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using mosfet as current sense resistor

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Simon:
Right, daring thought: I have a power control project that requires up to 6 paralleled soic8 mosfets. I also need to measure current in this project. Well  :-[ as I'm using soic8 mosfets so have multiple pins for the same connections (drain and source) I thought could I use the voltage drop on the mosfet to measure current ? the mosfets will be fully turned on. What sort of variation in full on resistance do you get on mosfets over different temperatures ? are they stable ?

Differencies in manufacture would not be a huge issue as I can fine tune the software providing it falls in a range I know about

jahonen:

--- Quote from: Simon on March 03, 2011, 06:33:17 pm ---Right, daring thought: I have a power control project that requires up to 6 paralleled soic8 mosfets. I also need to measure current in this project. Well  :-[ as I'm using soic8 mosfets so have multiple pins for the same connections (drain and source) I thought could I use the voltage drop on the mosfet to measure current ? the mosfets will be fully turned on. What sort of variation in full on resistance do you get on mosfets over different temperatures ? are they stable ?

Differencies in manufacture would not be a huge issue as I can fine tune the software providing it falls in a range I know about

--- End quote ---

Temperature is one variable affecting Rds(on), at maximum junction temperature (150°C or so), expect twice the value in room temperature.

Regards,
Janne

Simon:
sounds like that's rules out then

Neilm:
I do seem to recall a range of MOSFETS that allowed you to do this. I believe that they had internal compensation. I seem to recall looking at them many years ago when I was working on a SMPS design but decided that it was not safe, or was the wrong value or something.

What current are you pulling that you require 6 in parrallel?

Neil

Simon:
well starting at 20 amps but may push to 30 amps. The mosfets I'm using have only 2.5W dissipation but are 6.5mR full on. I'd probably be fine with 4 but choose 6 for safety. I've already implemented a sense resistor but was considering the shortcut if possible, maybe not a good idea. I could run up my prototype and take some measurements thought

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