Thank you very much for the reply,
The circuit is driving a resistive load (resistor ladder), and without any load the spike is active across the high side MOSFETS and across the output terminals of the bridge.
I have attached the circuit and the signals so it will be easy to understand.
The circuit is running at 700V, I could not find a H bridge driver that works up to 700 so I designed a simple circuit (555 timer and logic invertors as you can see in the attached circuit).
Because the circuit is so simple I could not set a dead time so I know the results will be overlapped between the gates (as you can see), I did not care about the overlap as the circuit running on 50mA maximum and the MOSFET I used is a 1KV, 5A, 250mJ.
The circuit is running fine but I am just confused how come the MOSFET did not blow up from the high voltage spike (btw the current spike is 0.7A only).
I tried snubber circuit but it did not work very well for some reason, I will appreciate if you can lead me to any good method for snubber design, I did not try fast varistor, I thought the varistor is a slow device.
Have a look at the driver signal you will see about 10uS overlap, am I right if I said this is the cause of all the problems? But even though how did the MOSFET manage to survive this spike as i was running the circuit for a few months now?
I did simulate the circuit and I decided to use PIC24 to control the circuit to stop the overlap however after I used the CPU the spike disappeared across the MOSFET but the spike across the H bridge still active.
I am wondering If I am wasting my time using the CPU to kill the high voltage spikes ( 2 - 4KV or more) as the 555 timer and logic gate seems to work ok with all the voltage spikes for few months now?
Below is the link to the image of the circuit:
Below is the links to the images of the signals: