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# Mosfet gate-source voltage

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#### chris414

##### New Member
I'm using mosfets for the first time and am having a bit of trouble. I need to drive a 24V motor with an H-bridge - i've got it all set up and it works when the logic circuit and motor use the same supply (eg. 10V), but when I connect the motor's power supply (24V) and leave the logic power supply at 12V, the motor only gets about 10V across it. Why is this? I've been trying to find an answer on the internet and somewhere it seemed to imply that the voltage applied to the gate had to be Vin + Vgs, ie. in my case that would mean I have to apply 34V to the gate. Does this sound right?

The FET gates need to be driven with-respect-to their SOURCES.

If using high-side PFets, their sources are tied to +24V.
To turn them off, the gate has to be at 24V.
To turn them on, the gate has to be at 0 to ~15V.

If using high-side NFets, their drains are tied to +24V.
To turn them off, the gate has to be 0V.
To turn them on, the gate has to be > 30V, because the source is pulled all the way up to 24V. This is why it is easier to use PFets for high-side switching.

Note that to properly turn on any FET being used as a low impedance switch (motor driver), then the gate to source voltage must be 2 to 5 times the Vt. If Vt is about 4V, then it takes 10V on the gate to get the advertised Ron.

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Gates can only handle ±20V with respect to the source though, so do not exceed this. 10V with respect to source is a "normal" turn on voltage. Logic level mosfets are designed to work at 5v gates, although 10V is still a bit better even for them.

If you are planning on doing PWM control you can use one of the I.C's on the market with boot strapped high side driver.

There is a level shifter to high side logic and driver. The high side Vcc cap provides the power for high side N-ch MOSFET driver. When low side MOSFET is low it charges the Vcc supply bootstrap cap. When low side MOSFET goes high the boot strap Vcc cap becomes boosted supply for high side driver.

Look up spec for MPIC2111. There are similar parts by other vendors.

You must always do some PWM cycling to recharge the high side bootstrap Vcc cap.

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• MPIC2111 MOSFET Drvr.pdf
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I just ran into my first mosfet that only had +/-15V max on the gate. Will have to make sure to check that from now on.

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