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PWM on mosfet gate

drkidd22

Member
The BSS123 has a max vgs of 20V. I'm driving the gate at 24V with a PWM wave at 22khz 50% duty cycle.

Circuit works as intended but is the gate seeing the average of the PWM? Or is the absolute max rating of 20V vgs on the fet a peak regardless of pulse width?
 

Beau Schwabe

Active Member
The reason for the 20V limit is due to the thickness of the gate oxide on the MOSFET transistor ... as a general rule, the dielectric strength of the oxide is slightly more than 3V per micron and the typical oxide thickness is about 6 microns. Exceeding the rated voltage will cause a spark to penetrate the oxide layer and once that happens the transistor becomes damaged, most often inoperable.
 

crutschow

Well-Known Member
Most Helpful Member
Circuit works as intended but is the gate seeing the average of the PWM?
It sees the average only if you have an RC low-pass filter averaging the PWM signal to get DC at the gate.
Otherwise the peak voltage may damage the gate oxide.
 

gophert

Well-Known Member
Most Helpful Member
But, if you do add an RC network to the PWM gate, your motor speed will not be a linear response to duty cycle and the motor will stop sooner as duty cycle is decreased.

You are much better off adding a voltage divider with two resistors or, better, use a 10v zener diode and a current limiting resistor.
 

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