LNK420EG Load Current at Low input voltage

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Hi ,

I have designed 60W driver (output current 1.5A) using LNK420EG.

I take reference of RDR-290 and Application Note AN49.

I have designed it for universal input volate range : 85VAC to 265VAC. But i am facing weird issue.

At around 90VAC to 180VAC load current is increased upto 2A and after 180VAC to 240VAC it got down to 1.2A.

Please find attached desing of 60W driver.

Can you please suggest any solution?

I have executed following experiments:

1. R1 and R2 value changed as per reference (R1-4M, R2-1.4M)

2. C6 value changed to 10uF

3. I have tried 0.1uF cap between V and S pin but it didn't worked.


Looking forward for proper solution.


Thanks & Regards,

Notion Electronics

LNK420.pdf368.23 KB


Please disconnect first Q1 and R4 just to make sure that the FB is not influenced by other components. Then measure the bias voltage (across C5). The bias voltage should not change drastically over line if the transformer is designed correctly. You may have to reduce or increase R6  in case the leakage spike is peak-charging C5.

If the regulation improves just by removing Q1 and R4, then there's leakage coming from these circuits.

R1 and R2 can help with the regulation but I have not seen such a huge variation on IOUT. Please monitor the drain current as well to make sure that there is no pulse skipping if ever ifb exceeds around 210uA.