the beginning of each cycle,subsequent changes in the EN/UV pin voltage or current during the remainder of thecycle are ignored. The Current Limit State Machine reduces the current limit bydiscrete amounts at light loads when Tiny Switch-II is likely to switch in the audiblefrequency range. The lower current limit raises the effective switching frequencyabove the audio range and reduces the transformer flux density including theassociated audible noise. The state machine monitors the sequence of EN/UV pinvoltage levels to determine the load condition and adjusts the current limit levelaccordingly in discrete amounts. Under most operating conditions (except when closeto no load), the low impedance of the source follower keeps the voltage on theEN/UV pin from going much below 1.0V in the disabled state. This improves theresponse time of the optocoupler that is usually connected to this pin.

5.8V Regulator and 6.3V Shunt Voltage Clamp :

The 5.8V regulator charges the bypass capacitor connected to the BYPASS pin to5.8V by drawing a current from the voltage on the DRAIN pin, whenever theMOSFET is off. The BYPASS pin is the internal supply voltage node for the TinySwitch-II. When the MOSFET is on, the Tiny Switch-II operates from the energystored in the bypass capacitor. Extremely low power consumption of the internalcircuitry allows Tiny Switch-II to operate continuously from current it takes from theDRAIN pin. A bypass capacitor value of 0.1μF is sufficient for both high frequencydecoupling and energy storage.

In addition, there is a 6.3V shunt regulator clamping the BYPASS pin at 6.3V whencurrent is provided to the BYPASS pin through an external resistor. This facilitatespowering of Tiny Switch-II externally through a bias winding to decrease the no loadconsumption to about 50mW.

BYPASS Pin Under-Voltage:

The BYPASS pin under-voltage circuitry disables the power MOSFET when theBYPASS pin voltage drops below 4.8V. Once the BYPASS pin voltage drops below4.8V, it must rise back to 5.8V to enable (turn-on) the power MOSFET.

IC operation:

Tiny Switch-II devices operate in the current limit mode. When enabled, the oscillatorturns the power MOSFET on at the beginning of each cycle. The MOSFET is turnedoff when the current ramps up to the current limit or when the DCMAX limit isreached. As the highest current limit level and frequency of a Tiny Switch-II designare constant, the power delivered to the load is proportional to the primary inductance


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of the transformer and peak primary current squared. Hence, designing the supplyinvolves calculating the primary inductance of the transformer for the maximumoutput power required. If the Tiny Switch-II is appropriately chosen for the powerlevel, the current in the calculated inductance will ramp up to current limit before theDCMAX limit is reached.

Enable Function:

Tiny Switch-II senses the EN/UV pin to determine whether or not to proceed with thenext switch cycle as described earlier. The sequence of cycles is used to determine thecurrent limit. Once a cycle is started, it always completes the cycle (even when theEN/UV pin changes state half way through the cycle). This operation results in apower supply in which the output voltage ripple is determined by the output capacitor,amount of energy per switch cycle and the delay of the feedback.

The EN/UV pin signal is generated on the secondary by comparing the power supplyoutput voltage with a reference voltage. The EN/UV pin signal is high when thepower supply output voltage is less than the reference voltage.

In a typical implementation, the EN/UV pin is driven by an optocoupler. The collectorof the optocoupler transistor is connected to the EN/UV pin and the emitter isconnected to the Source pin. The optocoupler LED is connected in series with a Zenerdiode across the DC output voltage to be regulated. When the output voltage exceedsthe target regulation voltage level (optocoupler LED voltage drop plus Zener voltage),the optocoupler LED will start to conduct, pulling the EN/UV pin low. The Zenerdiode can be replaced by a TL431 reference circuit for improved accuracy.

‘On/Off’ Operation with Current Limit State Machine:

The internal clock of the Tiny Switch-II runs all the time. At the beginning of eachclock cycle, it samples the EN/UV pin to decide whether or not to implement a switchcycle, and based on the sequence of samples over multiple cycles, it determines theappropriate current limit. At high loads, when the EN/UV pin is high (less than240µA out of the pin), a switching cycle with the full current limit occurs. At lighterloads, when EN/UV is high, a switching cycle with a reduced current limit occurs.

