Troubleshooting SMPS Problems (PWM Type)

Switch Mode Power Supplies (often called“ c h o p p e r s ” , s w i t c h e r s ” o r s i m p l y“SMPS”) are used extensively in manyAC-powered devices, such as computers,monitors, television receivers, and VCRs.This Tech Tip provides a 6-step procedureto use for isolating problems in the PWMtype of SMPS that are typically used inconsumer electronic equipment. Tech Tip#203 explains how SMPS operate.TechTip #204 provides information on how todetermine whether the SMPS is the causeof an inoperative chassis.

Why SMPS Troubleshooting IsDifferent Than Linear Supplies

Convent iona l l inear power supp l iesusually continue to supply an output, evenif a load shorts, a filter capacitor opens, orhalf of the bridge quits. Isolat ing thedefective component in a linear supplyusually consists of tracking too low, toohigh or lost DC voltages. The SMPS, onthe o ther hand, requ i res a d i f fe ren ttroubleshooting process. Here’s why:

First, replacing a burnt resistor, shorteddiode or bad transistor in a SMPS doesnot guarantee a fix as with the linearsupply. Simply fixing the SMPS supply byreplacing a burned part often causes thesame components to burn up again.

Secondly, when troubleshooting a linearsupply, a variac is used to slowly bring upthe AC line voltage while you monitor thecurrent draw. Not so with most SMPS.You have virtually no chance to back offthe AC power once the SMPS kicks in. Iteither puts out no current or full current.

P e r h a p s t h e b i g g e s t d i f f i c u l t y w i t htroubleshooting a SMPS is a direct resultof one of the SMPS benefits - it’s ability toprotect itself from over-voltage or over-

current condit ions by shutt ing down.Most SMPS component failures or loadchanges cause the SMPS to completelyshut down and produce a “dead chassis”symptom. This can make troubleshootingdifficult and confusing. Is the shutdowncaused by too much high voltage?; the B+be ing pu l led down?; too much loadcurrent?; a supply component failure?; ora defective safety circuit?

Wi thout a log ica l p rocedure , SMPStroubleshooting can be frustrating. Butyou can break the SMPS shutdown loopand quickly isolate the defective area in 6easy steps.

The Four Key Circuits

Before we look at these six steps, let’sbriefly review the four general sections(most important circuits or MICS) thatmake up a SMPS, as shown in Figure 1.(Refer to Tech Tip #203 for a completeexplanation of each MIC).

Fig. 1: A SMPS contains four key circuits.

MIC #1 - Unregulated B+ - This circuitincludes the linear power supply, standbys u p p l y , p r i m a r y o f t h e s w i t c h i n gtransformer, and the switching transistor.

MIC #2 - Startup & Drive - This sectionp r o v i d e s t h e c o n t r o l s i g n a l f o r t h eswitching transistor. The heart of MIC #2is the driver circuit. It can be a singles tage t rans is to r o r a cur ren t modecontroller IC.

MIC #3 - Secondary Ci rcui ts - Thesecondary circuits include the secondarywinding of the switching transformer andthe components (diodes capacitors, etc.)that provide power to the loads. MostSMPS have 2 to 5 loads.

MIC #4 - Feedback & Control - MostPWM SMPS feedback loops provide fourfunctions:l Output voltage sampling for regulationl High voltage monitoringl System control micro for power ON/OFFl Ground isolation through opto-isolators

6-Step Troubleshooting Procedure

The following six steps are proven to be asafe, effective method of isolating theproblem to a specific MIC. Combiningthese steps with dynamic componentanalyzing will get even the toughest SMPSup and running.

Always use an Isolation Transformer.All SMPS contain both hot andchassis (floating) grounds. You willcause damage to the SMPS and/oryour test equipment if you connecttest equipment to a hot ground, orattempt to tie the hot and cold

Keep the following things in mind whenperforming the SMPS troubleshootingprocedure:

l Always use the correct ground referencewhen making a measurement. Using thewrong ground reference will result in anincorrect reading.

l Hot grounds are usually found on theprimary side of the switching transformer.Use this ground for all MIC 1measurements.

l Chassis are found on thes 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. Use this ground for MIC 2, 3& 4 measurements.

l The opto isolator input (from the controlcircuits) is measured with respect tochassis ground.

l The opto isolator output (to primary sidedriver or controller stage) is measuredwith respect to hot ground.

l Be Prepared to Make All ParameterMeasurements. Efficient troubleshootingd e p e n d s o n y o u r a b i l i t y t o q u i c k l ymeasure different signals and voltages:DC from tenths of a volt to 16OV; signalvoltages from 2 VPP to >400 VPP; andfrequencies from 40 kHz to 150 kHz.

We recommend using the SC61, SC3080or SC3100 Waveform Analyzer, since theyallow you to make all measurements withjust one probe connection.

