I am facing this problem, which is causing the two push-pull MOSFET to extreme stresses.

I am using this to power TAS5630 amp. the boost converter is converting the 12V DC voltage to 36V DC (unregulated). As this is a car amplifier so the battery voltage can be 10V to 14.5V.

Everything is ok upto 13.5V DC input. But when the input voltage is higher than that, inductive spike is seen at the MOSFET drain and they becomes hot.
At 13.8V or higher the kick exceeds the max Vds 40V and the FETs are starting to runaway.

I have attached some screenshot and unsnubbed oscillation at MOSFET drain. Please help me to remove the inductive kick @15V.

 

Whats the circuit for the booster?

 

the schematic is inside the zip file. anyway I am again uploading...

 

what is the signal like on the gates when the voltage is altered?

You can put an ultrafast diode like MUR460 across the drains to batt positive to clamp it.

 

Are these scope photos with the power supply loaded or unloaded??

 

what is the signal like on the gates when the voltage is altered?

You can put an ultrafast diode across the drains to batt positive to clamp it.

Thanks Dodgy for your reply. Attached is the gate waveshape. I will try to add an ultra-fast diode to see if anything is improved. Thought the snubber will be sufficient to absorb everything (ringing ,spike etc.)

 

Are these scope photos with the power supply loaded or unloaded??

These are unloaded waveforms.

 

Look at the waveforms with the power supply loaded.

 

Probably this is caused by the primary leakage inductance. I am thinking about rewinding the transformer. Is there any tips or guide about the winding procedure so that leakage inductance is minimum ?