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    sg3525 feedback design

    hi
    i use this schematic to build a power supply with some changes(core is ETD39-frequency is 80KHz-target output voltage is 45*2-output capacitor is 2000uf-output inductor is 28uh)
    but i have a problem with feedback and by this feedback transformer start to produce a noisy sound like"tik-tik-tik" and voltage change beetwen 40-45
    this feedback is a crude feedback an dosn't control duty cycle continuously and just turn pwm on/off
    so i need a better feedback and by searching net i realized that i should use TL431 for feedback.
    now i want u help me design a good feedback using TL431 and 4N25.
    Click image for larger version. 

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    Re: sg3525 feedback design

    Hi,

    Your description makes me assume that the inductor is saturating.
    Did you check this.

    What about air gap on core?
    Did you wind it on your own?

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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    Re: sg3525 feedback design

    Quote Originally Posted by KlausST View Post
    Hi,

    Your description makes me assume that the inductor is saturating.
    Did you check this.

    What about air gap on core?
    Did you wind it on your own?

    Klaus
    for output inductor i use two EE25 core with about 0.5 mm gap and wind them on my own.inductor dosn't saturate
    without feedback smps work without any noisy sound and duty cycle is maximum and voltage is 155/turn ratio
    i connect a 300 watt load without feedback and circuit worked fine but voltage droped about 20 volt which is normal without feedback
    i just want a good feedback with TL431



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    Re: sg3525 feedback design

    Quote Originally Posted by yoosefheidari View Post
    transformer start to produce a noisy sound like"tik-tik-tik" and voltage change beetwen 40-45
    The irregular operation could be unwanted 'hiccup' mode, or maybe a case of squegging. It works okay for a few cycles, then a momentary lockup. As this repeats back-and-forth, the transformer can make audible noises.

    The noise is not necessarily arcing but it is natural to wonder if you're getting high voltage arcing in the transformer? Your schematic is not obvious whether it has a snubbing network. Its caption says 700W so the snubber should be able to handle 700W spikes.



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    Re: sg3525 feedback design

    I suspect that unsuitable feedback loop compensation is causing the observed instability. By connection a capacitor to comp pin you get pure I controller operation which is likely to give insufficient phase margin. Although TL431 circuit has additional features to tune the loop characteristic you'll probably need to modify the SG3525 compensation.


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    Re: sg3525 feedback design

    Quote Originally Posted by FvM View Post
    I suspect that unsuitable feedback loop compensation is causing the observed instability. By connection a capacitor to comp pin you get pure I controller operation which is likely to give insufficient phase margin. Although TL431 circuit has additional features to tune the loop characteristic you'll probably need to modify the SG3525 compensation.
    i think this problem caused by on/off state of pwm.in fact with this feedback transformer turns on and off continuously and cause that noise.
    so i need a feedback that control output by decrease/increase duty cycle.
    am i right?
    i cant design a feedback on my own because i havn't compelete information about poles/zeros/bode plot/transien response/phase margin/gain margin and......
    so i need much help.on the net i found this feedback
    is that good without any problem?can i use this feedback without any change?or nees some modification?this is type I or II ? is compensation good?
    Click image for larger version. 

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