Radio WC6W wrote:
>> A single IRF840 will do the job, although the
>unregulated
>>650V input is higher than its breakdown voltage - you should
>> reduce the input voltage if you can.
> The IRF840 should be fine in a properly designed shunt regulator at
>350V, as the transistor should never see more than that voltage across it
>-- discounting failures.
Failures, you can never discount!
It's very bad design to operate semiconductors from rail voltages that
are above their breakdown voltage - even if they're never "supposed" to
see that full voltage.
If the shunt regulator circuit *ever* goes into a condition where the
MOSFET draws no current, then the drain goes up above its breakdown
voltage and you lose the MOSFET. This is especially likely to happen to
circuits that involve feedback loops, like active voltage regulators.
There are ways to design around this problem, but the only reliable way
is to use a higher-rated device and/or a lower rail voltage.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
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