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Large Power Ripple at no load conditions

Posted by: mvdeventer on Fri, 08/27/2021

Using PI Expert I designed a SMPS supply with the TOPSWITCH TOP270KG.

Output Voltage: 36V

Output Current 1.5A

Everything is working well and the device performs very good except for the fact that when no or very little load is supplied the output voltage ripple exceeds 400mV at more or less 50Hz. Under load conditions the output ripple is from 20mV to 80mv from 0.1A to 1.5A. The reason for choosing the TOP270KG is because we have very little area for heatsinking. Only D8 and D9 ES3D output diode's is placed and for the snubber circuit only R6, R20, R25 and C4 is placed.

Please see attached schematic diagram for the design.




Attachment Size
Swich_PSU_0.pdf 491.55 KB
DH_PCB.pdf 944.08 KB


Submitted by PI-Wrench on Mon, 08/30/2021

I can see your schematic now. It appears that you might be using compensation capacitors that are too large. The electrolytic capacitor at the control pin  provides compensation as well as stored energy for start-up. Try reducing this cap to the default value of 47uF. The compensation capacitor  across the TL431 from cathode to reference is also on the large side. A value of 100nF might be a better choice.