At near maximum load, Tiny Switch-II will conduct during nearly all of its clockcycles at slightly lower load, it will “skip” additional cycles in order to maintainvoltage regulation at the power supply output. At medium loads, cycles will beskipped and the current limit will be reduced. At very light loads, the current limitwill be reduced even further. Only a small percentage of cycles will occur to satisfythe power consumption of the power supply.

The response time of the Tiny Switch-II ‘On/Off’ control scheme is very fastcompared to normal PWM control. This provides tight regulation and excellenttransient response.


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Figure 8

Power Up/Down :

The Tiny Switch-II requires only a 0.1µF capacitor on the BYPASS pin. Because ofits small size, the time to charge this capacitor is kept to an absolute minimum,typically 0.6ms. Due to the fast nature of the ‘On/Off’ feedback, there is no overshootat the power supply output. When an external resistor (2MΩ) is connected from thepositive DC input to the EN/UV pin, the power MOSFET switching will be delayedduring power-up until the DC line voltage

Exceeds the threshold (100V), in applications with and without an external resistor(2MΩ) connected to the EN/UV pin.

During power-down, when an external resistor is used, the power MOSFET willswitch for 50ms after the output loses regulation. The power MOSFET will thenremain off without any glitches since the under-voltage function prohibits restartwhen the line voltage is low.

Figures illustrates a very slow power-down timing waveform of Tiny Switch-II as in

Figure 9


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Stand by applications. The external resistor (2MΩ) is connected to the EN/UV pin inthis case to prevent unwanted restarts.

Applications of PWM switching IC :

The Tiny Switch-II is ideal for low cost, high efficiency power supplies in a widerange of applications such as cellular phone chargers, PC standby, TV standby, ACadapters, motor control, appliance control and ISDN or a DSL network termination.The 132 kHz operation allows the use of a low cost EE13 or EF12.6 core transformerwhile still providing good efficiency. The frequency jitter in Tiny Switch-II makes itpossible to use a single inductor (or two small resistors for under 3W applications iflower efficiency is acceptable) in conjunction with two input capacitors for input EMIfiltering. The auto-restart function removes the need to oversize the output diode forshort circuit conditions allowing the design to be optimized for low cost andmaximum efficiency. In charger applications, it eliminates the need for a secondoptocoupler and Zener diode for open loop fault protection. Auto-restart also savesthe cost of adding a fuse or increasing the power rating of the current sense resistorsto survive reverse battery conditions. For applications requiring under-voltage lockout (UVLO), such as PC standby, the Tiny Switch-II eliminates several componentsand saves cost. Tiny Switch-II is well suited for applications that require constantvoltage and constant current output. As Tiny Switch-II is always powered from theinput high voltage, it therefore does not rely on bias winding voltage. Consequentlythis greatly simplifies designing chargers that must work down to zero volts on theoutput.


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Experiment 3

Objective :Study of MOSFET switching device ICTNY268P.

Equipments Needed :

1. Oscilloscope


Procedure:



3. Connect Oscilloscope with (1:10) attenuator probeat ×10 position, at TP4 andobserve the waveform with respect to Ground (TP21), which is nothing but aPWM waveform, this signal is generated by Tiny switch IC TNY268P whichgenerates the PWM signal.

Note : To observe this waveform triggered on Oscilloscope, use Normaltriggering with Level adjustment at Time base position of 0.1 ms.

Figure 10

4. Now calculate the time period of this waveform which is the switching time(this will be of ms order) of this IC record this in Observation table.

From this experiment you can observe following parameter :

Parameter Value

Switching time of TNY IC268P ------------------ ms


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Optocoupler

An opto-isolator internal circuit

Figure 11

Schematic diagram :

In electronics, an opto-isolator (or optical isolator, optocoupler or photo coupler) is adevice that uses a short optical transmission path to transfer a signal between elementsof a circuit, typically a transmitter and a receiver, while keeping them electricallyisolated — since the signal goes from an electrical signal to an optical signal back toan electrical signal, electrical contact along the path is broken.

