Half bridge (class c chopper) problem!

[xaolong]

**broken link removed**

Help needed! Okay now i am working on controlling a DC motor (prime mover) connected to the 3 phase syn motor so as to control the frequency. Alright, currently i can control the dc motor speed without any problem like the speed of 3000rpm or 1500rpm with the class c chopper(pwm) but whenever i turn on the variable load connected to the syn motor like 0 to 400ohm, immediately the dc motor slow down and the mosfet at the low side shall break down as shown in the picture. I am not sure what is the cause of the problem. Is it the back emf but i did connect the resistor to dissapate the back emf? The dc motor is draw approximately 230V 2-4A and my mosfet(IXTN36N50) rating is 500V 36A and operating at 4khz (too slow?), dead time for pwm is 4us. Even without changing the pwm signal duty cycle leaving it constant like 92%, the dc motor is still able to run but whenever load is applied to the synchronous generator, the low side mosfet still fails and breakdown

 

[ronsimpson]

Your MOSFET is bigger than needed. It should be fine.
Keep C2 close to the MOSFETS. The back EMF should hit it first. Does the 350V get forced up above 500V?
IR2101 should be close to the MOSFETs. It will take 10 volts of "ground bounce" to kill the gate of the FET.
Do the FETs get hot?
Q2 is on only 8% of the time. Maybe less with dead time.
What size for C2?

 

[tcmtech]

Why are you using that type of PWM circuit on a DC motor?

When the lower Mosfet turns on it creates a dead short across the motors leads. Although the motor may only draw a few amps at 240 volts when dead shorted I suspect it can easily go well past your Mosfets design capacity.

I would reconfigure the design to only use the top one Mosfet and a simple fly back diode in place of the bottom one instead.

 

[ronv]

Like ron says it sounds like the supply is being "pumped up". C2 may need to be a few thousand Ufd. If you have a scope you could scope the supply voltage and see how it moves with a small load applied. Also think about a smaller higher frequency cap in parallel with the big one.

 

[crutschow]

I agree with tcmtech. The bottom transistor makes no sense for driving the DC motor with PWM. Just remove that transistor and connect a fly-back diode across the motor to snub the inductive current spike of the motor (anode to ground).

 

[xaolong]

Your MOSFET is bigger than needed. It should be fine.
Keep C2 close to the MOSFETS. The back EMF should hit it first. Does the 350V get forced up above 500V?
IR2101 should be close to the MOSFETs. It will take 10 volts of "ground bounce" to kill the gate of the FET.
Do the FETs get hot?
Q2 is on only 8% of the time. Maybe less with dead time.
What size for C2?

Thanks for all the advice posted by every one!
Didn't really notice if the DC link input hits 500V.Will take note next time May be is the voltage spike that killed it.
Yes the low side fet is quite warm.
The capacitor C2 is a high frequency capacitor each is rated 0.47uF 1000V and there are 6 in parallel. Can a Resistor replace c2 at the dc link able to dissapate off the emf?