Envelope-to: dave@picks.force9.co.uk
Delivery-date: Mon, 06 Feb 2006 17:04:13 +0000
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Date: Mon, 06 Feb 2006 17:03:24 +0000
From: Wolf DL4YHF <dl4yhf@freenet.de>
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To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Re: More smoke...
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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.