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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: MOSFET "Normalized On-Resistance" question
Date: Tue, 17 Dec 2002 02:31:30 GMT
Organization: MindSpring Enterprises
X-Server-Date: 17 Dec 2002 02:31:26 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
>>> There's also an increase in Rds(on) associated with increasing
>>> drain current, which like the tempco can also be substantial,
>>> well over 2x when near the FET's maximum pulsed current.
The IRL1404 I was checking out has a normalized Rds(on) of about 1.45
at 100-degrees C. at an Id of 160A and a Vgs of 10V (according to the
Disregarding the tempco, would the Rds(on) get any worse at a lower
Id, or can I assume that the 1.45 value is a worst case? Or are these
typically idealized numbers that are never really seen in actual use?
Regarding the tempco...
Is there anything else other than the "Normalized On-Resistance vs.
Junction Temperature" and "Max. Id vs. Case Temperature" graphs that I
can use to determine the tempco of a MOSFET? The linear derating
factor certainly affects what I can do, but it's not something I
typically associate with a device's tempco.
>>> John, when using FETs as linear loads these Rds properties don't
>>> matter, unless Rds(on) * Id grows to over the available voltage.
I am actually feeling pretty good that I thought about that. :-)
I was looking at some ABT linear MOSFETs that had what I thought were
VERY high Rds(on) ratings (around 0.1ohm) for TO-220 and SOT-227
devices until I realized that it doesn't matter as long as the rating
is low enough to allow the max current I need to flow (at the
available Vds) with all the assorted resistances wandering around the
I'm still trying hard to understand all of the graphs in these data
sheets. Some I don't need to worry about now (relating to short or
hi-frequency pulses), but it's been great learning more and more about
the stuff that helps me to better spec a MOSFET for my applications.
John "another virtual beer is on the way" Muchow
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