# Bulk Capacitance and Iripple

#### luca290799

Joined Apr 4, 2022
53
Hi! I need to choose an SMD bulk capacitor to put in my PCB in the power supply line. I used the following formula to calculate it:
Cbulk (min) = ( D*(1-D)*Iout / (Vripple*FreqSwitch) ) with D = Vo / Vin * efficiency

Now I have to choose which SMD capacitor to use and I am faced with the choice of two parameters: ESR and rated ripple current.
In this case (if I am not mistaken) I choose ESR as small as possible ( because P=R * I^2 ) ... what about Iripple?

In the lessons I am following, the following sentence is stated and a high Iripple is chosen:
"When the transient current exceeded the ripple current value, the capacitor current will saturate and it can cause heating, yes heating depends on its totally resistive component of it"

Can you explain this statement to me? And especially whether in this application the ripple current should be taken minimum or maximum :/

Thanks

#### Papabravo

Joined Feb 24, 2006
19,617
Which kind of DC-DC converter are we talking about?
Also, D = (Vo/Vin)*efficiency makes no sense as it would lower the duty cycle to correct for losses. You could have meant D = Vo/(Vin*efficiency) which makes some sense because it raises the duty cycle to correct for losses. Normally we would only do this in an open loop scheme, because in a closed loop arrangement such a correction for losses is unnecessary. I don't believe your formula for the minimum capacitance. I think for a buck converter it should be:

$$C\;=\;\cfrac{1-D}{8L(\Delta V_0/V_0)f^2}$$

Tell me if you are talking about some other kind of DC-DC converter.