# Calculate the Input/Output Capacitor Value on DCDC converter

#### stoui333

May 14, 2019
7
Hi everyone,

I have a general question on how to calculate the input and out capacitor values for a DCDC converter for filtering/smoothing.

My example:

DCDC Converter: Synqor NQ60W60QTC25NRS
Input Voltage: 36v
Output Voltage: 12v

Is there any mathematical formulas to find what capacitor values to use?
Is there any rule of thumb?

Also, i have read that for the specific purpose (filtering/smoothing) better to use a ceramic capacitor instead of an electrolytic, is that right?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,719
Is there any mathematical formulas to find what capacitor values to use?
See page 12 of the datasheet.
Also, i have read that for the specific purpose (filtering/smoothing) better to use a ceramic capacitor instead of an electrolytic, is that right?
No. Electrolytics and ceramics serve different purposes:
• Electrolytics with their high capacitance serve to buffer a lot ocg charge to provide high currents for a long time (comparatively).
• Ceramics with their low capacitance baut also very low ESR to filter primarily high frequency noise.
Usually a combination of both types (and even ceramics of different capacities) is used to cover a wide range of frequencies. See e.g. here.

#### stoui333

May 14, 2019
7
See page 12 of the datasheet.

No. Electrolytics and ceramics serve different purposes:
• Electrolytics with their high capacitance serve to buffer a lot ocg charge to provide high currents for a long time (comparatively).
• Ceramics with their low capacitance baut also very low ESR to filter primarily high frequency noise.
Usually a combination of both types (and even ceramics of different capacities) is used to cover a wide range of frequencies. See e.g. here.

Thanks Harald kapp that was en lighting!

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,719
I need to work on my spelling - or buy a new keyboard

Replies
4
Views
2K
Replies
1
Views
2K
M
Replies
14
Views
10K
Michael A. Terrell
M
Replies
5
Views
132
Replies
5
Views
827