Greetings all,
Jim wrote:
Power MOSFETs work like big zener diodes anyway – if you exceed the
drain-source breakdown voltage, avalanche occurs and the voltage is
clamped, usually about 20 – 40% above the BVdss value (60V in this
case). This does not harm the MOSFET directly, but does result in high
power dissipation – if breakdown occurs at 80V, and the clamping
current is 10A, 800W will be dissipated. This is OK if it only occurs
during short, infrequent voltage “spikes”, but if the high voltage
transients occur with a large duty cycle, the MOSFET will quickly
overheat. It might be possible to connect high-power zeners across the
MOSFET with a slightly lower breakdown voltage than the MOSFETs, but
then the problem would be how to protect the zeners!
Instead of super-duper zeners, you may as well use fast switching diodes
to protect the MOSFET. Use these diodes to discharge excessive drain
voltage into an electrolytic cap. Next, use a cheap 2N3055 or similar,
and a low-power zener diode, to limit the maximum voltage on the
capacitor. You may even measure the voltage as a tuning indicator - it
acts as a peak-detector.
(you may even use that "high-DC voltage" for something more useful: a
crude switching-mode step-down converter to feed it back into the power
supply :o)
Regards,
Wolf DL4YHF.
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