- Published: Saturday, 04 April 2009
- Written by Digital DIY
- Hits: 82570
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.
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 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.