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P-Channel MOSFET always on

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Newbie level 5
Jul 23, 2014
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I am using a MOSFET driver circuit with SI2303 as p-channel mosfet and SI2302 as n-channel mosfet.

I have given control signal (of 5V) on the gate of n-channel mosfet to turn-on and off the p-channel mosfet.

However, the p-channel mosfet is not getting off and I am constantly finding voltage on Output (marked as Point-A).

Please let me know what is the problem in this circuit.



the problem is that
* we don´t know whether you are talking about a real circuit or a simulation
* you don´t tell about the load
* you don´t tell how much voltage you see at the output
* you just talk about 5V control signal when ON, but you don´t tell what´s the control signal when OFF
* we don´t know which resistors you used at the control signal
* and how you get your voltages

Thus I can just guess
* a defective part
* a measurement error
* wrong test conditions
* wrong expectations.

My recommendation:
* give complete informations
* focus on the "faulty situation"
* measure V_GS of each MOSFET
* measure V_DS of each MOSFET


If you are measuring with a DMM, no output load, you will read a voltage
because of this -


There is still some drain current. And this does not account for leakage in the NMOS MOSFET Idoff
current at its Vgs = 0. Which can cause a non zero Vgs in the PMOSFET.

Regards, Dana.

Aside from all the other issues pointed out here, 100K seems awfully large to me for the (presumable) gate pull-up. There might be enough leakage in your circuit to keep that MOSFET turned on.

10uA*100K means -1V Vgs, not zero Vgs, on the PMOS. You
can substitute whgatever leakage current you measure (or
simulate as the case may be) on the NMOS drain.


As a rule-of-thumb, put a 1k or up to 10k resistor ground at point A. As danadakk mentioned, it might well be the 'off-state' leakage current which causes a 'phantom' voltage, very typical. Datasheets will state leakage current and can presumably calculate Rleak better than just the 1k to 10k habitual guesstimate.

How did you connect the MOSFET? No offensive, e.g. were the drain and source terminal reversely connected, it is always on.

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