Which (if any) capacitors for these DC/DC converters?

Thread Starter

tpw

Joined Oct 7, 2019
5
I’m trying to choose/work out if I need capacitors for the input and output of two DC/DC converters.

The first converter is a Murata PAE-29/3-D48, powering this along a ~30cm cable, and a bike headlight, using ~100ma with a ~40cm cable.

The second is a Mornsun VCB4805SBO-30WR3 (5v version), powering a wireless phone charger which uses ~1.3A with a ~60cm cable.

Both will be powered by a 48v lithium battery pack, on a bike which will be used in weather from -5 to 25 C.

My main point of confusion is the capacitor ripple current rating - I’m aware I should aim for it not to be exceeded, but both DC/DC datasheets only list ripple voltage and I don’t know how to calculate the current (or the frequency for that matter). The datasheets also mention choosing a low ESR capacitor, but I don’t know what counts as ‘low’, or why many capacitors don’t seem to list an ESR rating at all.

Finally, neither datasheet seems to mandate the use of input or output capacitors, and I’m not sure how to know whether they’re needed.

Thanks!
 

BobTPH

Joined Jun 5, 2013
4,298
I don’t see anything in the datasheet that calls for an additional capacitor. Are you seeing problems that make you think you need one?

Bob
 

Thread Starter

tpw

Joined Oct 7, 2019
5
The first does mention "All models are tested and specifi ed with external parallel 1 μF and 10 μF output capacitors and a 22μf external input capacitor .. These capacitors are necessary for our test equipment and may not be needed in the user’s application." (page 4) and "These converters will operate to specifications without external components, assuming that the source voltage has very low impedance. Since real-world voltage sources have fi nite impedance, performance is improved by adding external filter components" (pg 14).

And the second lists a diagram for a 'typical application' that includes input and output capacitors on page 4.

I have no idea if these will be necessary for my application (I know very little about electronics!), my issue was that I didn't know how to know, and how to choose them if I did need them.

I should note that I haven't bought these modules yet, I'm trying to buy everything I need in one go.
 

BobTPH

Joined Jun 5, 2013
4,298
Ah, I only looked at the first.

Neither of your uses is sensitive at all to ripple, so I would use the recommended ones for the second one, and none for the first one.

Bob
 

Thread Starter

tpw

Joined Oct 7, 2019
5
That's where I'm stuck though - the datasheet only gives capacitance values, so I don't know how to choose the other variables like type (electrolytic, ceramic etc), ESR, ripple current rating etc.
 
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