Top 264 OV trips depending on the load

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Good afternoon,

I'm using a TOP 264KG for a 40W 300- 950 VDC Flyback design. I've added a mosfet in series to the top switch in order to be able to withstand 1kV input voltage.
The top switch is behaving good in all the input voltage range. The problem started when I tried to set the OV at a desired voltage level.
I'm using 9 Mohm divider to the V pin, calculated value OV= 1008V.
At 2% load measured OV=940V
At 100% load measured value OV=1060 - 1080V.
I couldn't find any relation between the OV and the load of the flyback transformer.

A little help would be appreciated.

Thank you in advance,