Fig. 2: Follow these steps to quickly and safely isolate SMPS problems

The flow chart in Figure 2 outlines the six the SMPS can operate. (Not all SMPStroubleshooting steps. Perform the steps have standby supplies). Check for standby

in order. Each step funnels the problem to voltage with the chassis plugged into an

a specific MIC and suspect components. isolation transformer set for an output of

Any one of the troubleshooting steps may 117VAC, (such as the PR57 POWERITE)

isolate the SMPS problem. Often, you may but w i th the chass is power (chass isnot need to do all 6 steps because the ON/OFF switch) turned off.problem is found in one of the first steps.Following is an explanation of each step. Some chassis use a second, smaller

SMPS as the standby supply. The fact that

#1 - Check The Standby Supply the standby supply is working eliminatesmany suspect components. The IC Driver

The standby voltages to the driver and in MIC #2 is always a suspect in a

microprocessor must be correct before down condition, and is often needlessly

replaced. The standby switcher isphysically smaller, and has lower powerhandling capabilities than the mainswitcher, but it is driven by the sameDriver IC as the main switcher. Therefore,if the standby switcher is running theDriver IC is likely OK. The shutdowncondition is caused by something elsethat preventing the Driver IC fromsupplying a control signal to the mainswitching transistor.

If the standby supply voltage is correct,but the main SMPS is still not operating,move on to step #2.

#2 - Substitute For The Main Load

An important step in troubleshootingSMPS problems is to separate the SMPSoutput from the rest of the chassis. Thishelps you determine if the shut downsupply symptom is due to the SMPSsupply itself, or if the symptom is due toan outboard circuit or load. (A similarprocedure is given in Tech Tip #204 forisolating problems to the SMPS orexternal circuitry).

Most PWM SMPS will not operate withoutan adequate load current. Therefore youcan not simply disconnect the loads.Instead, most manufacturers recommendreplacing the main load with a light bulbthat has approximately the same wattagerating. The light bulb provides currentlimiting and provides a suitable, constantload for the SMPS.

The main B+ load is the output of theSMPS that contains the feedback dividernetwork. In a television receiver or

Fig. 3: Use a light bulb as a substitute loadfor fhe B+ output of the SMPS. Disconnect thenormal B+ load, then connect the bulb afterthe feedback takeoff point.

monitor this is the output that powers thecol lec tor o f the hor izonta l outputtransistor. By substituting for this loadyou ef fect ive ly d isable the safetyshutdown controls that come from theexternal circuits.

The size of bulb you use depends on theload you are substituting. For example, ifyou are substituting for the load on the130 VDC B+ supply in a television receiveror monitor, use a standard 60 watt, 120VAC light bulb. If you are substituting forthe 15 volt B+ output in a VCR supply, usea 12 or 18 volt bulb.

You will need to open the circuit path toremove the normal load. Make sure tobreak the circuit after the feedback takeoff point. Removing the horizontal outputtransistor in a television or monitor willbreak the circuit, but do not connect thebulb in place of the horizontal outputtransistor. The primary of the flyback isnot designed to handle a continuouscurrent. Connect the light ahead of theprimary, as shown in Figure 3.

After you substitute for the load, you willsee one of four conditions when you turnon the SMPS:

l Bulb lights and measured voltage isnormal

This means that the SMPS is workingproperly. Something external to the SMPSis causing the shutdown. Possibilitiesinclude excessive HV, excessive currentdraw by one of the loads, or a defectivesafety circuit.

l Light bulb doesn’t light (SMPS doesn’tstart)

l Light comes on but goes out (SMPSstarts but goes into shutdown)

l Light is very bright (indicates possibleregulation problems)

These last three conditions indicate thatsomething is wrong with the SMPS.Continue with the remaining steps untilyou locate the problem.

#3 - Remove the Drive Signal from theMain Switching Transistor

Open the signal path between the Driveand the gate or base of the switchingtransistor. You can easily do this byunsoldering and lifting any one of thec o m p o n e n t s i n t h e s i g n a l p a t h .

Disconnecting the input signal to the mainswitching transistor allows you to safelytroubleshooting the SMPS circuits, whilethe chass is is turned on, wi thoutaccidentally producing an output from theSMPS.

#4 - Check MIC 1 CircuitsMIC #1 includes all the B+ path from theoutput of the linear supply to the groundpoint of the emitter or source of theswitching transistor. Begin by checkingfor B+ voltage at the switching transistor:

1. Connect the Waveform Analyzer to theswitching transistor’s drain or collector.Set it to measure DC voltage.

2. Set the PR57 to zero volts AC output.Press the “0-1.5A/175W" output currentmonitor button.

3. Gradually increase the PR57's ACoutput while monitoring the outputcurrent.

You will observe one of the followingconditions:

l Low current, normal B+ (approximately160 VDC) with PR57 output at 117 VAC.

