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LM5146DESIGN-CALC: Input Filter Capacitor Selection

Subrahmanyam V
Intellectual 430 points
Part Number: LM5146DESIGN-CALC

Hi,

I have the following doubt: 

Scenario: Let us say, I need to design input filter (pi-filter) for a buck converter. The input switch side RMS current is 3A.

Now, let us say, after designing filter, I got a filter capacitor value of 47 uF on switch side of pi-filter. 

Now, I need a capacitor with 47 uF and 3A current rating!

But, for such high switching currents (Fs = 250 kHz), my ELCAP impedance curve doesn't support high frequency currents. 

I can proceed to select MLCC caps based on its impedance curve to support the current and ELCAP to support Farad value.

My doubt is: What should be the ELCAP current rating to be chosen? Any reference document for this?

Because, 47 uF 115 mA cap costs 3 times less than a 47 uF, 500 mA ELCAP.  Bothways, ELCAP cannot give me that ripple current!

Any thoughts are welcome.

Thanks,

Siri

  • 0
    TI__Mastermind 27030 points

    Hi 

    you can put multiple small value of MLCC paralleling with ELCAP.

    then You have to calculated the ripple current of MLCC and ELCAP . Simple way is do the simulation and see each cap's current. 2.2uF*4 with ELCAP may fitting you application . 

    You also can calculate the AC current go through each cap by change the capacitor +ESR value to the impedance@250Khz. 

    Thanks