Troubleshooting & repairing switch mode power supplies pdf


Troubleshooting &. Repairing Switch Mode. Power Supplies. Brought to you by Jestine Yong Troubleshooting & Repairing Switch Mode Power Supplies Brought to you by sheet) UC pdf (or pdf files) UC diagram UC marking UC not require an. I do suggest you read more on SMPS and how to repair them because it will definitely speed Troubleshooting & Repairing Computer Printers .

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Troubleshooting & Repairing Switch Mode Power Supplies Pdf

SMPS Trouble Shoot Guide - Download as PDF File .pdf), Text File .txt) or read Notes on the Troubleshooting and Repair of Small Switchmode Power Su. Troubleshooting & Repairing Switch Mode Power Supplies Brought to you by Jestine Yong Content Part I Introduction to SMPS 1. Introduction to Switch Mode. has over 46 years of experience in switch-mode power supply . Problem Areas Current-Mode Control and Subharmonic Ripple.

This article is written as per the request made by Mr. If charge full the green led will glow If charge low the red led will glow But now this charge not working Now the new diode also damaged Pls guide me sir In our area shop.. But my aim is not to download new one.. Pls help me sir

These may include high value resistors above 50K, or low value wirewound resistors above 1K. Similarly, capacitors which may be rated above V can be left unchecked unless one of these look somewhat damaged externally. Testing for a Burnt Inductor Transformer Every SMPS circuit will essentially include a small ferrite transformer, which this part can also possibly become the cause of a burnt SMPS circuit, although the chances of a damaged transformer can be too remote.

This is because the wires inside the inductor might require some time to burn, and before this can transpire the other more vulnerable parts such as diodes and transistors would be forced to blow off ,preventing any further damage to the inductor. So basically you can be rest assured that the transformer is the one element which might be the safest and the undamaged part in a given faulty SMPS circuit. If in a rare event the inductor burns, this would be distinctly visible from the burnt insulation tape which may be also melted and stuck with the winding.

An SMPS with a burnt transformer could be virtually irreparable, because a burnt transformer would mean most of the elements burnt out, along with PCB tracks uprooted. Time to download a new SMPS unit. The secondary side mostly will not require any checking as it is isolated from the primary and can be expected to be aloof from the dangers. Well, this concludes this article explaining tips to repair an SMPS circuit, if you think I have missed some crucial points, or if you have something important to add in the list, please tell us through your valuable comments.

It has to be measured by the ESR meter. ESR meters come in all shapes, sizes and brand. We found that a unit called Blue ESR meter designed by Bob parker to be the best value for our repair shop. Blue ESR meter is a powerful ESR meter, user friendly, robust, versatile and capable in locating bad or shorted electrolytic capacitor while still in the circuit.

Please visit the link at page to find out the capabilities of using Blue ESR meter to test on other components besides electrolytic capacitors http: Your precious time will be wasted because you could not effectively locate the bad component which is the coils.

This Blue ring tester is an inexpensive yet effective way to test any high Q inductive component. Another good reason you should get this blue ring tester is that it has the capability to perform in circuit test. If you are familiar with the equipment you are repairing and this will surely save your time without having to remove coils from the circuit.

Otherwise, you may need to solder out one leg to test the coil. The test leads are non-polarized-that means you can test the coils using either way from the test probes. For more information about Blue Ring Tester you may visit http: Instead of reading signals in numbers or lighted indicators, an oscilloscope will show voltage versus time on a graphical display. If you know what kind of signal to expect, and the scope shows you a different signal, you know something is wrong. The scope can be used to check the operating characteristics of parts like transistors and capacitors.

Oscilloscopes have being used for many years to troubleshoot power supply, amplifiers, and other analogue devices. For example, you need to check the presence of output waveform of a power IC.

It is also useful in checking the secondary output lines to see if there are any ripples or not.

Without an oscilloscope, it is difficult or almost impossible to trace those important signals in SMPS. The higher the frequency of the scope the more expensive it is. Oscilloscope of 50 to megahertz MHz bandwidth will serve you well. This allows you to compare the timing relationship of two related signal.

If you have used an oscilloscope before, then you probably know just how useful they can be. Every electronic technician must always take safety precautions before he or she starts work.

