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Distortion of start-up output voltage waveform

Posted by: imada0406 on Thu, 10/14/2021

We designed the power supply circuit shown in Fig. 1 using TOP253EN.
As a result of the test, it was found that there was distortion in the output voltage waveform at startup.(Fig. 2)
What could be the cause?
Please let me know.

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Submitted by PI-MrScott on Sun, 10/17/2021

Hi,

Thank you for showing interest in Power Integrations' products.
TOPSwitch-HX products (including TOP253EN have a soft-start feature.
For 17ms after the CONTROL pin reaches 5.8V, the soft-start sweeps both the current limit and frequency from low to high.
This is illustrated in its datasheet, design guide, and design example reports under the RESOURCES tab of the following page.

https://www.power.com/products/topswitch/topswitch-hx

You may also use our design tool, PI Expert to aide in transformer design and component selection.

https://piexpertonline.power.com/

Submitted by imada0406 on Mon, 10/18/2021

Thank you for your answer.
I am relieved to know that this is caused by the soft start function.