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From: "Phil Allison"
References: <email@example.com> <8npR9.16063$jM5.firstname.lastname@example.org>
Subject: Re: Low leakage parts
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Sat, 4 Jan 2003 13:10:56 +1100
NNTP-Posting-Date: Sat, 04 Jan 2003 13:00:59 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Winfield Hill" wrote in message
> Phil Allison wrote...
> > Tom Bruhns wrote
> >> Some of you may recall some postings I've made ... [ snip ]
> >> Sooo...I'd say that modern power mosfets also have pretty low gate
> >> leakage current. To hold the voltage above the nom. 3V required to
> >> turn the mosfet on, the average leakage must have been less than 50fA,
> >> assuming 21 days, 0.01uF and 9V delta (and no arithmetic errors).
> >> (It was cool, about 18C, and likely wouldn't do quite so well inside
> >> a car with the windows rolled up in Phoenix in the summer...)
> > ** 21 days is 1.8 exp 6 seconds, divided by 0.01 exp -6 gives
> > 1.8 exp 14 ohms as the resistance in the cap, fet and wiring etc.
> > I don't believe it - do you ??
> > I suspect there was a DC leakage path HOLDING the 0.01 uF cap voltage
> > up that was the real story here.
> Phil, having made similar measurements and calculations myself, I'd
> say that you could well be wrong.
** The issue is NOT the resistance inside the semiconductor or cap
di-electric (they are super high as you say ) but *stray* resistance and
therefore leakage on the surface of the plastic package, PBC material and
any coatings used for insulation of wires etc.
This resistance carries leakage current between any and all points in
a circuit. A super high impedance like a fet gate will float at the leakage
network's value at that point unless a deliberate resistance swamps the
network of all possible strays.
It is always good design to have a real resistor of at least one
order of magnitude LESS value than the stray resistances at the highest
operating temp and humidity expected.
I have seen the specific problem of a leaky PCB in poorly designed
audio gear ( both valve and SS) many times - it is a nightmare to fix.
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