Electricity must be handled properly, or else it can injure or cause fatalities. Here are some basic steps that show you how to avoid accidents from occurring. Always keep in mind that safety has to come first.

A serious shock may stop your heart and if a large amount of electric current flows through your body, you will receive serious burns. Here are some rules, which should help you to avoid electricity hazards. If you have to run tests while the equipment is operating, turn the equipment on, make your test carefully, and then turn the equipment off again.

Wear rubber bottom shoes or sneakers. Try to do the work with one hand, while keeping the other in your pocket. That keeps the possible current paths away from the heart. Use only plastic screwdriver for shock protection during service operation. But some resistors may fail and the capacitor can hold this charge even after you have turned off the equipment.

This capacitor has a range of about uf to uf at to working voltage. Before you start to work on a power supply, always turn off the power and discharge the capacitor. You can do this by placing a resistor across the two legs of the capacitor.

The resistor value can be around 1. It takes only a few seconds to fully discharge a capacitor. Double-check the capacitor with a voltmeter after every discharge. Do not discharge capacitor with screwdriver because: It may melt the tip of the screwdriver.

It will damage the capacitor and its terminal. If we are too near to the point of discharge, the heavy spark generated may cause injury to our eyes. Hot Ground Problem Figure 8. Hot ground is in the primary side of a switch mode power supply while the cold ground is the equipment ground and located mostly at the secondary side. Be careful when taking voltage measurements around these grounds. For example, if you want to check the primary circuit of a power supply with power on, always ground your meter black probe or scope to the hot ground, while checking the secondary side then you have to connect the black probe to cold ground.

If you want to test the primary side of SMPS and you accidentally connect the black probe to cold ground, the voltage measurement might not be correct and sometimes it may destroy your meter. Isolation Transformer Figure 8. These components can be easily damaged by static electricity. There are several techniques which can reduce the incidence of component damage, caused by static electricity.

Some suggest using a battery powered soldering iron when working on ESD circuits. Fire Before returning the equipment to the user, every reasonable precaution is taken to avoid fire hazards. Be sure to use only direct replacements and not one that defeats some safety measure.

For example, the fuse in your equipment is carefully designed and it must be replaced only with the same size, type and ratings. Should you install a fuse that is too large than the original rating, chances are that the equipment will be flammable.

Look at the problem carefully before you start any troubleshooting work. Even if it takes you a longer time to inspect the faulty SMPS-the time will be well spent! Ask your customer what is the problem with the SMPS.


If it is bad dead and silent with fuse blown then go further to test the bridge rectifier, power FET, Secondary diodes etc to see if the components have developed any short circuit or not. Figure 9.

Sometimes the fuse can blow by itself due its life span. Low output voltage It is easy to determine the low output voltage problem. If you did not measure the output voltages of the power supply you may have thought it was a no power symptom. Furthermore, a low voltage output problem could cause the power LED not to light up. You may ask whether the output voltages are within specification or not? If you measure the output voltage to the heater circuit and you got only 2 to 3 volts, this clearly indicated that the line have problem.

In other words, if you have 4 output lines and all of the output voltages dropped to half, then this proved that the problem is due to the low output voltage problem. If only one of the output lines has problem while the others are okay, then this shows along the faulty output line there must be some components that are faulty that pulled down the output voltage.

Most of the time, it was the bad filter capacitor along the line that causes the voltage to drop. Once you have identified that it was the low output voltage problem in the SMPS, then you can use the isolation method in chapter 14 to find out whether is the SMPS itself is faulty or the secondary side components problem that had caused the SMPS to produce low output.

High output voltage Instead of producing the correct output voltages at the secondary side, all of the output voltages have increased in value. This is very common in ATX power supply where the 12 volts output may increase to 14 volts; the 5 volts output increase to 6 volts and so on.

You can test the output voltage with your meter to confirm the reading. Please refer also to power shutdown problem in page as both of the high output voltage and power shutdown problems are quite similar. Some technicians refer to it as power pulsating problem. Look at the power LED of the equipment and if it blinks, then it is a power blinking problem.

If the SMPS does not have the LED power indicator stand alone type of power supply , then you can place an analogue meter red probe to the output diodes at the secondary section and black probe to cold ground voltage testing method. The SMPS tries to start up but because there are some components failures either at the primary or the secondary section, the power went down again and the cycle goes on. The best way to confirm the power cycling problem is to use the voltage testing method.

