Author Topic: Needed a P-FET selection suggestion  (Read 511 times)

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Offline saniTopic starter

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Needed a P-FET selection suggestion
« on: June 06, 2023, 05:59:39 pm »
Hello Everyone,

I want to reproduce a circuit that I provided in the attachment.
But it looks like I chose the wrong P channel mosfet.

I use IRF9540NPBF (here is the datasheet: ) and it never turns off  :(

So, do I correctly understand that VgsTh from -2 to -4 means that 5V(gates) - 8.4V(source) = -3.4V should turn off the mosfet, so there won't be a connection between batteries and the DC-DC?
Why this does not happen?
What am I doing wrong?

Could anyone correct me if I am wrong?
Also, if someone may suggest a good mosfet for me or tell me how to find a mosfet model that could solve my goal, it would be great.

Offline Terry Bites

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Re: Needed a P-FET selection suggestion
« Reply #1 on: June 07, 2023, 05:11:51 pm »
You've reversed drain and source, all current is flowing unimpeded through the body diode.
Vgs(th) is the voltage at which the mosfet turns on to some but not its full extents,


Offline saniTopic starter

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Re: Needed a P-FET selection suggestion
« Reply #2 on: June 08, 2023, 07:37:46 am »
Thank you for your response.

I am not sure is it correctly drawn on the circuit. But I use the image from the attachment to define the mosfet pin meaning and connected the Source pin(3rd one) to the battery BMS (8.4V), the Draine(2nd one) to the DC-DC and The Gate pin(1st one) to the 5v source. Additionally, I use 10k resistor for connecting the Gate pin to GND.

As a result, I always receive 8.4V on the DC-DC input no matter is 5v connected or not (

Is the connection right?

I am afraid that 5v is not enough to close the mosfet. Will the mosfet be closed if I use some step-up module for the gate pin, e.g. use 9v for the gate?

Also, I tried to fin another mosfet with the min Vgs(th) = -4v. I think maybe, in this case, the 5V on the gate will manage to close the mosfet.
But I haven't found anyone(

Offline Damianos

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Re: Needed a P-FET selection suggestion
« Reply #3 on: June 10, 2023, 05:14:40 am »
The numbering of the pins between the datasheet and the software that you use does not match! There are different packages and types of transistors with various pinouts, check it and correct the drawing.

The resistor should be connected between gate and source, so the transistor remains off without a control signal (Vgs=0).
The control signal (+5V in the schematic) must come from an open drain/collector circuit, for the Vgs to operate between zero and (-)VBATT. If it cannot be configured, use an additional transistor to implement it (this will invert the signal).

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