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SWTCHED - MODE POWER SUPPLY

Overview

In the simplest form a SMPS is a combination of transformers and voltage control devices that furnishes power to all the electronic components by converting them into different low voltage DC supply or different voltage ac supply according to the requirement.

Development and Introduction

With advances in electronics, need for dc power supplies for use in integrated circuits has increased manifold. For such electronics circuits (ICs)NASA was the first to develop a light-weight and compact switched mode power supply in 1960s for use in its SPACE VEHICLES.

Now this power supply became popular and presently, annual production of SMPSs may be as 70% to 80% of the total number power supplies produced.

Basic Concept of operationA switched-mode power supply (switching-mode power supply/SMPS, or simply switcher) is an electronic power supply unit (PSU) that incorporates a switching regulator in order to provide the required output voltage.

An SMPS is actually a power converter that transmits power from a source (e.g., a battery or the electrical power grid) to a load (e.g., a personal computer) with ideally no loss.

The function of the converter is to provide a reliable output voltage often at a different level than the input voltage.

Types of SMPS:

• Classification of SMPS can be done based on their functioning as switched models and averaged models. Further classification can be done based on the way they regulate voltage as:

• Conversion types:• § Forward conversion: uses power transformation for the required

output. • § Flyback conversion: stores energy during the switch conduction

interval and delivers it when the switch is not conducting.

• Control method:• § Voltage: varies the switching pulse width with the error voltage

level. • § Current mode: uses a combination of error voltage level and

output current.

• Operational modes:• § Continuous: current through the filter inductor never

reaches zero. • § Discontinuous: current is allowed to reach zero.

So Basically there are four types of SMPS available They are:

1. flyback .2. Pushpull.3. Halfbridge.4. full bridge.

Different Components of SMPS:

SMPS is combined of different electronicscomponents, from a simple diode to a microprocessor powered switch.

Basic components are:• variable resistor• breakdown diode• switch• diode• single winding inductor• capacitor

C

filter

How an SMPS works

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification.

The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage.

AC, half-wave and full wave rectified signals

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

Inverter stage • The inverter stage converts DC, whether directly from the

input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz (kHz).

• The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The output voltage is optically coupled to the input and thus very tightly controlled. The switching is implemented as a multistage (to achieve high gain) MOSFET amplifier.

• MOSFETs are a type of transistor with a low on-resistance and 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 mains power 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 output level on its secondary winding. The output transformer in the block diagram serves this purpose.

• If a DC output is required, the AC output from the transformer is rectified.

• The rectified output is then smoothed by a filter consisting of inductors and capacitors.

Regulation

• A feedback circuit monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator.

• The "Chopper controller" shown in the block diagram serves this purpose.

Power factor

• Simple "off-line" switched mode power supplies incorporate a simple full wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply.

• As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor.

• Putting a current regulated boost chopper stage after the off-line rectifier (to charge the storage capacitor) can help correct the power factor, but increases the complexity (and cost).

Operation In details: The SMPS we are going to describe that is based on the

concept of buck converter, so to understand the operation we have to first understand the concept of buck converter.

Operation of Buck converter: A buck converter is a step-down DC to DC converter. Its

design is similar to the step-up converter, and like other converter it is a switched-mode power supply that uses two switches (a transistor and a diode), an inductor and a capacitor.

Diagram

Operation of Buck converter:A buck converter operates in continuous mode if the current through the inductor (IL) never falls to zero during the commutation cycle. In this mode, the operating principle is described by the chronogram in figure .When the switch pictured above is closed (On-state, top of figure), the voltage across the inductor is VL = Vi − Vo. The current through the inductor rises linearly. As the diode is reverse-biased by the voltage source V, no current flows through it;When the switch is opened (off state, bottom of figure ), the diode is forward biased. The voltage across the inductor is VL = − Vo (neglecting diode drop). The current IL decreases.The energy stored in inductor L is

Therefore, it can be seen that the energy stored in L increases during On-time (as IL increases) and then decreases during the Off-state. L is used to transfer energy from the input to the output of the converter.The rate of change of IL can be calculated from:

With VL equal to Vi − Vo during the On-state and to − Vo during the Off-state. Therefore, the increase in current during the On-state is given by:

Identically, the decrease in current during the Off-state is given by:

If we assume that the converter operates in steady state, the energy stored in each component at the end of a commutation cycle T is equal to that at the beginning of the cycle. That means that the current IL is the same at t=0

and at t=T (see figure 4).Therefore,

So we can write from the above equations:

and

This yields:

This equation above can be rewritten as:

From this equation, it can be seen that the output voltage of the converter varies linearly with the duty cycle for a given input voltage. As the duty cycle D is equal to the ratio between tOn and the period T, it cannot be more than 1Therefore, This is why this converter is referred to as step-downconverter.

Operation of SMPS:

The relaxation oscillator produces a square wave. The square wave is integrated to get a triangular wave, which drives the non-inverting input of a triangular to pulse converter. The pulse train out of this circuit then drives the Pass Transistor. The output is sampled by a voltage divider and fed to a comparator. The feed back voltage is compared with a reference voltage. The output of the comparator then drives the input of the triangular to pulse converter.If the output voltage tries to increase the comparator produces a higher output voltage which raises the reference voltage of the triangular- to pulse converter. This makes the pulse that drives the base of the switching transistor narrower. That means duty cycle is reduced. Since the duty cycle is lower the output becomes less which tries to cancel almost all the original increase in output voltage.Conversely, if the regulated output voltage tries to decrease, the output of the comparator decreases the reference voltage of the triangular -to pulse converter. This makes the pulse wider and the transistor conducts for larger time and more voltage comes out of the L.C.filter. This cancels out the original decrease in output voltage.

Description:

For maximum efficiency the duty cycle should be less than 0.5. As long as the triangular voltage exceeds the reference voltage, the output is high. Since Vref is adjustable, we can vary the width of the output pulse and hence the duty cycle.Switching regulators are more efficient than conventional regulators as the power loss in the switching element is reduced to minimum as it conducts only for a fraction of a cycle.

Interior view of an switched-mode power supply.

A - bridge rectifierB - Input filter capacitorsC - TransformerD - output filter coilE - output filter capacitors

Applications:• Switched-mode PSUs in domestic products such as personal

computers often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with rated frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage "range" switch may be required). In practice they will operate from a much wider frequency range and often from a DC supply as well. In 2006, Intel proposed the use of a single 12 V supply inside PCs, due to the high efficiency of switch mode supplies directly on the PCB.

• Most modern desktop 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 -- as low as 0.8 V for a low voltage CPU to typically 1.2-1.5 V for a desktop CPU as of 2007. Most laptop computers also have a DC-AC inverter to step up the voltage from the battery to drive the backlight, typically around 1000 Vrms.

• Certain applications, such as in automobile industry and in some industrial settings, DC supply is chosen to avoid hum and interference and ease the integration of capacitors and batteries used to buffer the voltage. Most small aircraft use 28 volt DC, but larger aircraft often use 120 V AC at 400 Hz, though they often have a DC bus as well. Some submarines like the Soviet Alfa class submarine utilised two synchronous generators providing a variable three-phase current, 2 x 1500 kW, 400 V, 400 Hz.

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

Advantages:The main advantage of this method is greater efficiency because the switching transistor dissipates little power when it is outside of its active region (i.e., when the transistor acts like a switch and either has a negligible voltage drop across it or a negligible current through it).

Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency.

Disadvantages:Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.

Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase.

Non power-factor-corrected SMPSs also cause harmonic distortion.

Thank You