Beginner Question - Are these two caps interchangeable?

[cdonate]

Hello, I'm starting with PCB development and I need some help.

I'm trying to make a design a little bit more compact. This is the power supply of the system. It takes 220V AC and converts to 3.3V DC with a buck converter.

At some point on the circuit I have this CAP https://br.mouser.com/ProductDetail...a/RJ16DmbUEcBWCoqtKswETiCKebw9aAqfs2UgrMw9A== (ELXZ500ELL101MH12D) and I would like to change to https://br.mouser.com/ProductDetail...VE500ADA101MHA0G/?qs=9l4X0aOfMddzvH8UrKmeSw== (EMVE500ADA101MHA0G).

My question is, can I do that or this is a very noob thing to do? If I can't, is there another option?

I'm now only looking to make a smaller version of the circuit, and I'm only thinking about changing the caps. They take most of the space.

This is only a proof of concept.

The schematic is attached. At this point I would like to change the C3 Cap.

Any help is much appreciated.

Cheers!

 

[FlapJack]

Your schematic is missing the V in and out so i can not calculate duty cycle. Go to the Mouser site and search for the lm2594dadjr2g data sheet. On page 14 it shows how to calculate the rms ripple current the capacitor needs.

 

[FvM]

Your schematic is missing the V in and out so i can not calculate duty cycle.

Vin and Vout are about 35V and 3.3V.

The capacitor circuit is feeding about 12 mA average DC, so there shouldn't be a problem with capacitor ripple current in this application. The output current is respectively rather limited (< 100 mA).

 

[KlausST]

Hi,

While the fisrt one (THM) is rated for high frequency, with especially low ESR at high frequency the second (SMD) is not.
The second is more for line frequency applications and not for switch mode supplies.

I recommend to use a "faster" than the second one.

If the output current (DC) is limited to 100mA the second capacitors ripple current is specified with 250mA at 120Hz.
It is not very unlikely that the switch ripple current is (temporarely) in the range of 250mA.

The higher the frequency the less performance of the electrolytic capacitor. I usually add a ceramics capacitor in parallel to an electrolytic (even a fast one) to ensure to suppress the switching frequency with it´s overtones. (here maybe a 4.7uF)

Klaus

 

[FvM]

It is not very unlikely that the switch ripple current is (temporarely) in the range of 250mA.

It's not possible with the given design parameters. In any case, ripple current rating is thermal related and has respectively long time constants. I would usually say, use a low ESR capacitor, but it's not required for this mini-current switcher.

- - - Updated - - -

I just noticed that LM2594 is a very bad choice for this switcher because it consumes most of the available input current as quiescent current. Need to increase C1 or use a low power switcher.

 

[KlausST]

Hi,

True for continously switching switchers. But with that low load burst mode operation or pulse skipping mode is possible.

Then - I imagine (I really don´t know it for sure) - it could be temporarily (during switching) above 250mA.

For thermal considerations - i agree - that one will calculate less than 100mA ripple.

In either case: I recommend to use a capacitor that´s ESR is specified for high switching frequencies.
The first one is called "Low impedance, Downsized, 105℃" with ESR and ripple current specified at 100kHz.
The second is called "Standard, 105℃" wich is specified at 120Hz. --> I assume it´s use is for mains frequency.

Klaus

 

[pjmelect]

I think that it would be OK to use the second capacitor. It does have the 1uF capacitor in parallel to deal with the high frequency switching pulses, and that the main ripple is of mains frequency.