It could be a shorted secondary diode, bad filter capacitors, and faulty components in the regulation circuits i. However I have come across the Run DC circuit diode breakdown when under load in the primary side causing the power supply to cycle. It took me many hours to locate such fault. So in the future, if all of the secondary side components are tested good, I guess your next step would be to directly replace the Run DC circuit diode and retest the power supply even though the diode tested good with your ohmmeter Figure 9.

That means if you place your analogue meter red probe to one of the output diode and the output is a 15 volts line, you could see that the analogue meter needle will quickly kick up to more than 15 volts say 25 volts and then drop back to zero.

This is called power shutdown problem. The reason for it to shutdown was to protect the SMPS from continuous producing high output voltage that can burn the equipment or other electronic circuitry.

It is quite frustrating too because when you want to test out the problem, the SMPS works perfectly okay and when you let the SMPS runs, it starts to break down after few minutes or hours later! If it stopped once you hit the board, then you can conclude that it has intermittent problem.

Sometimes an open start up resistor in the primary power side could cause similar problem too and you need to test the start up resistor with your meter. Refer to page 29 on the function of start up resistor. The large filter capacitor in the power supply section can hold dangerous high voltage even if the unit has been switched off for a few days. It is advisable to discharge the filter capacitor first before you start to troubleshoot in order to protect yourself from being electrocuted.

Many power supplies have the bleeder resistors across them to discharge the capacitor voltage after the Main power is Switched Off but others do NOT have. A High DC voltage from the filter capacitor makes your muscles contract and if you touch it, you might find yourself unable to let go.

Thus it is necessary to take the proper precautions. The cathode ray tube monitor may break and cause the flying glasses to hit your body or eyes. Just be cautious when handling the power supply area. You can always use a meter to confirm whether there is still charged in the capacitor.

If there is a high voltage registered in your meter, then you have to discharge the capacitor. Otherwise you may proceed to repair the equipment. A typical value for this type of filter capacitor is to microfarad with a to voltage rating. Figure The reason for not using the screw driver to discharge a capacitor is because the printed circuit board or circuitry can be damaged due to the spark generated while discharging the high voltage in the capacitor.

I once had blown the power area using this method. However, if you know that the capacitor stored voltage is not too large after having confirmed it with a meter, you can easily discharge it with a small screw driver. Just place the screw driver tip to touch the two pins of the capacitor and within seconds the charge will be gone.

If the capacitor holds a heavier charge of electricity, then discharging the capacitor with a screw driver may melt the tip of the screw driver and the copper on the printed circuit board.

Sometimes a big spark may cause small disintegrated solder lead or copper to fly out from the circuit board and might injured your eyes or body. This method had been used by many electronic repairers around the world for the light bulb will act as an indicator to see if the capacitor still holds the charge.

If there is a charge, the light bulb will light and after discharging the light bulb will go off. In many cases, you would not be able to see the light due to the current being drained off too fast.

I still prefer the third method because in the second method you have to use both hands to touch the capacitor lead. You can use either a 1.

It is very simple to use and very effective too.

Repairing Switching Mode Power Supplies

It takes only a couple of seconds to fully discharge the capacitor. Please do not hold the two ends of the resistor with your finger, only hold the body of the resistor. Use only one hand to do the job while the other hand you can still hold on to the solder gun. I highly recommend to those who are using the screw driver to discharge a capacitor in power supply to consider the second and third method as these are the safest methods.

It not only protects the circuit but it also protects you. If you have discharged the capacitor and you are still not sure or have no confidence whether the charge is already gone, you can always use a meter to test and confirm it.

Once the charge is gone, you are now free to touch on the power supply section and begin troubleshooting.

Troubleshooting switch mode power supplies

With just a simple voltage test on certain components or area in the SMPS, you can easily conclude whether the component or the circuit section is faulty or not. Do you know that there are many experienced electronic repairers performing voltage test in electronic equipment in order for them to solve a problem fast? They use the voltage testing method on all kinds of electronic circuit which includes the SMPS circuit.

If you want to join them and be good in voltage testing, then grab this opportunity to learn from this topic as I will be guiding you step by step on how to easily perform voltage test in SMPS.

