Understanding Switch Mode Power Supplies

Switch mode power supplies (SMPS)have been used for many years inindustrial and aerospace applicationswhere good efficiency, light weight andsmall size were of prime concern. TodaySMPS (often called “choppers” or“switchers” are used extensively in ACpowered electronic devices such ascomputers, monitors, television receivers,and VCRs.

This Tech Tip explains the basic operationof the typical switch mode power suppliesused in consumer electronic equipment.We will cover both the Pulse Width(PWM) and Pulse Rate (PRM) types.Refer to Tech Tip #204 for information onidentifying SMPS problems, and to TechTip #205 for information ontroubleshooting PWM type SMPS.

SMPS Benefits

A SMPS offers three main advantagesover a conventional linear power supply:

1) high efficiency & less heat generation2) tighter regulation, and3) smaller size and weight.

Of these, greater efficiency is the biggestadvantage.

Fig. 1: The SMPS transformer (left) and theAC power transformer both provide isolationand various output voltages, but the SMPStransformer is smaller, lighter weight, andmore efficient.

Conventional “linear” power supplies areinefficient because they regulate bydumping the excess power in to heat. TheAC power transformer, operating at 60 Hz,also contributes to the inefficiency ofsome power supplies. When all theinefficiencies are added, conventional,linear power supplies are typically 40-50% efficient, while switchers haveefficiencies from 60 to 90%. This is veryimportant when the designer wants toreduce generated heat, reduce powercosts, or increase battery life.

Anther key benefit of a SMPS is theirability to closely regulate the outputvoltage. Switchers adjust for changes ininput voltage or load current, with littlechange in eff iciency. Switch modesupplies regulate continuously, and followload changes almost immediately. Inaddition, switchers have the unique abilityto maintain the correct output under lowinput voltage conditions. In fact switcherscan actually produce an output voltagethat is higher than the DC voltage appliedto the input.

A final advantage of switchers is theirrelatively small size and weight. Becauseswitchers operate at high frequencies, theparts are physically smaller than thoseneeded for a conventional, 60 Hz powersupply of the same power rating. Thetransformers, capacitors and coils areboth physically smaller and lighter. Thismakes them ideal for use in portableequipment.

Switchers Aren’t New

You may think that switch mode suppliesare relatively “new” technology. But quitethe opposite is true. One of the firstswitch mode power supplies was themechanical vibrator circuit used in early

automobiles. The vibrator “chopped” the6 volt battery voltage into an AC signalthat could be stepped up and down todeliver the plate and bias voltages neededto power the tube radio.

A more modern SMPS that you may befamiliar with is the horizontal output stageof a television receiver or computermonitor that develops the high voltage.Although this “flyback” circuit is notcommonly called a switch mode supply, itis a type of switcher.

Today’s more sophisticated SMPS stillemploy the same basic concept used inthe early vibrator supply: a DC voltage isconverted to an AC signal, the AC signal isstepped up by a transformer; and thestepped-up AC is converted back to a DCvoltage. Today’s supplies use feedbackand variable switching to provideregulation, and incorporate a solid stateswitch. Figure 2 illustrates the basicfunctions of a SMPS.

Basic Switcher Operation

Let’s take a closer look at the basicswitcher block diagram shown in Figure 2.The heart of all switch mode supplies isthe switching transistor and switchingtransformer. Raw (unregulated) B+ issupplied to the switching transistorthrough the primary winding of thetransformer.The switching transistor(SOT) is a switch. When the switch isclosed (the transistor is turned on) itprovides a path for current to flowthrough the transformer primary toground. As we’ll see a bit later, changinghow fast or how long the switch remainsclosed regulates the output voltages.

The current flow in the primary winding ofthe transformer produces an expanding

rate modulated (PRM). Televisionreceivers and computer monitors may useeither type, while VCR commonly usePRM supplies.

Pulse Width Modulated RegulationPWM regulators vary the “on” orconduct ion t ime of the swi tch ingtransistor. Figure 4 shows an example ofpulse rate modulation. Note that thefrequency of the signal remains constantand only the duty cycle varies.

As the width of the pulse is increased, the

Fig, 2: A basic SMPS consists of a DC voltage source, a switching device, a step up/downtransformer, (flyback) and a switching control stage.

magnetic field which couples to thesecondary windings through the core. Asthe transistor is switched on and off themagnetic field alternately expands andcollapses in all of the transformerwindings. By designing the transformerwith different secondary turns ratios,various amplitude pulses are produced atthe output of each secondary winding.The output of each winding is applied tohigh speed switching diodes and filterswhich produce the DC output voltages ofthe SMPS.

