Capacitance value

Thread Starter

pinkyponky

Joined Nov 28, 2019
194
Hi,

I would like to choose a capacitor to use it as filter at the output of the DC-DC component and output voltage is 15V. The capacitance value is 27uF and voltage rating is 35V as stated in the datasheet. So, I haven't found such capacitance value in the marked. So, Shall I use the two capacitors are in parallel instead one capacitor at the output of the converter, as shown below.

1629224113789.png
 

Audioguru again

Joined Oct 21, 2019
3,821
Many low dropout voltage regulators have a minimum output capacitor value so that it does not oscillate.
You did not provide a part number or post its datasheet for us to see.
An electrolytic capacitor has a wide range of value, maybe +50%, -20% so it must be allowed for.
Maybe your DC-DC device needs a 25uF capacitor for low frequencies and an additional 2uF capacitor for high frequencies?
 

Ian0

Joined Aug 7, 2020
3,752
33μF will be near enough.
It's very unlikely that 47μF would cause any problems.
A smaller ceramic capacitor in parallel will deal with the higher frequency components from the switching.
 

Papabravo

Joined Feb 24, 2006
17,235
You didn't specify the input voltage so we don't know what topology is used in your converter, but I would like to point out that adding capacitance to the output of an existing design may affect the performance in unexpected ways and you may not be happy with the results. It is difficult to tell without a schematic so that we can look for potential problems. If it were me I would be measuring some things about this thing both before and after you make the modification. If things don't improve as you expect you'll have some detective work to do.
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
194
You didn't specify the input voltage so we don't know what topology is used in your converter, but I would like to point out that adding capacitance to the output of an existing design may affect the performance in unexpected ways and you may not be happy with the results. It is difficult to tell without a schematic so that we can look for potential problems. If it were me I would be measuring some things about this thing both before and after you make the modification. If things don't improve as you expect you'll have some detective work to do.
The input voltage is 24Vdc and Output is 15Vdc.
 
Last edited:

Ian0

Joined Aug 7, 2020
3,752
I bet it's a misprint. Every other recommended value of capacitance is E6 series. I bet they meant 22uF.
Put a 1uF ceramic across it, and when you've added in all the other decoupling caps in your circuit you'll be close enough.
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
194
I bet it's a misprint. Every other recommended value of capacitance is E6 series. I bet they meant 22uF.
Put a 1uF ceramic across it, and when you've added in all the other decoupling caps in your circuit you'll be close enough.
You mean at the output I need to keep 1uF, Is that enough?. If so, then why they are listed high capacitance value?.
 

Ian0

Joined Aug 7, 2020
3,752
You mean at the output I need to keep 1uF, Is that enough?. If so, then why they are listed high capacitance value?.
Not quite. . . use 22uF, and put 1uF ceramic across it. Some ceramic capacitors are always a good idea for decoupling.
Then you might have a dozen 100nF in the rest of your circuit, that's 24.4uF.
Your 22uF might even be as high as 26.4uF as the tolerance is generally ±20%.

Notice that it says "recommended" values. They don't say "never use less than 10uF nor more than 33uF or the whole thing will oscillate". If it needed a precise value of capacitance, they would have said so.
 
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