Amplifier Update: Now Using an IRF630

I couldn’t figure out exactly what went wrong with the amplifier, so eventually I decided to try to replace the MOSFET with a more resilient one and hope it won’t explode like the first one. I know that I need to add SWR protection, and perhaps to improve the T/R switching, but I don’t have time to work on these right now. I hope that the more resilient MOSFET would be able to survive the conditions that caused the IRF510 to explode.

When I took the amplifier apart, I saw that the MOSFET blew up in a spectacular way.

I replaced the IRF510 with an IRF630. The IRF630 has a 200V and 9A rating, so it will be hard to destroy with a laptop 24V supply. The trade off in power MOSFETs is between improved rating (voltage, current, and on resistance) and input capacitance. The IRF510, with a 100V 5.6A rating, has 195pF input capacitance; the IRF630 has 600pF. This makes the IRF630 harder to drive. I had experience with another amplifier, an experimental class E amplifier, in which the driver chip could drive an IRF510 but not an IRF630. I was not sure that the 1W of the Softrock Ensemble would be able to drive the IRF630.

I soldered the IRF630 into the circuit anyway, connected the amplifier to the Softrock and a dummy load, and tried it out. Success! The IRF630 delivered about the same amount of power that the IRF510 did.

I then connected the amplifier to the “unusual” loop and ran WSPR on 14MHz. As expected, there was a significant difference between the signal reports I got with and without the amplifier (and also in the number of stations that heard me). I left the setup running for most of Friday. At the end of the day, the amplifier’s signal was received all the way from Sydny, Australia, through Singapore, India, and Europe, to the eastern United States. Not bad at all. And so far it has not exploded 🙂

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One Response to Amplifier Update: Now Using an IRF630

  1. Carl says:

    I had a similar experience with the VN67AF and IRF510 MOSFETs years ago when building the “Fun Amp” from 73 Magazine. I had bolted the MOSFETs to the back wall of the chassis with mica insulator. MOSFETs would burn instantly at 12VDC turn on. Dropping supply V to 1,2 volts allowed me to find that the MOSFETs were oscillating strongly at 230 MHz. Remounting the MOSFETs on PCB with Isolated heatsink and ferrite beads on Gate lead cured it. Then the amp was more Fun, hi hi ! WA0RLY Feb18,2013

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