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Design of the power components

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4

Design of the power components

4.1

Input capacitor selection

The input capacitor voltage rating must be higher than the maximum input operating voltage

of the application. During the switching activity a pulsed current flows into the input capacitor

and so its RMS current capability must be selected accordingly with the application

conditions. Internal losses of the input filter depend on the ESR value, so usually low ESR

capacitors (like multilayer ceramic capacitors) have a higher RMS current capability. On the

other hand, given the RMS current value, lower ESR input filter has lower losses and so

contributes to higher conversion efficiency.

The maximum RMS input current flowing through the capacitor can be calculated as:

Equation 13

function has a maximum at D = 0.5 and, considering

In a specific application the range of possible duty cycles has to be considered in order to

find out the maximum RMS input current. The maximum and minimum duty cycles can be

calculated as:

Equation 14

and

Equation 15

Where

The peak-to-peak voltage across the input filter can be calculated as:

Equation 16

Equation 17

D

2D

2





---------------

–

D

2



2

-------

+



=

+

–

+

--------------------------------------------------------------------------------------------------

=

+

–

+

----------------------------------------------------------------------------------------------------

=



-----------------------

1

D



----

–



 D



D



----

1D

–





+



+



=



-------------------------

1

D



----

–



 D



D



----

1D

–





+



=