# Input caps!!

Discussion in 'General Electronics Chat' started by supermankid, May 19, 2014.

1. ### supermankid Thread Starter Member

May 26, 2013
40
1
**Sorry for the short title....I couldnot use longer sentence because of the bug on the system.

I cant understand why the current rating(rms) of the input capacitor should be as high as

I.CIN.RMS= Iout sqrt(D*(1-D))

or

could some one explain why the current rating of the capacitor is high

and please also explain how this formula is derived. Many thanks in advance.

2. ### enggricha Member

May 17, 2014
66
1
what cant u understand about the higher current rating?

3. ### supermankid Thread Starter Member

May 26, 2013
40
1
I cant see any current going in or out of the input capacitor before the power supply circuit....

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,924
602
Is it DC or AC?

5. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
You can always leave what you want the title to be, then flag the post. Moderators will be happy to change the title for you until the bug is resolved.

6. ### crutschow Expert

Mar 14, 2008
16,230
4,341
If D is related to the duty-cycle of a switching regulator, then current is drawn in pulses from the capacitor as the regulator switch turns on and off at the duty-cycle rate to supply the average (filtered) output current. This pulsed current is an AC current (capacitor charging and discharging at the pulse rate). The duty-cycle of the regulator determines the ratio of the peak pulse current as compared to the average DC output current, which determines the RMS value of the pulses (see this).

7. ### supermankid Thread Starter Member

May 26, 2013
40
1
In simulation....or simple analysis....the current at the input for the regulator comes from the Vin....(I believe not from the capacitor). Where am I not seeing it clearly ?