The good news is there isn't much to SMPS and they tend to be similar in design.
SMPS (except low powered flyback types) usually work in the same way. They rectify the incoming voltage, feed it via two switching devices driven by a special oscillator driver to a transformer, and then the secondary output of the transformer is rectified, filtered and sometimes regulated.
If you trace out the key parts of the circuit and figure out what should be happening, compare it with what is happening, you will probably find your problem soon enough.
Please, if you're not suitably equipped for safety and comfortable working with high voltages, don't proceed with any live analysis. Do so at your own risk, etc. There are some things you could check without doing live analysis but it will limit your options somewhat.
An oscilloscope is useful but you should be able to get by with a decent multimeter in most situations.
I'm by no means an expert but this is how I approached a similar fault in a subwoofer SMPS, and I solved it in the end. YMMV.
Tricky SMPS fault in Wharfedale SPC 12
First visually inspect. If it looks burned, it's suspect. If the transformer looks burned (not just a little brown but really burned), proceed with caution, or abandon the job and buy a new PSU.
Unplug the amplifier board, power the PSU and check the PSU output is at the appropriate voltage (often it's marked on the board, but for an amplifier its going to be something like +- 50 to 70 VDC and there will be +-15 VDC rails). If all good, then the PSU may not be your problem. Otherwise carry on. This check is reasonably safe to do provided you keep well clear of the "mains supply" side of the board. Note that these voltages could still be enough to kill you in some circumstances, but they are at least isolated from the mains and therefore ground potential.
If the "big" rails are good but you don't have + or - 15vdc I would suspect the regulators, probably just 7815/7915. If all rails are dead or close to it, carry on.
If you can safely, check if there is a square wave around 100 to 300khz across the transformer primary or separately at both switching devices, whichever pin goes to the transformer. DON'T CONNECT BOTH TO DIFFERENT CHANNELS OF A SCOPE OR OTHERWISE AT THE SAME TIME. This swing will be large (170 or 340 volts depending on your supply). If it's there, then maybe the transformer is toast or something on the secondary side is toast (rectifier or something?) If it's the transformer gone, you're probably best to replace the whole thing.
If not check the switching devices (usually MOSFETs) are good, you'll probably need to take them out of circuit to do this. While they're out, you can check whether the gates are getting a square wave from the oscillator (they should be).
If they check out, and there's no signal at the gate, then make sure the line rectifier is good and you have DC at peak line voltage getting to where the switching devices should be.
Then make sure the oscillator IC has appropiate VCC supply. If not make investigations. This is often a simple dropper resistor and zener setup, sometimes a little more complicated. That resistor has to drop a lot of voltage so they can cook and go bad.
If that checks out, make sure nothing is "disabling" the oscillator (usually by pulling a pin low, check the IC datasheet). If you can identify the shutdown circuitry, you could try to isolate it from the driver IC as a test, this worked well for me and proved the problem into that part of the circuit.
If you get this far and no clues, It's possible the chip is bad. I'd suspect this last if you really cannot find anything else (check other components as much as you can first). It would be more suspect if one or both switching devices went bad. Removing it may help you find the problem, if you can do it without damage (I failed to do so, thankfully the part was only $4 to replace).
I found in my case, a poorly designed overvoltage protect had failed and caused the PSU to self disable immediately it was powered on. But it would have been very difficult to find without analysing it while powered.
Good luck!
SMPS (except low powered flyback types) usually work in the same way. They rectify the incoming voltage, feed it via two switching devices driven by a special oscillator driver to a transformer, and then the secondary output of the transformer is rectified, filtered and sometimes regulated.
If you trace out the key parts of the circuit and figure out what should be happening, compare it with what is happening, you will probably find your problem soon enough.
Please, if you're not suitably equipped for safety and comfortable working with high voltages, don't proceed with any live analysis. Do so at your own risk, etc. There are some things you could check without doing live analysis but it will limit your options somewhat.
An oscilloscope is useful but you should be able to get by with a decent multimeter in most situations.
I'm by no means an expert but this is how I approached a similar fault in a subwoofer SMPS, and I solved it in the end. YMMV.
Tricky SMPS fault in Wharfedale SPC 12
First visually inspect. If it looks burned, it's suspect. If the transformer looks burned (not just a little brown but really burned), proceed with caution, or abandon the job and buy a new PSU.
Unplug the amplifier board, power the PSU and check the PSU output is at the appropriate voltage (often it's marked on the board, but for an amplifier its going to be something like +- 50 to 70 VDC and there will be +-15 VDC rails). If all good, then the PSU may not be your problem. Otherwise carry on. This check is reasonably safe to do provided you keep well clear of the "mains supply" side of the board. Note that these voltages could still be enough to kill you in some circumstances, but they are at least isolated from the mains and therefore ground potential.
If the "big" rails are good but you don't have + or - 15vdc I would suspect the regulators, probably just 7815/7915. If all rails are dead or close to it, carry on.
If you can safely, check if there is a square wave around 100 to 300khz across the transformer primary or separately at both switching devices, whichever pin goes to the transformer. DON'T CONNECT BOTH TO DIFFERENT CHANNELS OF A SCOPE OR OTHERWISE AT THE SAME TIME. This swing will be large (170 or 340 volts depending on your supply). If it's there, then maybe the transformer is toast or something on the secondary side is toast (rectifier or something?) If it's the transformer gone, you're probably best to replace the whole thing.
If not check the switching devices (usually MOSFETs) are good, you'll probably need to take them out of circuit to do this. While they're out, you can check whether the gates are getting a square wave from the oscillator (they should be).
If they check out, and there's no signal at the gate, then make sure the line rectifier is good and you have DC at peak line voltage getting to where the switching devices should be.
Then make sure the oscillator IC has appropiate VCC supply. If not make investigations. This is often a simple dropper resistor and zener setup, sometimes a little more complicated. That resistor has to drop a lot of voltage so they can cook and go bad.
If that checks out, make sure nothing is "disabling" the oscillator (usually by pulling a pin low, check the IC datasheet). If you can identify the shutdown circuitry, you could try to isolate it from the driver IC as a test, this worked well for me and proved the problem into that part of the circuit.
If you get this far and no clues, It's possible the chip is bad. I'd suspect this last if you really cannot find anything else (check other components as much as you can first). It would be more suspect if one or both switching devices went bad. Removing it may help you find the problem, if you can do it without damage (I failed to do so, thankfully the part was only $4 to replace).
I found in my case, a poorly designed overvoltage protect had failed and caused the PSU to self disable immediately it was powered on. But it would have been very difficult to find without analysing it while powered.
Good luck!
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