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Author Topic: how to measure mosfet Rds on resistance .....?  (Read 8270 times)
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download_crux
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« on: February 10, 2010, 04:57:14 04:57 »

hi

how to measure mosfet Rds on resistance .....? ( mosfet on state drain to source resistance)

regards
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carbontracks
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« Reply #1 on: February 10, 2010, 05:01:25 05:01 »

Ideally you'd want a curve tracer to fully characterize the transistors' DC characteristics.  Otherwise, simply measuring the drain-source junction with a ohm meter while applying a gate-source voltage may yield results.  Or you could apply your own test voltage to the drain and measure current and calculate it yourself.  But keep in mind that Rds is only a crude characterization of a mosfet's behavior, which applies to small signal levels.  If you're planning on using the device for high current, then it may not be a useful metric.
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solutions
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« Reply #2 on: February 10, 2010, 07:39:51 07:39 »

You need a way to inject constant and measured current into the drain/source and measure the voltage across the drain/source.  Then it's Ohms law V/I. 

You also will want to control the gate/source voltage and measure it, as Rds is a function of this voltage as well.

To further complicate it, the Rds will vary with silicon temperature - you could calibrate the internal diode to measure the silicon temp. 

If I haven't scared you off already, then you have to realize that there's an (approximately) Gaussian distribution of Rds that depends on device processing as well, so what you measure on one device will not be that on others, so you may be wasting your time unless you measure DOZENS of devices from MULTIPLE lot codes.....good luck with that one.

The easiest thing to do, unless you are a really small fish hobbyist, is to beg your selected manufacturer for a characterization report (sometimes these are online), likely provided under NDA, so you can see all of the info without touching a single device.  Or you can get really lazy, like I am, and use the datasheet numbers, which are guaranteed by characterization or design when shown as max/min values (not "typical").  If the devices fall out of the datasheet range, you actually have someone to yell at, no matter how big a fish you are.
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download_crux
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« Reply #3 on: February 11, 2010, 05:57:55 05:57 »

ok

any test circuit please...............?

Regards
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MAXPAYNE
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It's a little funny......


« Reply #4 on: February 11, 2010, 11:35:52 11:35 »

best will be to refer the datasheet............
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Whats the Craziest Project u have done lately...?
carbontracks
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« Reply #5 on: February 11, 2010, 12:24:09 12:24 »

ok

any test circuit please...............?

Regards
do you have a function generator and oscilloscope?  If so then you can make more functional circuits (you could effectively make your own curve tracer).
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