Role of the Output Capacitor in Buck Converter in terms of frequency

[FreshmanNewbie]

I am using this DC-DC buck converter - Buck Converter.

Schematics :



Buck Converter Specifications :

Input Voltage - 18V to 32V

Switching Frequency - 300kHz

Output Voltage 9V

Load Current - 0mA to 200mA Maximum.

I have 2 questions :

1. What does it mean when a design recommendation states - " The output capacitor ESR and capacitance form a zero = 1 / (2 x π x Cout x ESR). This zero can move significantly if an aluminum output capacitor is used. Aluminum output caps ESR can change 10x over temperature."

Can someone explain me a little intuitive on what this recommendation states and why should I take care why designing a buck converter? How to test it or verify this recommendation?

1. Why should we care about the Output Aluminium Electrolytic Capacitor's Frequency vs Impedance graph while selecting the output capacitor of the buck converter (In this case the 47uF) ?

Can someone explain me a little intuitively on the answers for the above 2 questions?

 

[KlausST]

Hi,

the ESR and the "ideal cpacitance" are inside the capacitor and connected in series.

Thus for the voltage across the ideal capacitor there is a low pass behaviour.
The cutoff frequency is defined by: fc = 1 / (2 x π x Cout x ESR)

Due to high ESR of aluminum capacitors they are not useful for high frequencies.
Thus - if possible - use ceramics capacitors. They generally have far less ESR than aluminum capacitors and thus are more suitable for SMPS applications.

Mind: "switching" means there are overtones. A 300kHz SMPS will generate voltage/current/noise in the MHz range. If you want to suppress them you need (EMI, CE, low noise, low ripple...)you need to use suitable capacitors ...

...and a suitable PCB layout, too. Without a really solid GND plane and wide and very short traces you will get series inductance and series resistance and thus may cause bad suppression of switching noise as well as cause additional ringing by forming a resonance...

Klaus