Can High Side damage on an SMPS get across the Transformer to the Low Side?

[Briesmith]

I have a blown smps and I am looking for some reassurance as to the amount of diagnosis I have to do. It blew the instant I plugged it in not under any load.

As you can see from the attached pix, the damage is located around the main or bulk capacitor (since removed for testing) and the resistor array/network that drives it (the bunch of SMD components to the left of the YAMAHA YC775 AMPS logo). The burn mark on the frame clearly shows the pins of the capacitor. And, for the avoidance of doubt, there is no way the board could have contacted the frame, it was correctly mounted on the standoffs at the time the amp blew up.

If anyone has any comments on how far any damage is likely to have spread, I would be grateful.

[stj]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

looks like the mosfet shorted and took out the resistors.
maybe because of a bad cap.

secondary is probably fine, but thats a complex looking board.

[momaka]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

>>> Can High Side damage on an SMPS get across the Transformer to the Low Side? <<<
It depends on the SMPS type and topology... but generally, it's a very rare occurrence and typically may only happen if the PWM IC gets "stuck" in some way so as to ramp up the PWM to a much higher value than it should be, causing an OV condition on the secondary. But again, that's almost never the failure mode. The transformer itself does provide some limitation on how much power a "strong" PWM pulse can carry onto the secondary side, which in essence provides protection to the secondary side.

secondary is probably fine, but thats a complex looking board.

Looks like an SMPS with a class D amp all combined on one board.
The SMPS looks like a double-forward topology.

I agree about checking the small caps near the big one on the primary (that is now removed), since those looks like they are cheap brands.

Could also be that the clearance / distance between the primary cap's leads and the aluminum heatsink just wasn't big enough, so perhaps the HV arced over to secondary side ground. But then that doesn't explain why those resistors would blow. Those resistors are for current sense on the primary side (for the MOSFETs), so indeed one or both MOSFETs could be blown up... or their damper diodes.

[Gentian]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

I also agree that the 2 mosfets connected to those resistors are blown and no damage on the secondary side of the transformer. I would suggest checking the Greatz bridge (AC/DC) converter before the big capacitor on the ac side because it can be the reason for the big capacitor failure. You can replace it also it may looks good with a new one to be sure no problem on the ac side.

Good luck!

[Briesmith]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

Thanks everyone for replying. I'll let you know how I get on.

[Briesmith]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

Update: I removed the 25F60CPM MOSFET and it tests as a resistor so it's definitely defective. Is the advice still to remove the other MOSFET or can I leave it where it is?

[CapLeaker]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

Remove it and test it.

[Briesmith]

Re: Can High Side damage on an SMPS get across the Transformer to the Low Side?

Thank you CapLeaker and everybody else who has helped with this. I'll remove the other MOSFET and report what I find.