HELP NEEDED to calculate a snubber for the primary of smps

[Silvio]

Hi, can anybody help me calculate the values of a snubber needed for the primary winding of a half bridge smps?
At the moment I have 680pf 1kv with a series resistor of 150 ohms at 3watts but its getting rather hot!

Input voltage = 230 volts AC
Input dc voltage at transformer Primary is 155 Vdc
Output voltage is 45-0-45 volts dc
transformer used is EI 40
calculated output power is 350 - 400 watts
operating frequency is 75Khz
Mosfets used are IRF 740
Drive circuit with IR2153
duty cycle 50%

HELP IS GREATLY APPRECIATED.

 

[ir02x6]

R = 220 ohm
C = 470 nF 2 KV

 

[Silvio]

Thanks very much for your help. It is a bit late though as I figured it out by trial and error and found the best value to be 150 ohms and 220nf gave me the best results

regards Silvio