[SOLVED] [moved] 3.3v High Side P-channel mosfet switch

[newtonsrl]

Hi everybody.
I need to switch on and off the 3.3V power supply of one chip on my board using a pin output from a pic32 powered by 3.3V. The current needed is around 300mA. I'm using a 1.8 P-Channel mosfet FDN306P (here datasheet) and a PNP transistor connected like in the scheme below.
My problem is I cannot switch off the mosfet. When I have 0V on the base the transistor, the mosfet stay on, but when I have 3.27V on the base, the mosfet is not open, but I have 2.2V on +3.3V_GPU.
Could someone help me please. Thank you.

 

[Audioguru]

You are not using a PNP transistor, it is an NPN.
Your Mosfet is shown connected upside down. Its diode is conducting all the time because you have its drain at the positive supply. A P-channel Mosfet has the positive supply on its source and the load to ground on its drain.

 

[newtonsrl]

Thank you very much Audioguru,
You're right, I made a big mistake. Do you think I can replace the components with different type and get my circuit working, without redo my pcb? May be I can replace the p-channel with a n-channel. I'm confused and very tired... Could you be so kind to help me a bit?

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Can I replace the mosfet and the pnp with a Sziklai Pair / Compound or Complementary Pair? I've to drive 300mA at 3.3V. I did some tests with mmbt2904 and bc817 but the transistor overheat. May be the transistor I have used are not appropriated. Any advice from someone?
Thanks.

 

[Audioguru]

An ordinary bipolar transistor needs a base current of 1/10th the collector current for it to saturate well and not get hot.

 

[newtonsrl]

I will try it tomorrow. Thanks alot for your suggestion.

 

[KlausST]

Hi,

There are dedicated power switches. Small, cheap, easy to use.

They offer some benefits:
* relatively slow voltage rise, to give a capacitor time to charge
* current limiting
* logic on/off
* no current flow back
* single part solution
..

One name is AP2141. There are many others.

Klaus

 

[newtonsrl]

Hi,

One name is AP2141. There are many others.

Klaus

I know, but I would like avoid to make again the pcb.

 

[Pjdd]

You have the MOSFET configured the wrong way. If you can swap the drain and the source on the PCB, it should be OK without changing anything else.

For example, remove the MOSFET and rotate it anti-clockwise so that the gate is on the same pad as before, but the source pin is now where the drain was. Bend the drain pin upward away from the PCB, and use a small wire to connect it to where the source used to be. It will need some dexterity but it should be doable.

 

[newtonsrl]

And if I replace the mosfet with this pnp? It has very Low vcesat. I can do a compound pair. What do you think?

 

[FvM]

And if I replace the mosfet with this pnp?

Possible but not reasonable. Vce will be always higher than FDN306P on state voltage with 300 mA. Just fix the circuit error as suggested by Pjdd.

 

[newtonsrl]

Thanks so much Pjdd!! It works!!
Just a question: i see when the mosfet cut off the voltage need a long time to go to 0V. May be a need a resistor to help the mosfet to discharge?

 

[FvM]

I don't believe that the problem is discharging the MOSFET. There are most likely some bypass capacitors connected to the switched power supply node? A load resistor can help to discharge it faster if required for some reason, worst case a NMOSFET discharge transistor can be connected.

 

[Audioguru]

C7 slows down the turn off of the Mosfet. The very high value of R55 also slows down the discharge of C7 and the gate-source capacitance of the Mosfet.

 

[newtonsrl]

I removed the resistor and the capacitor, but same behaviour

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The timing is not crucial, and yes, there are capacitors on the line.
I removed the transistor so I can control the p-channel mosfet directly from microP and it works properly.
Thanks to everybody.

 

[Audioguru]

Why don't you understand? The 100k resistor value of R55 is much too high. Try 10k then 1k. This resistor discharges the gate-source capacitance of the Mosfet.

 

[newtonsrl]

I removed the transistor, the capacitor and the resistor, and I can control the mosfet directly from the microP. I think now it works correctly.

It is the first time I write on a forum asking for something, and really I did not expect all this collaboration. I really appreciate your help. Thank you all, and I hope to be of help to someone in some way.

 

[Pjdd]

I removed the transistor, the capacitor and the resistor, and I can control the mosfet directly from the microP. I think now it works correctly.

It is the first time I write on a forum asking for something, and really I did not expect all this collaboration. I really appreciate your help. Thank you all, and I hope to be of help to someone in some way.

Glad to hear that we could help. When you bypassed the BJT (BC817) and controlled the MOSFET directly with the uP, did you notice that it now works in the opposite direction? That is, when the uP output is high, the MOSFET now turns off whereas it did just the opposite before.

 

[newtonsrl]

Yes, but it is not a problem. I have inverted the logics in software.