I designed 300V-800V to 24V,4A flyback converter using L6566BH. With RCD snubber of 470kΩ,2.2nF values, the converter is fine except switch stress of about 1500V at full load. To reduce it when I changed snubber to 235kΩ,4.4nF, IC got failed.

[HSHAR.1]

Is there any probable reason and solution for it?