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UC2842: How to determine the value of the input capacitance of the flyback power supply？

xiao.pan
Prodigy 40 points
Part Number: UC2842
Other Parts Discussed in Thread: UC3842

Tool/software:

Dear experts，

      The flyback power supply input voltage is 24VDC ,Vout=16V  and the input capacitor is 9 x 10uF MLCCs in parallel.

      When the flyback power supply is loaded with 1A, the input voltage fluctuates violently PP-1.35V, and the yellow waveform is the input voltage ripple.

      Why does the input voltage fluctuate with the increase of load, and how to calculate the value of the input capacitance with DC input.I find a calculation method in UC3842's datasheet but it calculate with a rectifier bridge

   Thankyou

  • 0
    TI__Guru 59085 points

    Hi,

    If the input voltage fluctuates then check your input voltage source to see if that source has any problem or if its current limit too low.

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi Hong，There was no problem with the input power, and when I added a 22uF aluminum electrolytic capacitor, the fluctuations were much better. Is there a formula for calculating input capacitance? thankyou

  • 0 in reply to xiao.pan
    TI__Guru 59085 points

    Hi,

    If adding more capacitors can resolved the issue then the issue is due to your long lead connections. Then you need to decide what is your input connection impedance and your input current the calculate the capacitor value at your converter input terminal to make the capacitance big enough without the issue. You can also test to find out by setup your worst input connection impedance then find what capacitor value can work.