>
>
>-----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.
// Low screen-V tubes such as the 4cx800 (350V-sc.) suffer more from
reverse screen current than tetrodes with handles. However, even
4cx5000As and 4cx10,000s occasionally exhibit reverse screen current with
1200 - 1500 V on the screen. Bleeding 25mA seems to take care of the
problem.
>This led me
>to look at series supplies, which led in turn to thoughts about effects
>under fault conditions.
>>
// Did you look at the screen current curves for the 4cx800?
>>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.
>
// Amen. Steve
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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