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  1. #1
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    300W Push-Pull SMPS PWM Signals

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ID:	108807Hello friends,

    I designed a 300W SMPS with Push-Pull Topologies.

    Features:

    Input Voltage: 220V AC/ 50Hz

    Output Voltage: 30V/ 10 A

    PWM IC : UC3825


    However, I have a serious problem. Output voltage is normal. I can adjust with trimpot(feedback is normal), no problem. But UC3825 Pwm Out Signal's are very distorted. I have added pics. (OUTA and OUTB signals and Control Stage.)

    (Oscilloscope pics features: 5 Volts/Div, 5 us/ Div)

    Does anyone have an idea?

    Thanks,

    Regards.

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  2. #2
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    Re: 300W Push-Pull SMPS PWM Signals

    Your scope traces show ringing at 1 MHz. It starts out strong after a transition, then fades slightly until the next transition.

    Is that the distortion you're talking about?



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  3. #3
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    Re: 300W Push-Pull SMPS PWM Signals

    I think that, current sense pin (Ilim) signal of uc3825 has problem. When I turn off input ac supply, signals (pics of scopes) return normaly square wave, also signal of current sense pin return normally sawtooth (this event occur in 1-3 seconds and signals low zero). I used current transformer(1:100) for current sense.
    Thanks,

    Regards.



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  4. #4
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    Re: 300W Push-Pull SMPS PWM Signals

    Hello Friends,

    I still work about push-pull with uc3825 and problem still continue. However, I have observed something. If I apply voltage 100VDC and less than, PWM out signals are excellent. But if I apply voltage greater than 100V DC, Pwm out signals are very distorted and duty time is decrease as the pics.
    Thanks,

    Regards.



  5. #5
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    Re: 300W Push-Pull SMPS PWM Signals

    Is 100V a threshold for some component? Think, whether any device is set to do something different at 100V.

    Do you have any P-devices whose gates could respond unexpectedly, depending on the supply voltage?

    It looks as though there are additional sections of your project, which are not displayed in your schematic. You may need to test each section by itself, before hooking it up to other sections.



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  6. #6
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    Re: 300W Push-Pull SMPS PWM Signals

    If the turn ratio of the transformer is wrong and there are more turns in output coil than needed then it will produce higher voltage level in output. The feedback circuit will try to regulate it by reducing duty cycle and you will see a narrow pulse of PWM output especially when there is no load present.
    .



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