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From: Winfield Hill
Subject: Re: A protection problem
Date: 8 Jan 2003 05:13:54 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
Ian Stirling wrote...
> Ken Smith wrote:
>> I'm currently working on a voltage regulator with a major protection
>> problem. The specs are: Input 23 to 28V. Output voltage DAC
>> controled 0 to 25V. Output current limited at 100mA, isn't too hard.
>> Some moron may hook a (+) or (-) 600V supply to the output. This
>> 600V supply can supply a zillion amps so MOVs aren't the answer.
>> PTC devices don't seem to be able to handle this case either.
> Hmm. Let's say the pass element is a P channel MOSFET with a
> 800V 500ma rating in the positive rail. This should cope with a brief
> connection to a -600V supply, though it will explode into lots of
> little bits if the control circuitry doesn't shut it off rapidly.
> So, the remaining problem is +600V. Can you put up with a diode drop?
Ian, I think you've got a solution. There aren't any 800V p-channel
mosfets, but there are some hefty 500V parts that'll likely withstand
-600 volts, such as Fairchild's FQPF3P50 (otherwise, two series FETs
can be used). The required 100mA current limit can be of the foldback
variety, keeping power dissipation under say 25mA (15 watts) for 600V
A 1kV diode in series with the drain is needed for the +600V fault,
but this can be inside the feedback loop so its voltage drop isn't
a problem. A few other minor protective parts (e.g. two resistors
and a diode in the feedback path) complete the fail-safe circuit.
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