# DC coupling capacitor selection

#### gautam023

Joined Apr 26, 2021
19
Hi,
I am using LT3958 to design a SEPIC Buck converter to supply my sensor for BMS function. I have made the necessary calculations for required DC coupling capacitor. I need a 22uF @ 100V XR7/XR5. I went to digikey website to check for the components. I see a capacitor whose application says is in SMPS filtering costing about 20$and another saying Boardflex sensitive which is costing around 3$. Which one do I choose. I would like to go with the cheaper option. Will it effect my output in anyway

Datasheet for LT3985

#### Papabravo

Joined Feb 24, 2006
16,773
This may be an impertinent question, but how is a capacitor useful for coupling DC? Doesn't a capacitor block DC current?
Maybe you could provide a schematic and point to the capacitor you are talking about.
For an SMPS applications you usually want the capacitor(s) with THE LOWEST POSSIBLE ESR you can afford. Even if you can't afford them -- you still want them.

#### gautam023

Joined Apr 26, 2021
19
This may be an impertinent question, but how is a capacitor useful for coupling DC? Doesn't a capacitor block DC current?
Maybe you could provide a schematic and point to the capacitor you are talking about.
For an SMPS applications you usually want the capacitor(s) with THE LOWEST POSSIBLE ESR you can afford. Even if you can't afford them -- you still want them.
I am using the circuit. The only change is the input and output voltage. Based on that I have calculated the required C,R,L values. So you are seeing don't see the type of application just look at their ESR values and choose the lowest one

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#### Papabravo

Joined Feb 24, 2006
16,773
The capacitor is not passing the DC voltage from the input to the output, only the AC component that results from the operation of the switch. The reason for using LOW ESR capacitors in all cases is to minimize losses as much as you possibly can. this is especially true in high-current applications. I cannot imagine a case where a capacitor with a higher ESR would be chosen if other requirements are met like capacitance and working voltage.

The term "coupling" comes from amplifier designs where the purpose of the capacitor was to couple the AC signal and BLOCK the DC signal. They were commonly referred to as "blocking" capacitors. I apologize for jumping to a conclusion without seeing a schematic, which to be fair you did nor originally provide. You did use the correct terminology for the converter, but I forgot the fact that in such a topology this is not a bug, but a feature.