-----Original Message-----
From: Ian White, G3SEK <G3SEK@ifwtech.com>
To: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
Date: 29 January 2002 14:00
Subject: Re: [AMPS] Shunt regulated high voltage psu
>
>Steve Thompson wrote:
>>
>>>
>>>> However, it does show that the more you think about screen supplies,
>>>> the more potential failure modes you see, and the more complex the
>>>> design becomes to defend against them. Marv's is complex; Acom's is
>>>> complex; Alpha's is complex; and G4JZQ's and mine is complex. We all
>>>> did it differently, but we all agree that a handful of cheap zeners
>>>> won't cut it.
>>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.
>>
>There doesn't seem to be any real need for an active series element to
>feed an active MOSFET shunt regulator. The loop gain in the shunt part
>is already very high, so the series feed can be just a plain resistor.
Certainly, in that configuration. There's numerous valves where the reverse
screen current appears to low or non-existent (4CX800 for example), where
dumping 40+mA all the time in a shunt regulator feels wasteful. This led me
to look at series supplies, which led in turn to thoughts about effects
under fault conditions.
>
>Part of the trick of using MOVs for this application is to use several
>(they're cheap enough, and you probably had to buy 5, so why not?). Two
>in parallel seem effective to protect the 1000V screen bypass in the
>socket against a flashover from the anode, then one at the output of the
>supply, and then another one directly across the MOSFET. Because of the
>various series elements in the path of the flashover current, the MOVs
>in the supply see less voltage and current than the ones out at the
>socket, so they clamp at progressively lower voltages.
Good point. I've not searched for V/I curves for MOVs. Might be interesting.
I've seen a screen current meter taken out because of the current being sunk
in the regulator/MOV. Another place where protection diodes across the shunt
are are valuable.
Steve
>
>
>--
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.com/g3sek
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|