John Fielding wrote:
>-----Original Message-----
> From: Rich Measures <measures@vc.net>
>
> ? Bipolar transistors have a secondary-breakdown problem which makes
> them problematic in HV pass regulator service. Power FETs are virtually
> immune to secondary-breakdown. . From my experiences, National
> Semiconductor's 723-based "positive floating regulator" circuit (using
> FET pass transistors) is good for regulating screen potentials. 2000v
> regulated output is practical with 600v-rated FET pass transistors.
> Since FETs operate with zero gate current, paralleling FETs for more
> current is easy. . On my Web site, there is a diagram and an article
> about this regulator. My version of National's "positive floating
> regulator" circuit is short-circuit proof and RF proof.
>Series pass stabilisers do not absorb the negative screen current that
>tetrodes
>typically generate. Using a series pass stabiliser causes the screen voltage
>to
>go up and down in sympathy with the negative and positive screen current,
>which
>can cause the tube to flash-over with spectacular results!
>
>The only safe screen stabiliser is a shunt stabiliser. This can be made using
>either mosfet's or bipolar's, the tv industry has some good (cheap) devices
>which can safely handle 1500V. Today the tube shunt stabiliser is a bit of an
>oddity, but they can still be made to work, but transistors are a better
>choice.
The problem with a high-voltage shunt stabilizer using either bipolars
or FETs is that it's hard to find devices with a high enough breakdown
voltage. Power FETs pretty much give up at 1000V, although bipolars can
manage up to about 1500V as John says.
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.)
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.
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. But mostly it's a matter of design
style and preferences. Properly designed, either circuit can work well.
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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