# Dclink capacitor selection criteria Film vs Electrolytic

#### ak52

Joined Oct 15, 2014
228
Hello to all my power electronic Gurus,

https://www.kalbeck.com/asset/630/Whitepaper SalconeBond.pdf
on properly sizing a dclink capacitor, i understood most of it, but there were a few things that were still unclear.

In the example which was shown on Page 4, Vbus is 325v, Inductance is 100uH, and a switching frequency of 10 Khz,
If we calculate the ripple current at 50% duty cycle it comes to around 28.7Amps(RMS) as per the given formula:
delta_i = (0.25*Vbus)/(f*L)
The author further says that " If an electrolytic capacitor were sized for this application it would require a 5,000uF / 450V capacitor to meet the ripple current requirement at 10 kHz switching frequency". How did he arrive at the 5000uF value? I dont see any equation between Capacitance and ripple current.
Thanks Last edited:

#### nsaspook

Joined Aug 27, 2009
12,321

#### ak52

Joined Oct 15, 2014
228
Hi, thanks for the quick reply, but I still don't get it,how did the author derive the 5000uF value just by calculating the ripple current?He further gets the voltage ripple to 0.2vp,but that's only if he assumed the capacitance to be 5000uF.My question here is how did he asume the capacitance to be 5000uF in the first place? Shouldn't there be an equation which combines the ESR, capacitance and current ripple?

Looking at the voltage ripple equation (equation 29), it's clear how the author has derived the capacitance for a film capacitor. I am stuck understanding how has he done it for the electrolytic capacitor.

#### nsaspook

Joined Aug 27, 2009
12,321
Hi, thanks for the quick reply, but I still don't get it,how did the author derive the 5000uF value just by calculating the ripple current?He further gets the voltage ripple to 0.2vp,but that's only if he assumed the capacitance to be 5000uF.My question here is how did he asume the capacitance to be 5000uF in the first place? Shouldn't there be an equation which combines the ESR, capacitance and current ripple?

Looking at the voltage ripple equation (equation 29), it's clear how the author has derived the capacitance for a film capacitor. I am stuck understanding how has he done it for the electrolytic capacitor.
Looks like he used the dependable engineering feat of taking an educated guess.

Last edited:

#### Papabravo

Joined Feb 24, 2006
20,615
Hello to all my power electronic Gurus,

https://www.kalbeck.com/asset/630/Whitepaper SalconeBond.pdf
on properly sizing a dclink capacitor, i understood most of it, but there were a few things that were still unclear.

In the example which was shown on Page 4, Vbus is 325v, Inductance is 100uH, and a switching frequency of 10 Khz,
If we calculate the ripple current at 50% duty cycle it comes to around 28.7Amps(RMS) as per the given formula:
delta_i = (0.25*Vbus)/(f*L)
The author further says that " If an electrolytic capacitor were sized for this application it would require a 5,000uF / 450V capacitor to meet the ripple current requirement at 10 kHz switching frequency". How did he arrive at the 5000uF value? I dont see any equation between Capacitance and ripple current.
Thanks 