In message <RrLbfDAD4CU2Ewg6@ifwtech.demon.co.uk>, Ian White, G3SEK
<G3SEK@ifwtech.demon.co.uk> writes
>
>Rich Measures wrote:
>
>>>Note that the necessary breakdown voltage foir a shunt stabilizer device
>>>is not the output voltage, but the INPUT voltage, to handle transient
>>>situations where the device is completely cut off and the
>>>drain/collector voltage rises all the way up to the input voltage. (This
>>>is also why you can't use a transistor shunt stabilizer with a dropper
>>>resistor from a B+ supply of more than 1-1.5kV.)
>>
>>I disagree. It seems to me that there can never be more potential across
>>a shunt regulator than the regulated output voltage.
>
>Don't forget the transient conditions - like, how do you switch it on?
>Before the feedback loop stabilizes, the shunt device is drawing zero
>cirrent and is exposed to the full unregulated voltage (almost as if the
>dropper resistor wasn't there), and this may well destroy the
>transistor.
>
>Active (feedback) regulators are subject to a variety of other transient
>conditions, so it seems to me like a bad risk to use devices that
>wouldn't withstand the full voltage that could be applied; but once
>again that is a matter of design style.
If the applied voltage exceeds the device breakdown voltage, it isn't
necessarily terminal if the current is limited.
I guess that low frequency power bipolars are probably dc tested in a
similar fashion to rf bipolars. The latter are typically forced into
breakdown at some defined current (usually 10s or 100s mA) to establish
that the breakdown voltage is greater than the specification. This is
done both with emitter and base shorted and the base open. In a shunt
regulator the current limiting resistor might well limit the current to
a safe level. In the crudest sense, the transistor acts like a zener
diode. As long as a FET has a gate-source resistor, I think the same
analysis can apply.
If a separate low voltage supply is used in the regulator circuit it can
be arranged to be present before the screen hv, in which case problems
are unlikely.
Steve
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