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

xiao.pan

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

1 month ago

0 Hong Huang 1 month ago

TI__Guru 53245 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 xiao.pan 1 month ago in reply to Hong Huang

Prodigy 30 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 Hong Huang 1 month ago in reply to xiao.pan

TI__Guru 53245 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.

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