P mosfet

Oct 19, 2011 9 Replies
R

G'Day All,



I'm trying to use a P type mosfet (IRF 9540) as a PWM switch to charge a battery. I cant get it to turn on completely, as a test I have connected the + supply to the source and the drain to the + of the battery and the neg of the battery to the neg of the supply. If I take the gate to the neg it does not turn on completely, there is a volt drop of several volts across it at 2 amps. This happens with the 2 devices I've tried. They turn off OK when the gate is held high. What am I missing here ???



Cheers ....... Rheilly


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P

"Rheilly Phoull"

** What WE are missing is the battery type and voltage and the nature of the supply.

Devil, details .....

.... Phil

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R

Ya right of course, 12v battery and a regulated variable supply that I crank up to set the current flow. The real use will be with solar panels.

Cheers ......... Rheilly

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P

"Rheilly Phoull

** You have got a " stopper " resistor is series with the gate - right ?

Say 2.2 kohms - to stop HF oscillation.

... Phil

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R

Thanks Phil, I didn't but putting one in has not changed things. One other thing is that these are a batch from Hong Kong, I'm wondering if perhaps they are bodgy ??

Cheers .......... Rheilly

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P

"Rheilly Phoull"

** Looks a lot like it.

Rds(on) is about 5 times higher than the spec of 0.2 ohms.

... Phil

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R

Yup, beats me. I'll wander down town and get a known brand and see how that goes. Thanks for the help.

Cheers ......... Rheilly

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S

What's the measured Vgs when it is supposed to be turned on?

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R

Well I just grounded the gate so it was around 14v or 0 depending on how you view it. As it turns out I was ripped off once again by the fiendish oriental trader :-) I've just come back with a brand type one and all is well. (As Phil A and I were discussing)

Cheers ........ Rheilly

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A

There are a couple of Thai dealer who are very good BUT some of the honkers and Singa boys sell rubbish If you find a good one stick with him

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