[Milsurplus] FET Regulator: How It Do Dat?

[Bruce Gentry]

It will work fairly well. At startup, the gate voltage will be limited 
to no more than 18 volts above the source, but that is far more than 
enough to turn the MOSFET on. The output will then rise until the source 
has reached the voltage (usually a few volts below the gate voltage) 
needed to bias off the MOSFET. The actual output voltage will probably 
be a few volts below the voltage on the gate because MOSFETs like this 
usually need about 3-5 volts of forward bias between the gate and source 
before they will begin to turn on. The voltage will drift slowly about 2 
volts depending on the temperature of the MOSFET. Take care in choosing 
the MOSFET, power dissipation in this circuit is a far more important 
consideration than current rating because all the power difference 
between the unregulated input and regulated output is dissipated as 
heat. You may have to use a 20 amp MOSFET to pass a regulated current of 
half an amp or less in order to have a device big enough to dissipate 
the power.

          Bruce Gentry, KA2IVY

On 4/3/17 5:41 PM, David Stinson wrote:
> Dear Smart People:
> Here's a FET High-voltage regulator that works:
>
> https://goo.gl/photos/jbVaywJcWGTNWkbq9
> There's a bunch of "better ways" and improvements
> etc., but please- just want to address this circuit.
>
> I understand how the zener stack must be in conduction and you pick a 
> point in the stack for the reference voltage on the gate.
> If the input Voltage ever falls below the value
> needed to keep the zeners conducting,
> the FET shuts off.
>
> What I don't understand is exactly how the FET matches the Gate 
> Voltage and the Source Voltage, thus regulating.  It seems to me that 
> the 200V clamped down to 18V at the Gate should turn the thing full on 
> and dump the input voltage
> to the output with no regulation.
>
> Excuse my dummy-ness; how does the
> FET regulate?
>
> Thanks.
>
>
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