Arv K7HKL notes that the problem with IRF510 RF amplifiers failing seems to be a recurring one for those who are not quite careful with antenna matching, bias level, and drive level.
As a way to start looking into this situation Arv performed some on-line searches to see how others were working around this problem.
There are a number of potentially useful ideas contained in those articles and discussions, but nothing that obviously applies directly to the problem of blowing IRF510 devices at only a few watts of power if the antenna is mis-matched.
Marco – KG5PRT notes from one of the stories reviewed …
Once I got tired of replacing the FETs, I built a current sense circuit, which shuts off the bias once the amplifier draws more than about 3 amps from the PSU. I think this circuit is essential. You can build it into the Power Supply or into the Amplifier. I built it into the Amplifier because the power supply, which is also homemade, does not limit until 7 amps. With the current limit circuit the amplifier now survives transmitting into any SWR from an open circuit to a short.”
He notes that the current limiting circuit has only 7 parts.
And Arv K7HKL suggests implementing this bias control circuit used by AC2CZ in his 50 watt IRF510 linear. He suggests you could probably do this using 2N3906 transistors. Arv likes the fact that it includes a warning
Or this one:
Conclusions about protecting the IRF510 devices
Arv’s conclusions to date are:
- Power supply current limiting (2.5 to 3A) seems to work.
- Bias voltage shutdown on high current (3A) seems to work.
- Driver impedance to IRF510 gate should be quite “stiff”. Use a 50 ohm pad.
- Use a low resistance (4.7 ohms) in series with each IRF510 gate lead.
- Keep IRF510 leads short.
- Impedance presented to the IRF510 drain should be 12.5 ohms.
- RF output transformer should be 1:2 windings for 1:4 impedance transition. A Binocular core may work better…?
- Avoid the problem by using some other MOSFET.
- Use plastic fuses at 3A for self-healing action.
- Fuse rig power at 2 to 3A to avoid IRF510 failures.