General steps in SMPS troubleshooting?

[caphair]

Hi guys so I'm wondering if the real experienced experts can give a step by step in how they troubleshoot these supplies.

For example I always check if there's voltages on secondary but when there isn't I often don't know how to proceed.

Should one, check for shorted semiconductors with power off or look up ic datasheets for switching ics to see if vcc voltages are present, etc

I'm usually faced with supplies that have no secondary voltages but there'll be proper voltage at the main's filter cap

[tom66]

Re: General steps in SMPS troubleshooting?

There are a few things to check.

No secondary output voltages: are all rails dead? If so, probably related to a primary side fault, or there's a short on the output. For the primary side, check the Vcc voltage. If that's low, check the Vcc cap and for shorts on Vcc, could be a defective controller IC or a shorted zener or maybe the transistor which switches Vcc on and off is broken. Use your instincts - open resistors rarely fail open without cause, so check MOSFETs and diodes in the same path. Check snubber components for damage. Check fuses. Especially hidden, SMD output fuses!

Using a scope you can see the secondary output waveforms - with experience you will get to understand each portion of these waveforms. For a flyback, for example, you can read back the primary side voltage by looking at the negative period of the flyback voltage (it will be some ratio to the primary side.) This can help diagnose issues with excessive primary side ripple (bad primary side cap etc.)

[caphair]

Re: General steps in SMPS troubleshooting?

Thanks Tom!

So what's a faster diagnosis, voltage testing with the supply powered on or checking for shorted components with power off?

I'm also curious how one determines what side of the supply the fault is on, I remember reading once someone suggested removing the switching transformer and that will help you locate which side has a fault but I never understand how exactly that would help?

[budm]

Re: General steps in SMPS troubleshooting?

You always check for shorted circuit first before applying power and do good visual inspections, you do not know if the unit has been worked on by someone else before or what was done to it so you do not want to cause more damages.

[caphair]

Re: General steps in SMPS troubleshooting?

Thanks. Do you anything about the suggestion of removing the switching transformer to help isolate which side has fault? I don't understand why it needs to be removed to determine that

[budm]

Re: General steps in SMPS troubleshooting?

I do not how that will tell you which side is bad, if you remove the transformer then the primary side drive circuit will no longer has the transformer for it to drive.