>
>
>-----Original Message-----
>From: Radio WC6W <wc6w@juno.com>
>To: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
>Date: 30 January 2002 18:32
>Subject: Re: [AMPS] Shunt regulated high voltage psu
>
>
>>
>>
>>On Wed, 30 Jan 2002 17:40:50 -0000 "Steve Thompson" <g8gsq@qsl.net>
>>writes:
>>
>><snip>
>>
>>Hi Steve,
>>
>>> 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.
>>
>> The screen bypass caps would necessarily have to be rated to survive
>>whatever your MOV's will allow surgewise.
>>
>> The pass FET should be OK in this instance. You might want to add a
>>small R in series with the FET drain to make the time constant long
>>enough back into the screen supply cap (presumably 10-20uF) to side-step
>>the surge.
>
>I can visualise a scenario where the control circuit is pulling the gate
>down because the o/p voltage is too high. Depending on the design, it might
>pull to almost ground via a resistor, maybe it's only as low as 300V. If the
>source is up at 5-600 because of the arc, the gate-source insulation will be
>at risk. A high value g-s resistor won't protect if the resistance driving
>the gate is much lower.
>>
Excellent point, Steve. For largish tetrodes that require a
pass-regulator scrren supply, I choose a screen PS transformer that
delivers slightly more than the max rated screen-V. Thus, if the FET
pass element shorts, the screen potential won't be dangerous.
>...
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|