[AMPS] Screen regulator circuit needed

[Ian White, G3SEK]

Rich Measures wrote:

>>Note that the necessary breakdown voltage foir a shunt stabilizer device
>>is not the output voltage, but the INPUT voltage, to handle transient
>>situations where the device is completely cut off and the
>>drain/collector voltage rises all the way up to the input voltage. (This
>>is also why you can't use a transistor shunt stabilizer with a dropper
>>resistor from a B+ supply of more than 1-1.5kV.)
>
>I disagree.  It seems to me that there can never be more potential across
>a shunt regulator than the regulated output voltage.

Don't forget the transient conditions - like, how do you switch it on?
Before the feedback loop stabilizes, the shunt device is drawing zero
cirrent and is exposed to the full unregulated voltage (almost as if the
dropper resistor wasn't there), and this may well destroy the
transistor.

Active (feedback) regulators are subject to a variety of other transient
conditions, so it seems to me like a bad risk to use devices that
wouldn't withstand the full voltage that could be applied; but once
again that is a matter of design style.

>>
>>For higher input voltages I'd be inclined to go with Rich's series
>>stabilizer circuit. Rich does point out that it demands a shunt resistor
>>capable of accepting the full worst-case negative screen current without
>>driving the series stabilizer into reverse voltage, to avoid the very
>>real problem that John describes.
>
>Power FETs can not be driven into reverse-voltage because they have an
>internal diode between the Drain and the Source.

I didn't say it was "the power FET" that could be driven into reverse
voltage, but "the series stabilizer" as a whole. The point is that if
this happens, the stabilizer has no more effect and the negative screen
current is free to drive the screen voltage upwards. (The output
impedance of the screen supply was maybe a few tens of ohms when
actively stabilized, but then jumps to something like a few k-ohms,
determined mostly by the values of the dropper and shunt resistors.) 

The rising screen voltage increases the cathode current and also the
secondary screen emission, which drives the screen voltage up even
faster until ... !!

Even if the transient is very short and doesn't go into that runaway
situation, the spike of screen voltage will have a pretty dramatic
effect on IMD. 


>>
>>If voltage is not a problem, I generally prefer the shunt circuit
>>because the IC and associated components are usually close to chassis
>>potential, and easier to work on than the series circuit where they're
>>floating above screen potential.
>
>In large tetrode Class AB1 amplifiers, the screen is usually grounded.
>This means that the (pos.) Drain of the FET pass-regulator is also
>grounded.
>
That's true, and a point in favour of the series regulator in that case.

>>But mostly it's a matter of design
>>style and preferences. Properly designed, either circuit can work well.
>>
>A 1500V, 150mA shunt regulator requires a >>1500v transistor/s that
>dissipates 225w during standby.   A 1500v, 150mA series-pass regulator
>requires a 500v FET that dissipates perhaps 10w during standby.  

I agree about the voltage ratings, but you're only considering the
positive current. If you're rating the screen supply to either deliver
+150mA or sink say -100mA, the series regulator needs a shunt resistor
of 15k from screen to cathode, and is always passing 100mA more than you
assumed. The best solution depends on the tube in question, and how
likely it is to go into negative screen current.

>///  It
>seems to me that under c. 800 screen volts is the dividing line between
>shunt regulated and series pass regulated screen
> supplies.

Yes, I'd say that's about the limit for a shunt stabilizer using cheap
readily available power FETs rated at say 1000V.

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

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