A common implementation involves an LED and a light sensor, separated so that lightmay travel across a barrier but electrical current may not. When an electrical signal isapplied to the input of the opto-isolator, its LED lights, its light sensor then activates,and a corresponding electrical signal is generated at the output. Unlike a transformer,the opto-isolator allows for DC coupling and generally provides significant protectionfrom serious over voltage conditions in one circuit affecting the other.

With a photodiode as the detector, the output current is proportional to the amount ofincident light supplied by the emitter. The diode can be used in a photovoltaic modeor a photoconductive mode.

In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedanceand light intensity.

In photoconductive mode, the diode is connected to a supply voltage, and themagnitude of the current conducted is directly proportional to the intensity of light.

An opto-isolator can also be constructed using a small incandescent lamp in place ofthe LED; such a device, because the lamp has a much slower response time than anLED, will filter out noise or half-wave power in the input signal. In so doing, it willalso filter out any audio- or higher-frequency signals in the input. It has the furtherdisadvantage, of course, (an overwhelming disadvantage in most applications) thatincandescent lamps have finite life spans. Thus, such an unconventional device is ofextremely limited usefulness, suitable only for applications such as science projects.

The optical path may be air or a dielectric waveguide. The transmitting and receivingelements of an optical isolator may be contained within a single compact module, formounting, for example, on a circuit board; in this case, the module is often called anoptoisolator or opto-isolator. The photosensor may be a photocell, phototransistor, or


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an optically triggered SCR or Triac. Occasionally, this device will in turn operate apower relay or contactor.

Applications :

Several types of opto-couplers. The top left and far right detect the presence of anObjective in between them. They are interruptible. The middle one detects reflectionsfrom Objectives in front of it. The two on the bottom left are both opto-isolators.

Figure 12

A simple circuit with an opto-isolator. When switch S1 is closed, LED D1 lights,which triggers phototransistor Q1, which pulls the output pin low. This circuit, thus,acts as a NOT gate.

Among other applications, opto-isolators can help cut down on ground loops andblock voltage spikes.

One of the requirements of the MIDI (Musical Instrument Digital Interface)standard is that input connections be opto-isolated.

They are used to isolate low-current control or signal circuitry from transientsgenerated or transmitted by power supply and high-current control circuits. Thelatter are used within motor and machine control function blocks.

The classical ball computer mouse is a common application, using infraredemitter LEDs and phototransistors to form optocouplers. The ball of the mouseturns a pair of optical encoder wheels. These wheels periodically block theoptocouplers and thereby translate the motion of the mouse into a sequence ofpulses. These pulses are then used to record the motion. The principle ofoperation does not require infrared light, but the infrared sensor is less sensitiveto interference from common flickering visible light sources such as fluorescentlamps and CRT displays.


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Experiment 4

Objective :Study Optocoupler and its application in SMPS.

Equipments Needed :

1. DMM (Digital Multimeter).

Procedure :



3. Observe the input voltage of Optocoupler using DMM (at DC voltmeter range)at TP2 and TP3, which is around 1volt DC. This is the switching voltagerequired for its operation; record this voltage in Observation table.

From this experiment you can observe following parameter:

Parameter Value

Input voltage to optocoupler atTP2 and TP3 --------------------- V


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Regulation Section

After the output of transformer the switched Stepped down DC (approximate AC) isfed into rectifiers for DC conversion but these rectifiers don’t give smooth regulatedDC hence some sort of regulation is provided using zener diode and differentregulation ICs.

Zener diode regulator : When forward-biased, zener diodes behave much the sameas standard rectifying diodes: they have a forward voltage drop which follows the"diode equation" and is about 0.7volts. In reverse-bias mode, they do not conductuntil the applied voltage reaches or exceeds the so-called zener voltage, at which pointthe diode is able to conduct substantial current, and in doing so will try to limit thevoltage dropped across it to that zener voltage point. So long as the power dissipatedby this reverse current does not exceed the diode's thermal limits, the diode will not beharmed.

