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Subject: Re: A protection problem
Date: Wed, 8 Jan 2003 11:14:59 +0000 (UTC)
NNTP-Posting-Date: Wed, 8 Jan 2003 11:14:59 +0000 (UTC)
User-Agent: tin/1.5.14-20020926 ("Soil") (UNIX) (Linux/2.4.20 (i686))
Ken Smith wrote:
> I'm currently working on a voltage regulator with a major protection
> problem. The specs are:
> Input voltage 23 to 28V. This is easy enough.
> Output voltage DAC controled 0 to 25V (doesn't need to boost make 25V when
> input is too low). This isn't too hard.
> Output current limited at 100mA. This 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.
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?
Otherwise, a N channel depletion mode FET (do these exist?) on the drain
of the P channel one would let you turn things off from inside the 0-25V
supply, otherwise if you use an enhancement mode FET, you'd need a ~35V
low power supply.
You will need a zener diode and a resistor on the N-channel FETs gate, as
it won't really like -625V across it otherwise.
http://inquisitor.i.am/ | mailto:email@example.com | Ian Stirling.
If it can't be expressed in figures, it is not science, it is opinion.
-- Robert A Heinlein.
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