# Rds(on) graph in MOSFET data sheet

Discussion in 'General Electronics Chat' started by strantor, Sep 25, 2011.

1. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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I'm looking at the IRFP4110PbF datasheet and trying to figure out what my Rds(on) will be @ a junction temp of 120C. The "rated" typical value is 3.7mΩ and the "max" is 4.5mΩ, but when I look at the graph, 120C corresponds to about 1.75. That makes me think it is a multiplier, but if I multiply 3.7 * 1.75 I get 6.5mΩ. so what gives?

2. ### Pencil Active Member

Dec 8, 2009
271
38
I am just guessing, but my observation is that
the Rds On Typ and Max are given at Tj=25C,
and the graph passes through 1 at what looks to
be 25C. This supports your theory of the value
being a multiplier. The Rds ON of a MOSFET
increases with temperature, this is a characteristic
of MOSFETS that makes them ideal for paralleling to
share current, it helps to prevent thermal runaway
and "current hogging".

Just my observation.

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3. ### vrainom Member

Sep 8, 2011
109
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The typical and max value variation is from one device to another, measured at 25ºC. One device can have an Rds on of 3.7mΩ and the next one as high as 4.5mΩ at 25º but those values go up with temperature.

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4. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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Right, but go up to what; is what I'm trying to figure out.
If 4.5 is worst case then I assume, according to the graph, that figure would be multiplied by 1.75 for a Rds(on) of 7.87mΩ. do you concur?

Sep 8, 2011
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Oct 3, 2010
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