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TOP258YN thermal shutdown and output noise

Posted by: Abdullah on Tue, 04/27/2021

Hello there,

We designed 24V 6A SMPS on Pi expert. Design documents are attached. But when we load 5A at the output, it enters thermal protection within 10, 15 seconds and Voltage drops about 1V. What could be the reason for this? In addition, the unloaded D pin is in the 1st picture and the output voltage is on the 2nd picture. D pin in 1.6A output is in the 3rd picture and the output voltage is on the 4th picture. As you can see, there is too much noise at the output. I am waiting for your opinion on this topic. Input: 230VAC
Primary inductance of transformer: 314uH

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Submitted by PI-Wrench on Thu, 04/29/2021

Please resend design documents, as I do not see them here. In response to your thermal problem, are you using a heat sink on your TOPSwitch? At this power level, a substantial heat sink will be necessary. Also, pending review of your design files, it would be a good idea to check the TOPSwitch drain current with a current probe to make sure that the transformer is not saturating.

Regarding output ripple/noise, and pending seeing your schematic, I would suggest that a 2nd stage output filter would be necessary to reduce both switching frequency fundamental ripple as well as high frequency "grass" that shows up at inflection points of the output ripple waveform. If you are checking output ripple using a standard oscillosope probe with ground lead you will see excessive ripple/noise due to the pickup loop formed by the ground lead. It will be necessary to use a "noise probe" as shown in the attached file to get accurate ripple measurements.

Attachment Size
Ripple_meas_setup_0.GIF 85.81 KB