OVP VOLTAGE NOT CONSISTENT

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Sir,

     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.

Thanks

SDM

This could be due to inconsistent transformer coupling based on how the voltage at VR3 differs from unit to unit. Please review the transformer and see how you can improve the coupling.

 

For the 39V zener conducting at 35V, please check the zener datasheet. Please contact the zener manufacturer if you doubt that the zener does not perform based on the published datasheet. 

 

Hello,

 

Kindly have a look at zener tolerance also. This also leads to wide range of OVP thresholds.

 

Rgds,

Kassin.