Zener diodes are manufactured with zener voltages ranging anywhere from a fewvolts to hundreds of volts. This zener voltage changes slightly with temperature, andlike common carbon-composition resistor values, may be anywhere from 5 percent to10 percent in error from the manufacturer's specifications. However, this stability andaccuracy is generally good enough for the zener diode to be used as a voltageregulator device in common power supply circuit:

Figure 13

The image shows a simple zener voltage regulator. It is a shunt regulator and operatesby way of the zener diode's action of maintaining a constant voltage across itself whenthe current through it is sufficient to take it into the zener breakdown region. Theresistor R1 supplies the zener current IZ as well as the load current IR2 (R2 is theload). R1 can be calculated as -

5V

5V


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where, VZ is the zener voltage, and IR2 is the required load current.

This regulator is used for very simple low power applications where the currentsinvolved are very small and the load is permanently connected across the zener diode(such as voltage reference or voltage source circuits). Once R1 has been calculated,removing R2 will cause the full load current (plus the zener current) to flow throughthe diode and may exceed the diode's maximum current rating thereby damaging it.The regulation of this circuit is also not very good due to the fact that the zenercurrent (and hence the zener voltage) will vary depending on VS and inverselydepending on the load current.

The LM78XX series positive regulator :

Figure 14

The LM78XX series of three terminal regulators is available with several fixed outputvoltages making them useful in a wide range of applications. One of these is local oncard regulation, eliminating the distribution problems associated with single pointregulation. The voltages available allow these regulators to be used in logic systems,instrumentation, HiFi, and other solid state electronic equipment. Although designedprimarily as fixed voltage regulators these devices can be used with externalcomponents to obtain adjustable voltages and currents.

The LM78XX series is available in an aluminum TO-3 package which will allow over1.0A load current if adequate heat sinking is provided. Current limiting is included tolimit the peak output current to a safe value. Safe area protection for the outputtransistor is provided to limit internal power dissipation.

If internal power dissipation becomes too high for the heat sinking provided, thethermal shutdown circuit takes over preventing the IC from overheating.

Considerable effort was expanded to make the LM78XX series of regulators easy touse and mininize the number of external components. It is not necessary to bypass theoutput, although this does improve transient response. Input bypassing is needed onlyif the regulator is located far from the filter capacitor of the power supply.

For output voltage other than 5V, 12V and 15V the LM117 series provides an outputvoltage range from 1.2V to 57V.


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Features:

Output current in excess of 1A

Internal thermal overload protection

No external components required

Output transistor safe area protection

Internal short circuit current limit

Available in the aluminum TO-3 package

Voltage Range:

LM7805C 5V

LM7812C 12V

LM7815C 15V

The 79XX series negative regulator :

Figure 15

The LM7900 series of fixed output negative voltage regulators are intended ascomplements to the popular MC7800 series devices. These negative regulators areavailable in the same seven–voltage options as the MC7800 devices. In addition, oneextra voltage option commonly employed in MECL systems is also available in thenegative MC7900 series. Available in fixed output voltage options from –5.0V to –24V, these regulators employ current limiting, thermal shutdown, and safe–areacompensation – making them remarkably rugged under most operating conditions.With adequate heat sinking they can deliver output currents in excess of 1.0A.


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Experiment 5

Objective :Study of the regulation section

Equipments Needed :

1. DMM (Digital Multimeter).

Procedure :



3. Connect the DMM (at DC voltmeter range) at TP11 and TP13, which is theinput of IC7812 and output of the rectifier circuit.

4. Connect the DMM (at DC voltmeter range) at TP16 and TP17. It will be +12VDC, output of IC7812.

5. Connect the DMM (at DC voltmeter range) at TP12 and TP13, which is theinput of IC7912.

6. Connect the DMM (at DC voltmeter range) at TP18 and TP19. It will be -12VDC, output of IC7912.

7. Connect the DMM (at DC voltmeter range) at TP20 and TP21. It will be +5VDC due to 5V zener diode.


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Experiment 6

Objective :Study of SMPS with Variac input (Variable AC) – Regulation Test.

