IRFP260 switching FET failing regularly - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 20, 2015, 11:49:03 am
content: Schematic attached, a 1kW 136kHz Class D amp, works fine save for oneannoying issue I cannot get a handle on, maybe someone can have alook see please?I am now on my 4th IRFP260 switching FET in the 50 / 100V DC feed!I have the keying socket shorted allthe time, so it's not actually constantly switching as I have yet totry QRSS or similar. After a while I'll hear a sizzle and smellburning. Then the 50V / 100V is permanently on, even when I un-shortthe keying socket.The IRFP260 is dead, shorted between drain andsource, and sometimes gate as well. Also, every time I find the 470 Ohm resistorbetween the collector of the BF393 switching transistor and the gateof the FET has burnt up, but the BF393 lives on. I have no idea whyit's doing this, but it did it again last night, and I am down to mylast IRFP260. Is it marginally rated or could it be something else? The amp draws about 10 Amps DC. IfI need to buy more should I look for something higher rated? Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: October 20, 2015, 12:15:16 pm
content: 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 20, 2015, 12:19:10 pm
content: 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.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 20, 2015, 12:40:19 pm
content: 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.

author: Kjelt
author_rank: Super Contributor
timestamp: October 20, 2015, 12:52:38 pm
content: 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?

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 20, 2015, 12:57:39 pm
content: Sorry, my bad, a typo, or brain fart, I am actually using an IRFP460 as specced, apologies.

author: Ian.M
author_rank: Super Contributor
timestamp: October 20, 2015, 01:04:30 pm
content: 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.

author: Kjelt
author_rank: Super Contributor
timestamp: October 20, 2015, 01:06:40 pm
content: OkWell 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.