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Shown below is an EMI filter for an AC input. My doubt is when I analyse the inrush current, should I be considering the contribution due to all three capacitors?

a) Differential mode capacitors

b) Common mode capacitors

c) Load capacitors ( In our case, the load is rectifier with bulk caps at the output)

Since these currents might not be in phase, the straightforward addition may not be feasible*. Iam not sure how can the net effect be calculated?

(*I guess, the 90 degree phase difference between AC voltage (across) and current through a capacitor would not be applicable during inrush duration (Since the 90 phase difference is a steady state phenomenon). Please confirm.)

Also would I be getting different inrush values if I measure the phase leg or nuteral leg of AC supply - depending on the additive/subtractive nature of common mode current?

PS: In our existing design we are addressing only the inrush current due to rectifier o/p caps. Iam not sure why the inrush due to EMI filter caps is omitted.

enter image description here

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  • \$\begingroup\$ Have you tried to simulate it? \$\endgroup\$
    – winny
    Commented Feb 4 at 12:02
  • \$\begingroup\$ @winny: Unfortunately no. I will try once I get the time to develop the model. \$\endgroup\$ Commented Feb 4 at 15:37
  • \$\begingroup\$ You already have a model, the schematic you posted. It should be good enough for 99 % of cases, save for EMI compliance at high frequencies. \$\endgroup\$
    – winny
    Commented Feb 4 at 15:54
  • \$\begingroup\$ @DivyaK.S are we done here? Do you have an answer? \$\endgroup\$
    – Andy aka
    Commented Feb 22 at 8:59

1 Answer 1

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My doubt is when I analyse the inrush current, should I be considering the contribution due to all three capacitors?

and

In our existing design we are addressing only the inrush current due to rectifier o/p caps. I am not sure why the inrush due to EMI filter caps is omitted.

Compared to the value of the bulk capacitor used in most power supplies I expect that the contribution to inrush from the EMI filter capacitors is negligible. However, if you wish to analyse inrush and include these capacitors then that's your call.

Since these currents might not be in phase, the straightforward addition may not be feasible. I am not sure how can the net effect be calculated?

I wouldn't hesitate to use a simulator.

I guess, the 90 degree phase difference between AC voltage (across) and current through a capacitor would not be applicable during inrush duration (Since the 90 phase difference is a steady state phenomenon). Please confirm.

Correct.

Also would I be getting different inrush values if I measure the phase leg or neutral leg of AC supply - depending on the additive/subtractive nature of common mode current?

Yes you would because neutral is tied to earth at some point and that imbalances the capacitors on the load side of the filter. My best advice is this: use a simulator.

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