Equipments Needed :


2. Variac

Procedure :

1. First of all make sure that power supply of Variac and Trainer kit is ‘Off’.

2. Now select ‘AC Mains/From Variac’ switch of the Trainer kit at ‘From Variac’position and all Fault switches at ‘Off’ position.

3. Adjust the Variac at zero output position.

4. Now connect Variac across the terminals P, N and E provided on the kit.

Note : Make sure that you are connecting earth to the kit and there is no looseconnection otherwise it may cause hazards.

5. Connect DMM (at voltmeter range) at output of the kit TP16 and TP17.

6. Now switch ‘On’ the Variac supply and Trainer.

7. Adjust the Variac input slowly from 0V to onwards and continuously observethe DMM. Note the point where SMPS gives the desired output i.e., +12V.

At this point measure the output of Variac at AC voltage position i.e.,-----V AC.This is the minimum voltage required for SMPS to work properly.

8. Continuously increase the voltage and observe maximum voltage of Variac,where SMPS stops to give response, record this voltage as --------------V AC.

From this experiment you can observe the regulation voltage from -------- V AC to --------- V AC.


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Experiment 7

Objective :Study of the different faults in SMPS circuit and the systematic procedure oftroubleshooting.

Equipments Needed :


2. Oscilloscope


Procedure :

1. Before performing experiment first select ‘AC Mains/From Variac’ switch ofthe Trainer kit at ‘AC Mains’ position and all Fault switches at ‘Off’ positioni.e., no fault is inserted.

2. Now connect the Mains cord into the kit and switch ‘On’ the power supply.

3. Test the outputs with DMM (at DC voltmeter range) at TP1-TP21, TP16TP17,TP18 TP19 and TP20 TP21 which are desired outputs i.e., 230V, +12V, 12Vand +5V respectively.

4. Now switch ‘On’ the fault switch 1, and observe voltage on Oscilloscope at TP1with (1:10) attenuator probe at ×10 position. Since the input of the SMPS is cuttherefore no output will be displayed hence the fault occured.

5. Now switch ‘Off’ the fault switch 1 and switch ‘On’ the fault switch 2, andobserve voltages using DMM (at DC voltmeter range) at various test pointsfrom TP1 to TP21 and find the fault created by the switch. Since the output ofthe rectifier is cut from the filter hence the problem occurs.

6. Now switch ‘Off’ the fault switch 2 and switch ‘On’ the fault switch 3 and try toidentify fault using same testing procedure. This switch will affect thetransformer outputs (Due to feedback, the input of transformer will also beaffected).

7. Now switch ‘Off’ the fault switch 3 and switch ‘On’ the fault switch 4.Applying the same testing procedure you will find that this time input of IC7812is not as desired. Hence there is no putput at TP16.

8. Now switch ‘Off’ fault switch 4.

9. In this way you can study step by step fault finding procedure.


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Warranty

1) We guarantee the product against all manufacturing defects for 24 months fromthe date of sale by us or through our dealers. Consumables like dry cell etc. are notcovered under warranty.

2) The guarantee will become void, if

a) The product is not operated as per the instruction given in the operating manual.

b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party.

d) Any attempt is made to service and modify the instrument.

5) The non-working of the product is to be communicated to us immediately givingfull details of the complaints and defects noticed specifically mentioning the type,serial number of the product and date of purchase etc.

4) The repair work will be carried out, provided the product is dispatched securelypacked and insured. The transportation charges shall be borne by the customer.

List of Accessories

1. Attenuator Probe (1:10) .................................................................................1 No.

2. Learning Material .........................................................................................1 No.

3. Mains Cord ....................................................................................................1 No.


Documents

SMPS Trainer - Specjalistyczne Testery, Przyrządy i ... Nvis Technologies Pvt. Ltd. 3 Introduction NV7002 SMPS trainer is a very adaptable kit has been designed to explain a very