i have a problem using mosfet & pwm,
i want to control a dc motor with high current approxmitly 40 amper
so i parallel two IRFP250
in high duty cycle my circuit works fine but when i decrease the duty cycle my mosfet start hoting or burning.
what is my problem?
PWM Ferquency:1Khz
i used two bdx 53 & 54 to bias the gate(in complementry mode)VCC of this two transistor is 12

 

We are going to have to see the schematic before we can offer help.

 

Pwm wave inject to r1 and output of IRFP250 get to motor.

 

No one is here?

 

Your resistor is in the wrong place. You need a low value resistor directly at the gate of the MOSFET to prevent ringing at that point. The dual voltage follower (AKA common collector) BJTs don't have a problem with it.

It could easily be ringing that is causing your problems.

You want more help you need to give more information, such as the load of the MOSFET, as well as what is driving the bases of the BJTs. I don't see a swamping diode either.

Seems we've gone down this road, or am I misremembering?

 

Actually, the 100 Ohm resistor is OK where it is, but another resistor is needed between the transistors' emitters and the gate, as close to the gate as possible; somewhere around 10 to 15 Ohms.

One of the problems you are likely having is that the Rds(on) is quite low when the MOSFETs are turned ON, but when they turn OFF, current is still flowing in the motor's windings, and that current then flows through the body diodes of the MOSFETs.
Datasheet for the IRFP250: http://www.irf.com/product-info/datasheets/data/irfp250.pdf
Basics: Vdss=200V, Rds(on)=0.085 Ohms, Id=30A, Qg=140nC
I don't know what your operating voltage is, but you pay a penalty of a higher Rds(on) and higher gate charge (Qg) for the high Vdss rating.

The body diodes will drop somewhere between 0.6v and 2v depending on how much current is flowing through them; at 30A the max is 2v. 30A*2V=60 Watts, which is a LOT of power to dissipate. When you are running at 100% duty cycle with the two MOSFETs in parallel and the Rds(on) is 0.085, and assuming that each MOSFET gets 20 out of the 40 Amperes, you'll have a voltage drop of 1.7v, and power dissipation of 34 Watts which is still very significant.

I don't know what your voltage supply is. You should tell us.

How large are the heat sinks attached to the MOSFETs? They would need to be pretty large to get rid of all of that heat.