LNK-304GN Output Oscillates on Step Load Change

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I have an LNK-304GN design that has been shipping in a product for over 10 years without issue. It supplies 18V to a varying load that averages 55mA, but can see steps of about 3X that current. With no changes to the load design or the power supply, the output of the power supply has suddenly begun to oscillate at about 5kHz when a step change in the load occurs. It does not appear that any supplier's components in the circuit have changed. i am baffled as to how to troubleshoot such an issue. The schematic is attached, and as you can see it follows the design outlined in the chip spec and the App Note. And again, it has had no issue for nearly 10 years. Any suggestions as to where to start looking for the source? Thank you.

Power Supply Schematic372.2 KB

Does no one have any thoughts on this? Thanks.


Could you send me the waveforms of the oscillations? Have you tried probing the drain-source voltage? Thanks.


HI - I will attempt to get you a scope shot of the oscillation. It is at approximately 4-5kHz and decays after the start impulse. I have attached a shot of the +18V supply (pink line) when the event that triggers the oscillation (light blue line) occurs. I note that most sample designs have larger output caps. Would a low ESR 100uF cap likely help with this droop? I have also attached the inductor input (light blue trace relative to circuit common). I presume that this is the standard pulse skipping we are seeing? Thanks for your assistance.

18V Droop23.63 KB
Inductor voltage27.77 KB


May I know what you are probing for the light blue line? What is that signal? This is for the 18V Droop screenshot.

I can't say for now if it is in fact pulse-skipping. Please try probing the drain-source voltage of the MOSFET (use a differential probe) and the output voltage(short probe) simultaneously. I think we can get more information from these. If possible, you may want to check the drain-source current as well. Thanks.


Hi - thanks for your response. The light blue line in the 18V droop is the gate drive for an IGBT, hence it causes a load surge. I will get your requested traces shortly.