[Milsurplus] FET Regulator: How It Do Dat?

[Richard Solomon]

Looking at that circuit, you have
a voltage drop of 150 volts across
that FET. At 0.5 Amps that's 75
Watts that puppy must dissipate.

I suggest a "robust" cooling system.

73, Dick, W1KSZ

On Tue, Apr 4, 2017 at 12:24 PM, Bruce Gentry <ka2ivy at verizon.net> wrote:

> 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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