>
>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.
Indeed.
>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.)
I disagree. It seems to me that there can never be more potential across
a shunt regulator than the regulated output voltage.
>
>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.
>
>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.
>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. /// It
seems to me that under c. 800 screen volts is the dividing line between
shunt regulated and series pass regulated screen
supplies.
- later, Ian.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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