IRFP260 switching FET failing regularly

[Chris Wilson]

Schematic attached, a 1kW 136kHz Class D amp, works fine save for one
annoying issue I cannot get a handle on, maybe someone can have a
look see please?

I am now on my 4th IRFP260 switching FET in the 50 / 100V DC feed!
 I have the keying socket shorted all
the time, so it's not actually constantly switching as I have yet to
try QRSS or similar. After a while I'll hear a sizzle and smell
burning. Then the 50V / 100V is permanently on, even when I un-short
the keying socket.

The IRFP260 is dead, shorted between drain and
source, and sometimes gate as well. Also, every time I find the 470 Ohm resistor
between the collector of the BF393 switching transistor and the gate
of the FET has burnt up, but the BF393 lives on. I have no idea why
it's doing this, but it did it again last night, and I am down to my
last IRFP260. Is it marginally rated or could it be something else? The amp draws about 10 Amps DC. If
I need to buy more should I look for something higher rated? Thanks.

[Ian.M]

Vgs is right on the limit. Replace D7 with a 12V Zener to clamp the gate at a safe voltage.  With 220nF to ground on the gate, it could be oscillating in common gate.  Add a ferrite bead on the gate, and preferably a 4R7 series resistor.

[Andy Watson]

Is D7 a zener? The 220nF on the gate will be forming a high-Q oscillator circuit with the inductance around the FET - introduce some series resistance into the gate lead (10-500 ohms).

Edit: +1 Ian's reply.

[Chris Wilson]

Actually, I just checked and in the build notes D7 is not specified at all, and I used an IN4007, not a Zener, for sure! Thanks for that! I know nothing about ferrite beads, are they pretty generic, as in if it will go over the lead it's OK? Or are they specced like toroids by material? There's a lot of RF floating around, with occasional mismatches due to weather changing antenna impedance, I have a feeling these failures could have occurred around the time antenna matching was poor.


What rating does the Zener need to be please? I have a few, but probably nothing very hefty. Many thanks for looking at this, I would never have considered this aspect without help.

[Kjelt]

Your schematic specifies a IRFP460 and you choose to use a IRFP260 ?
Ever compared the datasheets and saw what the differences are and if those could be the source of the problem.
Just had a very quick look and the 460 has a max. Vgs of +/-30V and the 260 only +/-20V. There could be more differences, why not use a 460?

[Chris Wilson]

Sorry, my bad, a typo, or brain fart, I am actually using an IRFP460 as specced, apologies.

[Ian.M]

For a IRFP460, I'd clamp the gate with a 22V Zener.  It wont conduct in normal operation, but its still a good idea to clip any transients that might exceed the 30V Vgs rating.

[Kjelt]

Ok 
Well just thinking further if the problem persists did you check temp. specs? 
Measure the temperature in action preferrably with a thermocouple glued to the device , second option IR thermometer.
Just to get an impression if it gets enough effective cooling.