Same circuit, different PCBs (layout) = different output voltage - why?

Posted by: travelerdbg on 05/02/2012

I'm using an LNK500P with the same circuit on 2 different PCBs with different output voltage results. The second PCB yields an output voltage 2VDC higher than the first PCB. I can't fully compensate for the difference by adjusting the feedback resistor. The best I can do is get it 1.75V higher than where I want it.

Granted, the LNK500 layout is different between the two PCBs. If my feedback resistor won't fully compensate for the difference in output, is there some other component I can change besides the transformer? And/or what in the layout could account for this issue?

Thanks in advance,
David

Files

schematic_1.pdf (25.92 KB)

Comments

Would you like to swap the two transfoemrs first? if outputs of the two boards are same, then double check the resistor in the AC side of bias winding (R2 in datasheet Figure 5). If the transfomers and R2 are same, then the leakage caused by the PCB makes the difference. (Leakage inductance with long lead or bigger PCB loop for the bias winding is bigger than the leakage inductane with shorter lead or smaller PCB loop).

Thanks for your reply.

Unfortunately, I had to ship the first board with the proper output voltage to a client. I only had the 2 sample transformers, but am expecting more in a week or so. R2 is the same between the 2 boards. The transformers are of the same design and tested close to each other by the manufacturer. Assuming for a moment that the transformers behave the same, is there a way to compensate for PCB leakage differences?

Schematic an help me a lot!

I just uploaded it.

Looks like polarity of C3 and C4 are not correct. If the output is good after you correct the polarity, the issue is caused by the different leakage of the cap.