IRF530 max input frequency

[Umit45]

Hi everyone,

I am using IRF530 in a half h bridge DC motor driver. The MOSFET is behaving erratically if the PWM frequency is increased above 20 KHz. I checked the datasheet thoroughly but could not find any thing related to maximum operating frequency. Is input capacitance related in any way to the max input frequency(It has input capacitance of 670pF).
View attachment IRF530.pdf

Thanks,

 

[dick_freebird]

The input capacitance of the FET will affect how well
or fast your FET driver can smack it around. 670pF is
not large, as power MOSFETs go. But what are you
driving it with? How good is that device's decoupling?
You'd want a minimum of 10X the driven load I would
say (like 10nF) and more wouldn't hurt.

Which FET is having the problem? High side, low side,
both? What does "erratic" mean, specifically?

Putting a 'scope to the FET gate drive pins might be
informative.

If you are using a bootstrap style half bridge driver,
you may need to parallel a better bootstrap diode than
is provided on-chip to ensure that the fly-cap gets its
charge fully restored on every cycle. And the fly-cap
should be sized, again, appropriately to the driven
gate load and the max high time period expected (see
the app notes for your driver, for that).

 

[Umit45]

Thanks dick_freebird for the reply!

I am driving it with IR2104 IC. could you tell me more about the device coupling, as I could not find much on it in the datasheet.
I am doing PI control of a PMDC motor. When I give a PWM frequency above 25KHz, it losses the PI control and starts running at a constant speed much lower than the reference velocity. Changing the Vds of MOSFET or reference velocity has no effect on the speed of the motor.
I think both the sides are creating the problem. As the actual velocity is much lower than the reference, the PI is giving 99% duty cycle. The constant 99% duty cycle is arriving at the input of the MOSFET but at the source, the pwm duty is varying a lot from 50 to 95%.
I am also using 1N5822 as the diode connected across the MOSFET( is this bootstrapping?). Its datasheet also did not mention anything about its max operating frequency.

 

[Truedeal]

Do you have a resistor from gate to ground? It could be latching in the on condition.

 

[Umit45]

could you elaborate on its effects, I have 22K ohm on each side.

 

[FvM]

99% duty cycle at 25 kHz correspends to 400 ns on-time for the low-side driver, which means that the low-side most likely doesn't turn on at all. Bootstrap driver operation will fail then, which might well explain the observed "erratic" behaviour.

You need to limit the duty cycle to a considerable lower value, e.g. 95 %.

IRF2104 has no bootstrap undervoltage lockout, a brownout of bootstrap voltage can easily kill output transistors.