Amps
[Top] [All Lists]

[AMPS] Screen regulator circuit needed

To: <amps@contesting.com>
Subject: [AMPS] Screen regulator circuit needed
From: measures@vc.net (Rich Measures)
Date: Mon, 16 Nov 1998 00:17:37 -0800
>
>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  


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm


<Prev in Thread] Current Thread [Next in Thread>