Ringing when using low value ceramics in SMPS.

[eem2am]

hello good friends,

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

I am using large (4u7) ceramic caps 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?

The ringing i speak of is that ringing that would occur between the low_ESL ceramic caps and the ESL of the big bulky (4u7) ceramic caps.

-Also, the ringing that would occur between the low value ceramic capacitance and the PCB trace inductance that would be "seen" through the boost diode capacitance and the Mosfet DS capacitance.

 

[FvM]

As a brief answer: No.

 

[eem2am]

thankyou FvM,

The thing is, here is the ripple current in a bulk 4u7 cap at the output of a boost converter where NO high frequency caps have been used at the output

and here is the bulk cap current when HF caps have been used

as you can see, when high frequency caps (~1nF ceramics) are used, the ringing in that capacitor current is very much worse.

so do you think its best to just leave out high frequency caps at the output of a boost converter?