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From: "Fritz Schlunder"
References: <3E2476F6.email@example.com> <7z3V9.3139$%W2.firstname.lastname@example.org>
Subject: Re: Selecting a heatsink?
Date: Wed, 15 Jan 2003 06:16:20 -0700
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> Is there a reason you are not using a mechanical switch or relay?
> That could save you power if this is simply an on/off device.
Err... MOSFETs are cheap these days and you can get a lot of bank for your
buck with them. Relays on the other hand have limited switching cycles/life
expenctancy, subject to strong vibration upsetting performance, often cost
much more than a comparable MOSFET, are physically very large, make audible
noise when switching, have slow switching performance, aren't readily
available for very high current low voltage use, and consume non-neglible
A much better question in my mind is why doesn't the OP use a lower on
resistance MOSFET such that no heatsink is necessary?
A MOSFET such as the IRFZ48V
(http://www.irf.com/product-info/datasheets/data/irfz48v.pdf) or similar
MOSFET could very easily handle 6.5amps DC continuously with no heatsink or
other special thermal provisions at all (even with significantly high
ambient temps, say over 80 celcius). Digikey will sell you ten of these for
a mere $0.84 a piece. Surely the price of the 0.04 ohm MOSFET + heatsink
cost + extra assembly cost + extra board space + mounting hardware +
decreased product reliability (more components to go wrong, IE, assembly
worker named "Bubba" doesn't mount MOSFET on heatsink properly and thus is
ineffective and causes thermal destruction), + power savings, etc. all sums
up to a quantity far greater than $0.84 (or less in larger quantities) a
Something to keep in mind though regarding MOSFET on-resistances. They go
up when the junction temperature goes up. Some MOSFETs on resistances can
as much as triple or so at 175 deg. C junction temperatures. As such at
increased temperatures you can't blindly use the on resistance figure at 25
deg. C to calculate MOSFET losses at high temperatures. I've already taken
this phenomena into account when I make the claim that the IRFZ48V will
handle 6.5amps continuously at over 80C ambient temperatures.
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