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Basic Electronics » p-channel mosfet not turning off.

May 21, 2010
by gnkarn
gnkarn's Avatar

Hi, Im using p channel mosfet IRF9640 ( big i know but is what have at hand), to power (multiplex) a led matrix driven by a TLC 9640 constant current led driver.

Im not able to turn it off properly, the gate is directly connected to the uc port pin, drain directly to +5 source to the common anodes, and cathodes connected to TLCs output pins.

the problem seams to be that when turning the gate off ( pin to +5v), its taking forever (500us) for the mosfet to switch off, How could I improve this?, can I do it with this Mosfet ?, by looking at the data sheet, it should work...

May 23, 2010
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi gnkarn,

When you mention the source/drain orientation, are you sure that you haven't reversed them? For a p-channel FET, the source should be kept at a higher voltage than the drain for proper operation. (This is the opposite of a n-channel FET.) So the source would connect to +5, and the drain would connect to the LED anodes.

Also, if you see very slow switching, it's useful to quickly verify that you've configured the microcontroller pin as an output. Otherwise, you may be only very weakly charging and discharging the MOSFET's gate.

Hope that helps!

Mike

May 23, 2010
by gnkarn
gnkarn's Avatar

Thank you Mike, it appears to be working ok now, my two multiplexed layers are mixed, but I think it is something else, I´m now looking in detail at the interrupt routine and verifying if there is any timing issue,

Gustavo

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