It is important to remember that all of thewindings are mutually coupled by themagnetic field. If more power is applied tothe primary winding, more power isdelivered to the secondary windings; if thevoltage at one secondary windingincreases (or decreases) the voltages atthe other windings change by an amountequal to the turns ratio.

Achieving Efficient Regulation

Regulation is important in most powersupplies. Good regulation is needed tomaintain a constant output voltage forchanges in load current and changes in

Fig. 3: A conventional, series-pass regulatordissipates the unused power as heat.

the input voltage. Switch mode powersupplies provide fast, efficient regulation.To better understand why, let’s reviewconventional power supply regulation.

Conventional power supplies regulate byusing a series-pass regulator operating inits linear mode, as shown in Figure 3. Theseries pass regulator maintains a fairlyconstant output voltage with changes ininput voltage and load current. But, inorder to maintain regulation for all loadconditions, more power is applied toregulator than is needed by the load. Thisunused power is dissipated as heat.

The key to making the regulator moreefficient is to produce only as muchpower as the load consumes. Then therewill be no extra power to dissipate. Thiswould be easy if the load current neverincreased or decreased, or if the sourcevoltage remained constant. Of course thisnever happens.

Switchers are very efficient regulatorsbecause they only produce as muchpower as is needed by the load. Voltageregulation is achieved by sampling the DCoutput voltage and comparing it to areference. (If the SMPS has severaloutputs the main supply is sampled). Theresulting correction voltage is used tocontrol the frequency or “on” time of theswitching transistor, which in turndelivers more or less power to the load.

The regu la tors used in consumerequipment fall into two types; 1) pulsewidth modulated (PWM), and 2) pulse

Fig. 4: The PWM regulates by varying the“on” time of the switching transistor

switching transistor stays on longer, andmore energy is applied to the switchingtransformer. This produces an increase inthe DC output voltage (Unless the load isshorted, or draws excessive current).Likewise, as the pulse width is madenarrower, the transistor is on for a shorteramount of time, and less energy is appliedto the transformer.

Pulse Rate Modulated RegulationFigure 5 shows an example of pulse ratemodulation. The PRM regulator varies therate (frequency) at which the switchingtransistor is turned off and on. Notice,however, that as the pulse rate increases,

Fig. 5: The PRM regulates by varying thefrequency of the pulses, which varies theswitching transistor’s conduction time.

the “on time” decreases. Thus, if theoutput voltage is too high, the switchingtransistor is turned on and off at a fasterrate. This results in less energy beingapplied to the switching transformer. Atyp ica l PRM supp ly may opera te a tfrequencies as high as 90 kHz under a “noload” condition, but slow to 40 kHz underfull load.

Putting It All Together

Figure 6 shows a block diagram of atypical SMPS. While specific switchersmany vary somewhat from this diagram,all SMPS have these basic functionalblocks. Some switchers, for example, mayconnect the System Control l ine to adifferent point in the control loop.

The blocks can be grouped in to foursections or “most important circuits”(MICS): 1) Unregulated Bt, 2) Startup &Drive, 3) Secondary circuits, and 4)Feedback & Control.

MIC 1 - Unregulated B+The SMPS needs a source of power. Thegray shaded blocks in Figure 6 providethe unregulated B+ to power the switcher.In an AC operated device the AC linevoltage is rectified by a conventional fullw a v e p o w e r s u p p l y . ( T h i s i s w h yeverything on the primary side of theswitching transformer is at “Hot ground”potential). Often an RF filter network isused to prevent the high frequenciesproduced by the switcher from getting onthe AC power line.

Most microprocessor controlled chassisinclude a standby power supply to keepthe micro running when the main powerto the unit is turned off. This allows themicro to respond to the “on” commandfrom the remote control, or front panelswitch. The standby supply may be asmall, conventional supply, or it may be asmall SMPS.

The primary of the switching transformerand the switching transistor are also partof the unregulated B+ stages. A field effect

transistor (FET) is usually used for theswitch. FETs offer several advantageso v e r b i p o l a r t r a n s i s t o r s f o r S M P Sa p p l i c a t i o n s . ( A h o r i z o n t a l o u t p u ttransistor is a bipolar transistor). First,FETs are voltage operated and can bedriven directly without a driver stage.Secondly, FETs are bi-directional (conductwith voltage of either polarity applied tothe source and drain) which eliminatesthe need for a damper diode. Thirdly,FETs operate efficiently at high switchingfrequencies.