Warning - Before you begin the voltage test, make sure the AC Voltage from the Main is from an isolation transformer otherwise quit this test. If you have the AC voltage coming from an isolation transformer then let us begins with the test! There are four points in SMPS where you can perform the test. The points are: Testing this point will quickly let you know if the AC input is present or not. Do follow the pictures shown in the next page on how to perform the voltage test on bridge rectifier.

If you get zero voltage or very less voltage then you have to check the circuit before the bridge rectifier. Suspect these problems if there is no AC input to the bridge rectifier: The AC power cable has broken internally. Main fuse open circuit or loosen. Circuit track opens or have dry joints. The EMI coil may have open circuit.

Not all SMPS have the same sizes of bridge rectifier and not all use a single package of bridge rectifier. If the designs have four individual diodes that form the bridge rectifier then place your test probes as seen in figure In order to test the voltage at the filter capacitor, the black probe has to be at the negative pin side and the red probe to positive side as shown in figure If you get zero volts when measuring the filter capacitor then troubleshoot the area to locate if there is any problem with the circuit like dry joints, broken track and etc.

If the DC voltage measured is lower then it should be, a high chance that the filter capacitor may have problem and you have to test it off board with a Blue ESR meter or a digital capacitance meter to check on the capacitance value. Please discharge the big filter capacitor before you remove it and test it with the meters otherwise if the capacitor has charge in it, it will blow the meter! If you accidentally slip either one of the test probes and it touches on the other pin, a great spark would develop and blow the Main fuse and sometimes destroy the primary section components too.

Once you have found the VCC pin, place the red probe of the meter to the VCC pin remember to set your meter to DC volt range and the black probe to HOT ground primary filter capacitor negative pin as seen in figure In figure Here are three possible results that you can expect from the supply voltage test: You should now perform the next test which is the secondary output test refer to page If the power IC has pins that are too close with each other like SMD power IC legs then I suggest that you place your red probe along the supply voltage soldering pad and not on the supply voltage pin VCC pin.

This is because the red probe would not accidentally touch on the other pins and blow the power supply. Therefore there is no chance for you to accidentally touch on the hot side section.

However, be extra careful too even though you knew that the secondary side is not as dangerous as compare to the primary side. Some SMPS have two outputs while some have more than 5 outputs. Generally the testing method is the same regardless of how many outputs the SMPS has.

Before you start checking on the output voltages you should know what the expected output voltages are, so that you can compare the result of the testing with what you are expecting.

You should get a positive DC voltage reading from the test and also from all the other secondary output diodes if the test probes touches on the cathode of other secondary output diodes.

You can measure it from behind the board or from directly on top of the secondary output diode as seen from the photos in the next page. If the output diode is connected the other way round, then place your red test probe on the anode side and the black probe still in the cold ground and expect a negative voltage as seen in figure Please do not place your red probe at the cathode side of the diode under test because the cathode side now is on high pulse AC voltage produced by the SMPS transformer.

The test method is quite the same. Simply place the red test probe to the centre pin where the two Schottky diodes symbol face each other and the black probe to cold ground and measure the output voltage. When testing a SMPS makes sure there is a load. If you are measuring output voltages of SMPS in equipment like TV, Monitor and etc then this means the output of the SMPS is already connected to loads like the color circuit, high voltage circuit, vertical circuit and etc and you need not worry of connecting another load to it.

For your information, I had came across some dot matrix printer supply that does not need a load motherboard in order to function. You can straight away plug in the AC supply and expect voltages to be present at the output. If you are not sure whether one needs to connect a load when measuring the output voltages of SMPS, you just connect it to remove the guessing game.

If you continue to spend your time finding bad components in the SMPS even though you have already got the correct output voltages, you will be wasting your time. From experience, the current sense resistor that had change in resistance value and high ESR value in secondary filter capacitor could also contribute to such problem. Do not overlook that the load problem can also pull down the output voltages.

This problem could be caused by an open resistor or resistor that had turned into high Ohm in the feedback area. Check the Optoisolator IC and the corresponding components as well. If only one of the output lines has increased in voltage, suspect a bad inductor in the output line or dry joints in the load area of that line. This could be the cause in primary, secondary, feedback circuit Optoisolator circuit and also in the load slight leakage in the load components.

