Burning MOSFET with AC load

Thread Starter

versor

Joined Jan 15, 2020
4
I'm pulling my hairs out, trying to understand why the following circuit is burning MOSFETs.

I'm trying to build a simple AC switch that would have minimal losses. SI8751 provides galvanic separation from the uC that controls the circuit. T4 and T5 are 800V rated MOSFETs, theoretically protected by Transil (D10) with Vbr @400V. Theoretically, because after a few on-off cycles (probably depending on the timing on the mains) one of the MOSFETs always die. Switching cycle is very long, on the order of seconds.
My supply is 230V AC, I'm trying to key live wire for a small PSU (60W) that powers LED strip, so it's a bit hard to say, but definitely has significant capacitive / inductive load.

I would appreciate any input how to improve the circuit or at least pointers where to start the investigation.

Thanks!
burning_mosfet.PNG
 

Thread Starter

versor

Joined Jan 15, 2020
4
Yes, I have good reasons for that :) This circuit is part of a uC controlled switch/dimmer - which I want to have trailing edge capability.
I was hoping to build something generic, that would allow me to switch or dim depending on the load it is controlling.

For linear load (dimmable LED) it works well and did not malfunction even once, yet connecting it to a PSU made me miserable. Just to be clear, I'm not trying to dim the PSU. When connected to it, it's purely on/off mode.
 

jpanhalt

Joined Jan 18, 2008
10,087
There is a recommended circuit similar to that in the datasheet:
1596656951225.png

Is the load within specifications. Can you check the gate with an oscilloscope to be sure it discharges?
 

Thread Starter

versor

Joined Jan 15, 2020
4
Thanks. I've checked before and the gate is discharging. Overall the circuit works well and I'm able to switch load on and off a couple of times. When in stable state, it's not even hot ( I also have a 5A fast-fuse F2 in series with the load, MOSFET is rated for 8,6A)

My guess is that it's a problem with inductive/capacitive load from the PSU, but I have no idea how to protect this circuit better than with the TVS.
 

Thread Starter

versor

Joined Jan 15, 2020
4
Sorry for not being clear, I should not say burn - but breakdown instead. The malfunction is drain-source breakdown of T4/T5
 

jpanhalt

Joined Jan 18, 2008
10,087
Drain source breakdown? Try a mosfet with double the voltage rating -- or at least 50% more.

Remember, your AC voltage value is not peak to peak, but RMS.
 

Alec_t

Joined Sep 17, 2013
11,522
The pulsed drain current rating for that FET is 34A. Could the switch-on surge current of your PSU be exceeding that?
 
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