[AMPS] Shunt regulated high voltage psu

[Ian White, G3SEK]

Tom Rauch wrote:
>> There are ways to design around this problem, but the only reliable
>> way is to use a higher-rated device and/or a lower rail voltage.
>
>.......or to clamp the shunt side of the series resistor  with a large
>zener with a capacitor across. You can use a zener clamp rated
>below the maximum voltage breakdown of the shunt device, and
>above the highest desired regulated voltage.
>
It doesn't make sense to spend money on a high-power, high-voltage zener 
(don't forget the heatsink) when you can uprate the shunt MOSFET for 
much less cost.

The only exception would be if you insist on running the screen from the 
B+ supply, and wasting a large amount of power in a 100W high-voltage 
resistor.

>This actually is a good idea in ANY case, because the shunt diode
>with protect the screen system if the main regulator goes on
>vacation.

The protection isn't as good as it seems at first sight. Because of 
component tolerances, the zener voltage has to be several tens of volts 
above the correct screen voltage, so the screen dissipation can easily 
be exceeded if the shunt regulator goes open-circuit. It's also a fault 
that could quite easily be missed in the excitement of a contest or a 
pileup. Unfortunately an overheated  screen grid often leads to a 
permanent increase in negative screen current, which may then make the 
tube impossible to operate at full power.

After over 10 years experience of being forced to run small tetrodes on 
the razor's edge of screen dissipation (as the only cost-effective way 
to get UHF power in Europe, before the Russian tubes came in) I've found 
that the only truly effective form of screen protection is a current 
monitor, completely independent of the voltage regulator, that 
switches-off the screen in milliseconds if there's a fault. It means you 
can forget the amp and concentrate on the operating.

-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)
                            http://www.ifwtech.com/g3sek

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