This is most probably due to the problem in the primary, secondary, feedback circuit Optoisolator circuit and also in the load too. Carefully go through this chapter again as the voltage testing method is one of the fastest way to locate fault in SMPS and in any other electronic equipment circuit.

Normally, technicians use an oscilloscope to check for the right waveform input or output waveforms in a particular IC or circuit to determine if the IC or the particular section is working or not. It depends on the complaint of the SMPS. There are actually two critical test points in any SMPS. The first would be the output waveform from Power IC and the second one would be the DC output waveform from all the secondary output diodes.

Searching from the Internet would give you the latest information on many IC data.

Let say the part number is UC, go to Google dot com and key in one of these keywords as shown below to look up for the data: Once you found the datasheet of the Power IC, look for the diagram to see which pin is the output.

In the example given below, pin 6 is the output because it goes to the gate pin of power FET. Just before you begin to place the scope probe to the testing point, make sure the AC supply is from the isolation transformer. The oscilloscope ground black colour probe of the oscilloscope has to be connected to the primary power supply ground HOT ground. The best location is to clip the probe at the negative pin of filter capacitor as shown in figure If you want to check the secondary side then clip the probe at the cold ground.

Of course some power supply designs may have a slightly different waveform but it has to be a pulse square wave. Hot ground and Cold ground is different! There is one question- why should we measure this output waveform of power IC and not other waveform in the primary side of the SMPS?

It is because the output waveform test result could tell us if the SMPS is fully working or not. This means if you got a good output waveform, it indicates that the Power IC is receiving the correct supply voltage and also the corresponding components are working fine.

Your next step would be to locate faults in some other areas like in the secondary section. Sometimes dry joints or track broken between the output pin of the power IC to the gate pin of power FET could cause the power FET to stop switching and the result will be no power.

It might be a defective power IC itself, shorted power FET or power transistor that dragged down the output waveform, dry joints, faulty corresponding components, shorted primary winding in the power transformer, shorted components in secondary side that shutdown the power IC through feedback circuit and etc. What you should do now is to place the scope probe onto the drain pin of the power IC as seen in figure What does this clean DC mean?

The waveform should have one horizontal line without any distortion or ripple. This indicates a clean DC output as seen in the below photo. Just before you perform this test, make sure the scope setting is set to DC and not AC otherwise you would not get the result as you would expect. Select the voltage per division to get the desire output waveform. Once you got this kind of clean DC waveform you will immediately know that the primary filter capacitor and the secondary filter capacitors are working fine.

Different SMPS may produce different types of waveform and the above waveform is just for reference purposes. You could also place the probe of a Multimeter that has the frequency range on top of the power transformer to test the frequency and to see if the SMPS is working or not. Please refer to figure You will surely know if a particular IC or electronic circuit is working or not with the help of an oscilloscope. If you are on a tight budget, you can always download a used analogue oscilloscope from site dot com.

This is much cheaper than downloading a new one. If you have the budget you may get a more sophisticated one like the digital oscilloscope. With all these waveforms test given I believe troubleshooting SMPS would be much easier and faster for you.

Not only that and if the power supply components blow again, I have to recheck the power circuit and replace the blown components and retest it. For your information all these testing and checking are a waste of time and money if the power supply components blow again.

Are we going to recheck it until we found the cause of the problem? There must be a better way of dealing with this kind of problem. The series light bulb trick for testing repaired power supply had saved me thousands of dollars in parts because the new replacement parts would not blow even if there are still components shorted in the power supply!

Question 1- What is the reason for using the series light bulb in the power circuit? The reason is simple- to limit the current to power semiconductors in power supply such as bridge rectifier, power FET or transistor, power IC, and diodes primary or secondary diodes so that the components would not blow.

Without it and if there's still a problem in the power supply, the semiconductors would surely get hot and blow again if there is short circuit somewhere in the SMPS. Questions 2- What is the light bulb wattage used and how do I connect the light bulb in the circuit? If you want to troubleshoot a bigger set of equipment such as a big amplifier and projection TV you may use the light bulb from to watt.

The connection has to be in series with the AC line. Assuming you have replaced the bad components in the SMPS, now connect the two wires across the two side of the fuse holder as shown in figure You may solder the two wires of the light bulb behind the board as seen in figure Light Bulb Off Figure The reason for it was because at first it charges up the filter capacitor rapidly and then gradually slows causing the light bulb to go dim and turns off.

