Dclink capacitor selection criteria Film vs Electrolytic

Thread Starter

ak52

Joined Oct 15, 2014
199
Hello to all my power electronic Gurus,

I was reading this paper
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.
Could someone please explain this...
Thanks :)
 
Last edited:

nsaspook

Joined Aug 27, 2009
9,991
"In this same example, if an electrolytic capacitor was used, the capacitance would be based solely on ripple current"

5,000uF is the value needed for the specified ripple voltage at the specified ripple current for a electrolytic capacitor due to the poor electrical characteristics of electrolytic capacitors vs film.
From your link.
1654611428366.png
 

Thread Starter

ak52

Joined Oct 15, 2014
199
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
9,991
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
18,804
Hello to all my power electronic Gurus,

I was reading this paper
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.
Could someone please explain this...
Thanks :)
The relationship is between capacitance and ripple voltage and is given by Equation (29). A quick algebraic rearrangement gives an expression for the capacitance as a function of L, the switching frequency f, and the ratio of the ripple voltage to the bus voltage.
the value of 5000μF is for very sμmall ripple voltage and the value of 308μF is for a ripple voltage that is about 1% of the bus voltage. Which one you choose depends on the requirements and specifications.
 
Last edited:
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