-----Original Message-----
From: Radio WC6W <wc6w@juno.com>
To: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
Date: 30 January 2002 16:48
Subject: Re: [AMPS] Shunt regulated high voltage psu
>
>Assorted snips from the last 24 hours of the regulator discussion...
>
>
>On Tue, 29 Jan 2002 11:49:58 -0000 "Steve Thompson" <g8gsq@qsl.net>
>writes:
><snip>
>
>> A couple of thoughts. MOVs are widely used to clamp screens in the
>event of
>> a flashover. At 10s amps, the clamping voltage will be in the regions
>of
>> 600V for a 375V MOV. Many designs survive this ok, but I wonder how
>many
>> times it's luck rather than design. There can be hidden side effects -
>for
>> example, if the supply has a series element using a Nchannel FET,
>there's a
>> big parasitic diode which will carry the 600V from source to drain.
>> Similarly, a bipolar series element will suffer reverse emitter base
>> breakdown and make 590ishV available at the base.
>>
>
> The FET is self protecting in this instance. The surge will be
>conducted back to the screen supply filter cap which will likely not
>charge to that the above mentioned potential, given the limited duration
>of the flashover, with any reasonable impedance in the rest of the screen
>circuit.
>
> And a bipolar should have diode externally connected (CE) for
>protection in this event similar to IC 3 terminal regulators that may
>experience reverse voltage.
I'm going to have to build this up and hook the storage 'scope on! The way I
worked it was to assume 2kVish arcing to screen limited to about 40A by a
glitch resistor. This gives a time constant in the region of a couple of ms.
At the screen, the source impedance is quite low if there's 40A flowing and,
given that we want to keep the screen voltage stable at audio frequencies,
we should have a low impedance path to the regulator for a pulse of 2ms
upwards so I was expecting a lot of the 600V to arrive there with plenty of
current capacity, in which case the screen supply cap could end up with a
significant top up. If there's any significant capacitance hanging on the
gate/base of the series device, or if the control circuit tries to pull the
gate/base down to try and regain regulation even 10's V could be enough to
cause breakdown.
Maybe I'm being pessimistic, and these problems don't occur in practice. I'd
be delighted to know I'm being over paranoid but I don't know as I've not
come across any reports that such circuits did (or didn't) survive
anode/screen flashovers. Shunt circuits by G3SEK and GW4FRX et al are known
to survive repeatedly.
73, Steve
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