I am trying to switch stw12nk90z N channel MOSFET as high side switch at 20khz frequency with Vds=30V, Rload=18ohm, and Vgs=24V (Rgate=100ohm, Rpull_down=1kohm). I am using TLP250 for level shifting and obviously, An isolated power supply for the gate signal.
Unfortunately, the voltage between the drain and source pin (represented by the graph in blue in the picture) was not square like the input (represented by the graph in red in the picture), it had the straight rise-up and down but had a curve like a sin wave at the top and not stable. I have also added the snubber and anti-parallel diode with the gate resistor but it didn't resolve the issue. Moreover, the MOSFET is also getting heated rapidly even at the current of 500mA.
This is testing for the prototype circuit. The real application will have Vds=600V with Id=5A.
Kindly let me know what's wrong with the circuit or strategy. I am using the following circuit (note that TLP250 is 8 pin IC, I have used this schematic just for the sake of reference), just skip the transistors.