My CMOS always destroyed..

[mahmoud shendy]

Dear all, nowadays I develop a design that controls speed of 220Vac universal motor, the control basically depends on rectifying the mains voltage using a full wave bridge, then applying the rectified voltage to the motor and switch it using IRF740 "n-channel CMOS" in a PWM fashion to control the speed, despite of connecting a Free Wheeling Diode across the MOTOR, the CMOS get destroyed many times!! How to overcome this problem?
Also, how to smooth current of the motor as I use 7KHz chopping frequency and the mains of 50Hz..

Thanx a lot ...SHENDY

 

[Hero999]

The device you're using isn't CMOS, it's a single N-channel MOSFET. A CMOS device contains both N-channel and P-channel MOSFETs that operate alternately.

What are you using to drive the MOSFET?

What current does the motor draw?

Please post a schematic.

I think the motor might pull a large current when starting up which is destroying the MOSFET.

There also might be a cheaper/simpler solution to your problem. Have you considered putting a simple dimmer switch before the bridge rectifier?

 

[eblc1388]

I would say your IRF740, which has absolute max. voltage rating of only 400V is too low for your purpose.

I would use a 800V rated device or at least a 600V rated Mosfet.

 

[mahmoud shendy]

the schematic in the attached file, what is its problem?

 

[mahmoud shendy]

This is the attched schematic!!!!

 

[Papabravo]

My guess is that with only 5V to the gate you are unable to turn the IRF740 on very hard. That you have a push-pull driver is a good thing but it should be working from 12-18V or so to adequately turn the device on.

Vgs(th) is listed as 2V Min and 4V Max with no typical. This means that at let's say 3V the MOSFET is just beginning to turn on. The resistance is still quite high and the IRF740 is no doubt dissipating considerable power. You have to admit that 300 Volts across a couple of ohms is an eye popping amount of power.

 

[Nigel Goodwin]

It also looks a very poor way to try and control an AC motor, you don't have a smooth DC supply for it, so your PWM is going to be useless. The way to do this is to use a triac or thyristor and phase control - as it's always been done.