Tip - Logic Level FET's (P-Channel)

img311Note: This device is hard to find, please consider the alternate part number listed below.

For a more detailed guide on MOSFET's in general, also have a read of the N-Channel FET page.

This guide is designed to explain how to use P-Channel FET's, as there are a couple of tricks to remember when using them. P-Channel FET's are useful for switching the positive supply of a target circuit on and off. Particular attention must be placed to the target circuit if the supply voltage is greater then the micro controller's logic voltages.

If for example, your target device is being powered by 12 volts, and your logic high state from the micro controller is 5 volts, then the MOSFET will never turn off, as Vgs will either be -12V or -7V (remember that this guide is designed for logic level MOSFET's). Anything over -3 volts will generally drive the logic level MOSFET on (varies between types of LL MOSFETs).

The solution is easy - if you are driving a greater voltage then the micro controllers logic high voltage, then use a NPN (2N3904) transistor to control the Gate of the MOSFET, tying it to ground with a logic high to create a (in this case) -12 Vgs, and 0 Vgs when a logic low drives the transistor to turn off.

P-Channel MOSFET Example

The reverse biased diode in parallel with the motor should be used when ever you are driving inductive loads, but is not required with purely resistive loads.

If your target device is being driven from the same voltage levels as the logic high output of the micro controller, then no driving circuit is required (note that the MOSFET's control is inverted);

P-Channel MOSFET, no driving circuit

p channel fet

Here's the Logic Level MOSFET that I use;

  • P-Channel Logic Level MOSFET
  • NDP6030PL *** Note, this part number is hard to find nowdays, please consider the NDP6020P, its specs:
    • -24 A, -20 V.
    • RDS(ON) = 0.05 W @ VGS= -4.5 V.
    • RDS(ON) = 0.07W @ VGS= -2.7 V.
    • RDS(ON) = 0.075 W @ VGS= -2.5 V.

Finally, keep in mind that FET's are very sensitive to static, so handle with care. I am yet to damage one while "hobby-handling" though.


Posted: 8 years 6 months ago by wilson16 #6413
wilson16's Avatar
to me, its very informative!
Posted: 8 years 3 months ago by jasplc #7509
jasplc's Avatar
Hi Graham,
I went looking to buy some of these parts you use for a P channel Fet application, but I can't find them anywhere. I tried element14, RS and mouser.

Do you have a replacement part that behaves the same way as what you describe in this post?

Thanks.,
Posted: 8 years 3 months ago by Graham Mitchell #7510
Graham Mitchell's Avatar
Hi Jasplc. Thank you for the heads up - I've found a variant that should do the job fine. Details are in the article
Posted: 6 years 11 months ago by dk-info #11751
dk-info's Avatar
I would like to use this circuit to fire a small pyro charge to eject a parachute on a high power rocket project. I would use an i/o pin from a PIC uC to fire the charge. I am looking at the IRF7424, I like the SO8 package and Voltage/Current specs.
What I am worried about is the moment I power up the PIC, I am not guaranteed what the state of the I/O pin will be. I don't want the ejection charge to go off in my face when I power up the PIC on the launch pad.

If I have a weak pull down on the I/O pin (say 47k) to digital ground, is that going to be problematic with the gate resistor of the '3904?
Posted: 6 years 7 months ago by lespic #12875
lespic's Avatar
Thanks for the article, not used FET's before , I wanted a soft ON button and a PIC controlled shutdown . on my modified circuit the button turns on P-MOSFET by grounding the gate, this in turn supplies the V-in to a chipKIT UNO and PIC16F1939. On power up
and after a few u-sec a port then turns on the transistor, the button now cannot turn off the supply, this allows the UNO to save data to an I2C EEPROM and control the power down when required.

Forum Activity

  • No posts to display.

Member Access