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P-Channel MOSFET always on

aankur

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Hello

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.

Thanks
MOSFET.png
 

KlausST

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Hi,

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

Klaus
 

danadakk

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

1652696377266.png


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.
 

barry

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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.
 

dick_freebird

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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.
 

d123

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Hi,

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.
 

koreansud88

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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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