Ceramic caps in SMPS to improve EMC?

[eem2am]

hello good friends,

please , i speak in reference to a boost converter 12V in, 24V out, 1 Amp out.

I am using electrolytic capacitors at the output, but i also want to use some small value ceramic capacitors there too. (eg 1nF)

You see, it is told that putting some low_ESL, ceramic capacitance straight after the boost diode in a boost converter means that the higher frequency components of the switching harmonics then traverse a loop with a smaller area....this in turn means less radiated noise.

However, would this low capacitive value , low ESL, ceramic capacitance then not just act as a resonator and create terrible ringing at the cathode of the boost diode?

 

[betwixt]

Ringing would only occur under two circumstances, the 'Q' of the output circuit was sufficient to allow resonance to occur and the switching frequency was close to the loop self-resonant frequency.

Although you are right in assuming electrolytics generally have high ESR/ESL, it is still low enough to damp any resonance to minimal levels. Your switcher frequency is also probably only a few 10s of KHz while the self resonant frequency of your output filter loop will be hundreds of MHz.

It is good practice to parallel ceramic capacitors with electrolytics but only to counter the ESR itself. Generally, ESR increases with frequency as it has an inductive component but the ceramic capacitor will have a falling reactance at the same time.

Brian.

 

[eem2am]

Thankyou Brian,

I now realise your excellent answer is very true in this case.