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# gate voltage on mosfets

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#### Thunderchild

##### New Member
I'm after a P channel mosfet that is going to be run by a pwm signal so I just need it full on or off. Now datasheets are stating a Vgs (sometimes specifying the drain resistance in these conditions), is this the minimum voltage require to bring the mosfet full on or is it the maximum supported ?

Datasheets for Mosfets have one page for the maximum ratings. The maximum allowed gate voltage for most Mosfets is 20V.

The Characteristics spec's have pages of listings:
1) The "threshold" gate voltage is from 2V to 4V for most Mosfets where the drain current is only 0.25mA.
2) The Vgs for most Mosfets to turn on fully is 10V and the max on-resistance is shown at 25 degrees C.
3) The Vgs for "logic-level" Mosfets to turn on fully is 4.5V.

well I have a mosfet datasheet stating -4.5 V for 0.2 ohm drain resistance which should be full on

P-CH mosfets are sometimes characterised showing negative voltages. So you take the gate negative with respect to the source to turn the FET on.

Normally PFETs are used with the source at the input voltage, your modulate the gate and take the output off the drain.

Another point to consider in your design, if your PWM signal is 5V, you will need to level shift the PWM signal to fully turn on and off your FET if your source is at greater than 5V.

Catches some people out...

I'm after a P channel mosfet that is going to be run by a pwm signal so I just need it full on or off. Now datasheets are stating a Vgs (sometimes specifying the drain resistance in these conditions), is this the minimum voltage require to bring the mosfet full on or is it the maximum supported ?
Many newer logic level FETs have on resistance numbers guaranteed for VGS of both 4.5V and 2.7V. They are designed to be driven from logic level sources.

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Are you using the Mosfet to drive a motor?

no birdman but similar, I am making a regulator circuit for a vintage car (more in another thread) so I will be PWM the field coil of the dynamo to control the armature output voltage. I'm using a P channel mosfet so yes its a negative VGS and yes i may well be using a pic so will use a transistor to negate and convert the 4-5 V output to 12 v from the battery

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