I am designing a push-pull power supply to generate isolated 24V.
On primary side, center tap is connected to 24VDC.
The switching is done with pair of NMOS, and those gates are driven by PWM on ADuC7023.

The design is same as Push Pull power supply on this website.
http://pe2bz.philpem.me.uk/Power/- Inverters/D-101-Converters-etc/smps.htm

Both 24V and IC power are generated from a same power supply.
I realized when I turn off the power supply, once in a while, one of the MOSFET gets burned.

I am thinking the problem is PWM stops too early and transformer does not have path to discharge itself.
What can I do to make sure that energy stored in transformer get discharged even when PWM suddenly turns off?

Thank you,
Toru

 

It is likely that the PWM is disrupted as the supply voltage falls. Have you set up the power supply monitor to stop the PWM signal with both outputs low (to switch off the MOSFETs) before the voltage falls too low?

 

Without your complete schematic it is impossible to say.

ak

 

It is likely that the PWM is disrupted as the supply voltage falls. Have you set up the power supply monitor to stop the PWM signal with both outputs low (to switch off the MOSFETs) before the voltage falls too low?

Thank you for your reply.
We don't have code to stop PWM on power down.
I will start debugging and see if that is the cause of the problem...

 

Hello,
We found the solution to the problem.. Sorry for the late reply.
Putting pull down resistor on gate of the MOSFETs solved the problem.

So we were following the design below, which was obtained from the link attached on question of this thread.

The problem was not on power down, it was on power up.
As AlbertHall mentioned, IC was pulling the PWM line to VDD line until IC (and PWM) turns on.
The pull down resistor we placed makes sure that MOSFETs stays in cutoff on power up.

Thank you for your help!
Toru