Lastly, FETs have a low “on” resistancewhich contributes to the efficiency of theSMPS, and reduces heat build up. (Ther e a s o n t h a t FETs a r e n ’ t u s e d a shorizontal output transistors is that theycan not handle the large reverse voltagesp ikes , 900-1100 VPP, p roduced inhorizontal output stages).

MIC 2 - Startup & DriveThe pink shaded blocks in Figure 6 areresponsible for the signal that drives theswitching transistor. The driver block is

Fig. 6: The four most important circuits in a switch mode power supply are: Unregulated B+(MIC1), Startup and Drive (MIC 2). Secondary Loads(MIC3), and Feedback and control (MIC 4).

either a PWM or PRM, and the necessaryoscillator and control. This block may be as i n g l e I C , o r m a y i n c l u d e d i s c r e t ecomponents.

This stage receives its initial DC voltagefrom a resistor divider on the B+ supply.After the SMPS is running, the “runvoltage” for the driver stage may comefrom one of the secondary windings onthe switching transformer.

One important signal that is applied to thedriver comes from the output of the optoisolator. This feedback signal is a sampleof the output voltage of the SMPS. It iscompared to a reference level in the drivercontrol block. (The reference may be adiscrete zener diode, or may be internal tothe dr iver IC) . A cor rec t ion or e r rorvoltage is produced which changes thepulse width or pulse rate to regulate theSMPS.

Several other control signals may beassociated with the driver, depending ont h e s p e c i f i c S M P S : o v e r c u r r e n tshutdown, over voltage shutdown, and/orunder voltage (low B+) shutdown. Thesecontrol signals may be applied to thedriver block via separate opto isolators;they may be combined in to a commoncontrol line and then applied via an optoisolator; they may be appl ied to theFeedback & Control blocks; or they maynot be used at all.

MIC 3 - Secondary circuitsThe secondary circuits are indicated bythe black blocks in Figure 6. A SMPS may

have five or more DC outputs, dependingon its application. There are several thingsyou need to keep in mind about thesecondary circuits.

First, a problem in one of the secondaryloads (such as the horizontal outputstage) may cause what appears to be aSMPS problem. Before you troubleshoot a“dead” SMPS symptom, be sure todetermine if the problem is the load or theSMPS.

S e c o n d l y , m o s t S M P S w i l l n o t r u nproperly without the main load connected.The main load is the ou tpu t tha t i smonitored by the Feedback & Controlcircuit for voltage regulation control.Usually this is the highest voltage output,or the output that has the most currentdrain. (In television receiver and computermon i to rs the main load powers thehorizontal output stage). Refer to Tech Tip#204, “Identifying SMPS Problems” formore information on isolating a SMPSproblem from a load problem.

Lastly, keep in mind that SMPS typicallyoperate at frequencies from 40 to 150kHz. This means that you can not replacethe rectifier diodes with conventional, ACpower supply diodes. The diodes used inthe outputs of a SMPS are special highfrequency, quick recovery types. Theelectrolytic capacitors should have lowequivalent series resistance (ESR) andleakage.

MIC 4 - Feedback & ControlThe Feedback & Control blocks shown in

Figure 6 (red) couple a sample of theoutput voltage back to the driver stage.Two major parts of this functional area arethe feedback divider network and the optoisolator.

The feedback divider network usual lyconsists of two precision resistors whichdivide the output voltage. The divideddown voltage is coupled to the Driverblock through an opto isolator. The Driveruses this feedback voltage to regulate theswitching transistor. Some SMPS mayuse a separate winding of the switchingtransformer to produce the feedbackvoltage.

An opto isolator is needed to maintaini s o l a t i o n b e t w e e n t h e c o l d g r o u n d( s e c o n d a r y s i d e o f t h e s w i t c h i n gtransformer, and the hot ground (primaryside), while coupling the DC feedbackvoltage. An opto isolator consists of aphoto transistor and an LED. As thefeedback voltage changes the brightnessof the LED changes. The changes in lightcause the conduc t ion o f the phototransistor to change. (The voltage at TP10is supplied by the Driver, and is pulled toground by the conduction of the optoisolator).

Refer to the Tech Tip #204 for informationon identifying SMPS problems, Tech Tip#205 for information on troubleshootingPWM type SMPS, or Tech Tip #206 forinformation on troubleshooting PRM typeSMPS.

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