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  1. #1
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    uc3842 application note

    I'm trying to make a flyback converter using uc3842. My design is based on an application note.
    http://www-us2.semiconductors.philip...tes/AN1272.pdf
    At first try it didn't work. I tried to design my own transformer. The problem may arise from this point. I want to ask some questions that aren't clear to me.
    I want to convert 300VDC to 20VDC. I used an ETD29 core to design transformer. UC3842 is running at 86KHz. Primary of the transformer is 120 turns. auxiliary is 7 and seconder is 8 turns. I used a small airgap in the transformer. It didn't work. Clock seems ok. Mosfet also runs at 86khz, it seems ok. Duty cycle is low, may be, because of no load. But VCC is oscillating and i couldn't fix it at 16V. I tried without transformer Vcc is a sawtooth shape waveform. My questions:

    1- Doesn't it work only with the startup circuit (a resistor connected to the 300VDC)?
    2- Are there any important details about the airgap and transformer winding?
    3- Auxiliary and seconder windings are less in number. Should i wind them in to the center or distribute them equally on the middle leg of the transformer? or what?

    Do you have any other suggestions? What may have gone wrong?

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  2. #2
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    uc3842

    Duty cycle is low, may be, because of no load
    This kind of supply must always operate with load. In general connect some kind of power resistor, to ensuare 30-50% of nominal load. This is first step in my opinion



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  3. #3
    Advanced Member level 2
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    uc3842 application

    I solved the problem. It was due to the auxiliary winding. I increased its turns. Uc3842 doesn't become active until the VCC voltage exceeds 16V. Now i have another problem. There is a big dip at the output voltage at the switching frequency of uc3842. How can i eliminate this? Also output is much more than my calculations. I was expecting 22-25V but its about 34V. What may be the reason for this?



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