This is the result light bulb off that we are expecting if the SMPS is working fine. If it still does not function even with the light bulb is turn off, then it could be some other components in the power supply that you did not check.

You may have to recheck the power supply again for bad components like an open resistor, bad capacitor and etc. Please refer to page 39 for more information about feedback circuit. At least now you are very sure that the SMPS would not blow the fuse because in the earlier test, the light bulb was off.

Light Bulb Very Bright Figure Discharge the big filter capacitor and begin to troubleshoot until you have found the shorted component. A shorted SMPS transformer primary winding could cause the light bulb to glow very bright and you need to use the Blue Ring Tester to test the primary winding.

The power supply is actually trying to come up but is shutting down most probably due to the light bulb wattage which may be too small for the equipment not enough current flowing to the SMPS. Before you put in the fuse, there is one test that you should make. Place the red probe of your analogue meter to one of the secondary side diode output diode and the black probe to cold ground please refer to chapter 11 on how you can connect the test probes for voltage testing.

You can conclude that the power supply is actually working. The reason why the bulb pulsates is because the power supply is not getting enough of current. In this case you can remove the light bulb and put in the fuse and retest the SMPS or equipment without worrying that it will blow the components because the secondary output voltages are already present in the secondary side.

If you get very little voltage or no voltage at all in the secondary side, I guess you might need to troubleshoot the SMPS again until you locate the faulty parts. Once you see that the bulb did not go off even after a few minutes indicating short circuit in power supply what you need to do is to turn off the power and pull out the AC plug and quickly discharge the big filter capacitor in the power primary section.

Once you have done that then, place your finger on the semiconductors, SMPS transformer or any suspected component in power supply. The hottest component in the power supply is the one that had caused the bulb to light brightly because the shorted component consumes more current! Generally, components would only run warm and not hot.

I do not know about others but after so many years of being involved in the electronic troubleshooting line, I could conclude that the 5 methods that I had used for repairing SMPS were very effective in identifying fault in the power supply section. If you repair a SMPS and found the following problems namely: That means if you see that the Varistor split opened, you can conclude that only that area is faulty and chances are very slim for the secondary side to be faulty.

Your next step would be to concentrate in finding bad components in the corresponding area Varistor circuit area. After you have replaced the bad components, you will begin to retest the power supply-am I right?

The problem now is after opening up the SMPS cover or the equipment cover, you did not see any physical signs of component failure and you do not know where to begin to check. The fault could be in the primary section, secondary section or could even be in both sections!

The five methods that I have used could help narrow down your search and you can conclude at which section is the cause of the problem. From there you can use your testing electronic components knowledge to locate the faulty components! It consists of primary and secondary section and has different types of components in it to make the power supply to work. If any components in either one of the section is found faulty primary or secondary , it may cause the power to blink, no power, power shutdown or low power problem.

Understanding switch mode power supply theory, tutorial or operation will make us better in repairing the unit. In this first method, I will show you a way to troubleshoot switch mode power supplies even faster.

By using the Blue Ring Tester, you can test the secondary output diodes without lifting the diode from the circuit. The Blue Ring Tester is very handy and versatile when it comes to finding fault in the primary and secondary section of a switch mode power supply. By placing the Blue Ring tester probes to the primary winding of the switch mode power transformer, a good measurement will usually indicate between LED lights up.

If it indicates only one or two LED or the tester LED goes off no light , this prove that there are problems in the power supply. The primary winding, Power FET and secondary output diode could have been shorted. Make sure you have discharge the large filter capacitor and unplug the ac before placing the test probes. It is very simple to locate the primary winding of the switch mode power transformer.

Connect one probe to the drain centre pin of the field effect transistor power FET with the other probe to the positive side of the large filter capacitor. If you carefully trace the drain and the positive pin of the filter capacitor, it will eventually bring you to the two pins of the switch mode power transformer. These two pins is the primary winding of the switch mode power transformer. A good primary winding will have 4 to 8 LED lights up.

Please refer to page That means when testing the primary winding of the power supply and if you notice that the LED goes off-then suspect one of the diode in the secondary side may have shorted. With this testing method, it can save you lots of time. Sometimes a small secondary diode can go shorted at the secondary section of the flyback transformer G1 or Vcc supply circuit.