This means that the B+ supply is good.But there still might be a problem in theMIC 1 circuitry:

Check the switching transistor to makesure it is not open. Use a dynamictransistor, such as the TF46 “SuperCricket.check."

Also check the resistor(s) in the emitter orsource lead of the switching transistor. Ifyou suspect they may have changed invalue, r e p l a c e t h e m w i t h t h emanufacturer’s exact replacement. Theseresistors are precise tolerance, and arecritical to safe operation of the SMPS.

If the transistor and resistors are good,proceed to Step #5.

l No DC and no AC current drawThere is an open in the B+ supply. Checkthe fuses, safety resistors, diodes, andswitching transformer primary.

l No (or low) DC & increasing AC currentLow or missing DC voltage along withincreasing AC current is caused by ashort in the B+ supply itself, or some-

where within MIC #1. Check the switchingtransistor, bridge, and filter capacitor.Also check the primary winding of theswitching transformer for a DC short tothe core or to another winding.

#5 - Check Driver Circuit

First, confirm that the Driver IC has start-up voltage. In most switch mode powersupplies the start-up voltage is obtainedfrom a resistor divider network off thelinear unregulated B+ supply. Alwayscheck for start-up voltage before checkingif the oscillator is running. Connecting aprobe to the oscillator test point can“kick-start” the oscillator.

Secondly, check all of the oscillator testpoint waveform parameters: DC, PPV,and frequency. The oscillator frequencymust run at the switch mode supplyfrequency. If the frequency reads high(the oscillator waveform may be noisy andcontain with glitches) confirm thefrequency using the Delta Frequencyfunction on the waveform analyzer. If thefrequency is more than 10% too high, thecontroller IC is defective.

NOTE: Usually a current mode controllerwill not output a drive signal to theswitcher with the SMPS disabled. Makeyour measurements at the oscillator testpoint on the controller IC)

#6 - Perform a Dynamic Check of TheFeedback & Control Circuit

This final troubleshooting step confirmsproper operation of the Feedback andControl circuits, MIC 4. Often failures inMIC #4 are caused by a defectivetransistor which shuts down the entirefeedback loop. This dynamic feedbackcircuit check will quickly isolate anyproblem in MIC 4.

To check these circuits you will need toapply an external voltage (equal to thenormal, main B+ output of the SMPS) andconfirm that the circuits respond properlyto it. Use a variable, current limited DCpower supply, such as the leakage powersupply in the Sencore LC102 “AUTO Z”.

1. Disconnect the substitute light bulbload from main B+ output of the SMPS.

Fig. 4: Apply an external DC voltage to the feedback circuit to confirm the operation of MIC 4.

2. Connect the LC102 power supply to theB+ point, after the feedback dividernetwork. (The same point where the lightbulb was connected).

3. Connect the Waveform Analyzer to thecontrol input of the Driver. (Output of theOpto Isolator) Press the “DCV” function.

4. Set the PR57 to 117 VAC output andturn the chassis on.

5. Vary the LC102 supply voltage fromfive volts below the normal B+ voltage tofive volts above the normal B+ voltage,while monitoring the DC reading on theWaveform Analyzer.

If the feedback circuitry is working, youwill see an increase in the DC voltage atthe Driver input as you increase theapplied voltage above the normal B+ level.A 1 VDC change in the applied voltagetypically may only show a .1 VDC changeat the Driver.

If there is no change at the Driver input,check the opto isolator. (Remember toreference the correct ground.) With achanging DC into the opto isolator thereshould be corresponding change in theoutput DC voltage. Continue checking theMIC #4 feedback loop until the defect islocated. This includes the “Power On”command from the system controlmicroprocessor, and the output from thesafety circuits, such as over-voltage andover-current shutdown. Be sure to checkthe electrolytic capacitors for ESR.

SMPS Component Analyzing Tips

Don’t use general replacement parts.The switching transistors and diodes usedin SMPS must operate effectively at highcurrents and high frequencies. The mostreliable results are achieved using themanufacturers’ original parts.

ESR is a critical capacitor parameter.Due to the high operating frequency ofSMPS, ESR (equivalent series resistance)is a common failure mode of electrolyticcapacitors used in SMPS. High ESRcauses an array of symptoms, fromshutdown to wrong regulation. Replacinga suspect cap with an off-the-shelf partmay only make matters worse.Test anysuspect capacitors with an ESR tester,such as the Sencore LC102.

Test the Switching TransformerSMPS switching transformers, like flybacktransformers, are high Q transformers.They are responsible for transferring alarge amount of energy in a short periodof time. A single turn short will shut downthe SMPS. Be sure to use the patentedSencore ringing test to test any suspectswitching transformer.

Form 4926Printed In U.S.A.