Mosfet doesn't switch off

Thread Starter

piratefishfinges

Joined Nov 7, 2012
2
Hi,
I have a STP12PF06. (P channel)
Drain is connected to 5.25v,
load (~0.02A) is connected source to 0v

Datasheet claims gate threshold voltage 2-4v.

I was under the impression connecting the gate to 0v would give my load the full voltage while connecting gate to 5.25 supply would give ~0v at the source.

Connecting Gate to the 5.25v supply actually gives a voltage at the source pin of 4.87v.

I havent used mosfets before but i thought this would work. Am i misinterpreting something here?
Thanks.
 

m1ch43l

Joined Aug 16, 2012
63
Your link to the datasheet is broken (complains about some gateway problem...)
Anyway, don't know much about P-MOS. but seems like in your scenario, you are not switching the device with some pwm.
Why are you connecting the gate to 0v? It shouldn't turn off in the first place! PMOS are by default ON. So, feed the gate with a negative voltage to create a 'constrict' across the channel. That way, it will function as desired.
What you need to do is get some switching mechanism; an ordinary pwm will do; only that, you invert the output of the pwm to give a negative signal which you feed to the gate of your P-MOS.
That's my take on this...
cheers.
 
Last edited:

JMac3108

Joined Aug 16, 2010
348
Pirate,

Post your schematic and we can solve your problem.

If you're using the P-channel mosfet as a switch (which is what you describe), the the usual connection is as follows...

Connect source to the voltage you want to switch (5.25V in your case)
Connect the drain to your load.

Driving the gate high (all the way to 5.25V) perhaps with nothing more than a pull-up resistor, will turn off the mosfet.

Pulling the gate to ground will turn on the mosfet.

See the attached quick sketch of a typical circuit (ignore the mosfet part numbers - I just grabbed anything to show the circuit).
 

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Papabravo

Joined Feb 24, 2006
14,701
Aside from having it hooked up backwards, what is important is NOT the voltage on the gate with respect to GND -- oh no Mr. Bill! It is the voltage from the GATE to the SOURCE. For a PMOS FET to turn on, this must be less than the voltage at the SOURCE. For example if the source is at GND the gate must go to -5V to turn the part on.
 

Ron H

Joined Apr 14, 2005
7,014
Aside from having it hooked up backwards, what is important is NOT the voltage on the gate with respect to GND -- oh no Mr. Bill! It is the voltage from the GATE to the SOURCE. For a PMOS FET to turn on, this must be less than the voltage at the SOURCE. For example if the source is at GND the gate must go to -5V to turn the part on.
And the drain needs to be more negative than the source.
 

Audioguru

Joined Dec 20, 2007
11,249
Yes, the Mosfet was connected upside-down.
The Mosfet is not a "logic-level" one so it barely turns on with only 5V from gate to source. It is designed to completely turn on when it has 10V from gate to source.
 

JMac3108

Joined Aug 16, 2010
348
Not true that the drain has to be more negative than the source for a p-channel mosfet to work as a switch. A common circuit is to connect the mosfet in reverse: drain-to-voltage and source-to-load. Then the intrinsic diode conducts and puts a voltage on the source side which allows the mosfet to turn on (depending of course on the gate voltage). You can make a reverse polarity protection circuit like this. See attached.
 

Attachments

Ron H

Joined Apr 14, 2005
7,014
Not true that the drain has to be more negative than the source for a p-channel mosfet to work as a switch. A common circuit is to connect the mosfet in reverse: drain-to-voltage and source-to-load. Then the intrinsic diode conducts and puts a voltage on the source side which allows the mosfet to turn on (depending of course on the gate voltage). You can make a reverse polarity protection circuit like this. See attached.
True. I just didn't want to get into that amount of detail with an obvious novice.
 

Thread Starter

piratefishfinges

Joined Nov 7, 2012
2
I did have it upside down, my assumption was that source connected to the lower potential as with an n type. The orientation of the diagram on the datasheet only fuelled my error. (+ is always at the top).
It was a bit of a faff changing the layout but it works great now.
Thanks all.
 
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