PWM on mosfet gate

[drkidd22]

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?

 

[ronsimpson]

The Gate to Source (peak) voltage should not be more than 20V.

 

[Beau Schwabe]

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]

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]

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.