Negative voltage spike on Drain to Source of my flyback mosfet.

The yellow waveform is the drain to source voltage waveform for my flyback mosfet. The blue one is the magnetizing current. My Vin is 36V, and the transformer is 1:6, with magnetizing inductance of 170uH and leakage inductance of 1.5uH. The flyback primary is connected between J9 & J11 and secondary of flyback is connected between J10 & J12. When I increase my Vin beyond 23 Volts, a negative spike is seen across my MOSFET which increases very sharply with further increase in input voltage. I have used an RCD clamp with Rc=733ohm and Cclamp=.1uF. The switching frequency is 50kHz. I have not used a snubber across output diode. A 284 ohm resistor as load is connected across the output capacitor C1.