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MOSFET Vgs

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clw

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What are the advantages and disadvantages to driving a power MOSFET with a Vgs significantly higher than the threshold? Most 600V ~40A MOSFET I'm looking at have graphs with Vgs up to about 10V. The optocoupler gate driver I'm thinking of using indicates 24V as the high-side (half bridge motor drive) gate voltage in several app notes. The max Vgs is spec'd at +/- 30V in the MOSFET datasheets.
 

tcmtech

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At some level you will just blow right through the Mosfets gate insulation and destroy it. Most have do have built in zener diodes to help bypass static discharges and small over shoots on the gate itself.
However these are very small and quite limited in how much power they can handle.

Once your over the minimum required voltage their are really no great gains to be found in most applications. If the Mosfet has a 5 volt rating with a 20 volt peak I would design for no more than a 10 - 15 maximum input voltage.

If you have more than that just trim it off with a resistor and zener diode on the gate drive output.
 

Sceadwian

Banned
It is important to keep in mind that the threshold voltage is NOT the minimum required voltage, the minimum gate voltage you should be driving your FET at needs to be determined from the VDS and the DS currents based on the datasheet for your specific fet it is very application dependant. Increasing the gate voltage (keeping it in the fets safe area of course) will lower RDS/on but if the gate capactiance is large it will also take longer to turn off.
 

mneary

New Member
The threshold is the voltage at which I describe as the device begins to leak.

As Sceadwain says, you need to bias it into conduction which is very different. For "ordinary" MOSFETs, this is Vgs of about 10V. For "logic level" MOSFETs this would be lower, in the neighborhood of 4 or 5V.
 

Sceadwian

Banned
4 or 5v for low currents for higher currents you still need the higher voltages it can only be tweaked so much.
 
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