It can cause the switch mode power supply to blink, shut down when power on and low output power. It is important to check on all secondary output diodes and primary winding first before checking on other components. Troubleshooting and repairing switch mode power supply can be frustrating if you do not know the right way.

If you do not have the Blue Ring tester, make sure you get one in order to speed up the repair. This section only explains one part of the way to repair switch mode power supply. Please keep on reading on the next four methods. You need an absolute way to find out which section is actually given you the problem. Once you know which section is having problem then it will be easy for you to spend all your time on that section so that you can easily nailed down the culprit. The first thing that you need to do is to follow the steps below: You can find out the power IC drain pin from datasheet downloaded from the Internet.

Not all SMPS will flick and some will flick higher than others. You need to test the result on different SMPS in order to familiarize with the flicking.

If you have confirmed that the primary side is the cause of the problem then check all of the components in this area. Once you have located the bad components, replace it and retest to see if it flicks or not. If it is flicking, do switch off the power, discharge the big filter capacitor and reinstall the SMPS transformer and power on the equipment again. The power supply faults can be caused by only one bad start up resistor and it can also be cause by more than ten bad components heavy lightning strike.

Sometimes a no power, power blink and low output power symptoms are due to the problems in the main circuit board load.

It is not necessary the power problem must come from the primary power section and shorted secondary output diodes. The fault can be further down the secondary output line which is in the main board. If you have measured all the components in the power supply section and could not find the defective component, then try troubleshooting components beyond the power supply section.

For your information a shorted small ceramic capacitor in the main board can cause the whole power supply to malfunction. All SMPS have outputs at the secondary section. Some designs have four outputs, others five and so on. Each of the output is connected to an individual circuit.

For instance, a 6. The Fairchild KA shows up in a number of devices see Photo 2. This was a Samsung design, now sold by Fairchild since they took over the Samsung facilities and product line. It is vaguely similar to the UC although with a different pinout. Again, there are a couple other vendors for this so you should check for multiple datasheets. One very popular IC that you will find in lower powered single output devices is the TOP series of chips made by Power Integrations.

Add a transformer, a couple rectifiers, and an optoisolator and you have a complete switching supply in a box. Of course, they fail frequently but are fairly easy to diagnose.

A similar but less popular device is the MC A lot of less expensive Chinese-made products will use the AP control IC, and this chip is manufactured and sold under dozens of different names by dozens of different companies in China.

The documentation on it is poor, but if you ever encounter a cryptic-looking PWM controller where pin 6 is not being used, it is likely to be an AP or a copy. The English language datasheets for this product are skimpy at best but once you have some idea of the pinout and how it works you should be able to figure out what is going on.

Photo 2: The Fairchild KA has been used in several devices. Encountering the Unexpected Not every supply is a single switching supply in a box. Sometimes you will encounter systems with multiple switchers in the same box providing multiple output voltages, each regulated. This is very common for things like video monitors and computers. If you see a lot of small discrete transistors all over the place, a good guess is that they are involved with automatic shutdown systems, to shut down in case of high or low voltages or currents at one or more place.

Troubleshooting these circuits without a manual can be a real nightmare since it can be difficult to figure out at what voltages individual parts trigger. Every once in a while for audio or other low noise applications you will see linear series regulators for a little additional smoothing, located after the switching supply.

Repairing Switching Power Supply

Since these can run hot, they are a common source of trouble but fairly easy to diagnose since you can see power coming into and out of them.

Fixing the Problem If you have documentation on the power supply, half the work is done for you. If not, you know the basic block diagram and you can work out the individual parts within each block by hand.

Often the PWM chip will have multiple sources. For example, you can get the common PWM controller from at least four different vendors. If the power supply comes on but immediately crowbars, the first thing to do is check or replace all the filter capacitors on the secondary side of the transformer. Other things can cause this like a leaky rectifier on the secondary or a bad resistor in a current sense circuit, but they are far less common.

When in doubt, change the caps and then take the diagnosis from there. If the power supply was working, was shut off, but then would not restart at all, replace the kickstart capacitor. The high voltage capacitor sometimes two capacitors on the primary supply, which filters the line directly seldom is seen to be failed in the US. However, in Europe where the line voltage is twice as much and where the same multi-input power supplies are used, those capacitors are frequently found to be bad.

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