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I am working on a 15w led driver using der278 ckt. The only change is the transformer core made with EE20 instead of RM10. But the driver gives only 300ma instead of 500ma.Also current changes with led string voltage changes!(27-33v) What could be wrong?

The output LED current is a function of Vpin current and FB pin current. Assuming you did not change the value of Vpin resistor, one possible reason why you are only getting 300mA is because the FB pin current has been reduced. Please check if the bias voltage decreased when you re-design with RM10 core. If so, please adjust the feedback pin resistor in order to get the same feedback current as the original design. Alternatively, please adjust the bias winding. The goal is to ensure that you get the same FB pin current before and after the transformer change.

With respect to current changes with LED string voltage changes, this is normal because when the output voltage change, so is the bias voltage which directly affect FB current and thus the regulation will change slightly. But, the change should not be drastic. Please check the line regulation data on DER-278 as a reference (fig. 7 and 8).

Thanks for the reply. From application note AN49 I understand that the over voltage protection circuit is optional. But In my driver using der278, when I disconnect ovp ckt using Q1,R12,C12,R11,VR2,C14 and D7 output current is not regulated.Load current varies from 300ma to 400ma for an input variation of 90-230v.But with ovp ckt driver is giving 320-350ma constant.Please tell me what is wrong with this

The regulation should not be affected even if you disconnect the OVP circuit. Please monitor the bias voltage if it varies a lot as you sweep the input from 90V to 265V. If it does, then this would explain why you are getting huge variation on the output current. Please ensure that the bias winding has good magnetic coupling with the secondary winding and it's tightly wounded in order to minimize the leakage inductance which I suspect is the cause of the problem.

thanks. The measured bias dc voltage is 16.6-17.6v for an input variation of 90-240 ac input.Also primary leakage inductance is measured as 10uH.Please tell me whether the problem is with leakage inductance.

The first problem you have is that you are not able to get 500mA output current after changing the transformer to EE20. I advise that you verify first if the problem still exist even after using the same transformer design in DER-278 which is an RM10 core. This is to rule out that the problem is with the transformer design. I don't have info on your transformer design but please check if there is no sign of transformer saturation with the EE20 design you have.