Meet UCC28C44 flyback start-up problem

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Chuoi
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Meet UCC28C44 flyback start-up problem

Postby Chuoi » Thu Jan 12, 2017 1:10 am

I have a problem with a design of a flyback converter for a power supply. I am using UCC28C44 PWM IC(UCC28C44 datasheet http://www.kynix.com/uploadfiles/pdf65976/UCC28C44D.pdf). Converter should star up with at least 200V and VDD should be charged to 14V when 15V should appear on the output and that output should take over the supply of the IC. However, as soon as VDD reached 14 or 15V it starts going down to 9V, which is the lower threshold for the IC to turn off. The IC turns of, the VDD is again charged to 14V, it goes down again and so on in cycle. I have notices that there is no current flowing in the 15V secondary winding so that might be a problem. There is a load on start-up, so I am not trying to run a no load start-up since I've seen that some people had issues with no-load startup.

Does anyone have an idea what is happening and what is wrong?

The schematics is below:
Image

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