we follow der286 for our 36w driver and it is working fine. But in the OVP section, 39V derived from bias section for turning on VR3 (39V ZENER) during over voltage conditions is changing with unit to unit. We use PQ2625 TRANSFORMER and number of turns are same for (prim, sec,bias) all transformers. Out of 10pcs tested, 4pcs having value 32v at the cathode of VR3 and other 6pcs give only 28v. Bias voltage fed back to BP and FB pins are same for all the 10pcs(22v). Because of this variation of OVP sensing voltage, we need to change the value of ZENER DIODE unit to unit. Is it the problem with CORES of the transformer? When we change the core pairs , value of OVp sense voltage change accordingly.
Also please note that 39v zener conducts once the OVP sense voltage reaches 35v. Why this early conduction?
Please guide us to get a solution to make the OVP sense voltage consistent unit to unit.