AN2459 datasheet(28/35 Pages) STMICROELECTRONICS

28 / 35 page

Components calculation

AN2459 - Application note

28/35

Appendix A

Components calculation

This appendix presents guidelines for the calculation of power components. The content is

based on the design process defined in AN966 (3).

A.1

Input capacitor

The input high frequency filter capacitor (C3) has to attenuate the switching noise due to the

high frequency inductor current ripple (twice the average line current, Figure 9). The worst

conditions occur on the peak of the minimum rated input voltage. The maximum high

frequency voltage ripple is usually imposed between 1% and 10% of the minimum rated

input voltage. This is expressed by a coefficient ‘r’ (typically, r = 0.01 to 0.1):

High values of C3 alleviate the burden to the EMI filter but cause the power factor and the

harmonic contents of the mains current to worsen, especially at high line and light load. On

the other hand, low values of C3 improve power factor and reduce mains current distortion

but require heavier EMI filtering and increase power dissipation in the input bridge. It is up to

the designer to find the right trade-off in their application.

A.2

Output capacitor

The output bulk capacitor (Co) selection depends on:

●

the DC output voltage;

●

the admitted overvoltage;

●

the output power;

●

the desired voltage ripple.

A voltage ripple (

∆Vo = 1/2 ripple peak-to-peak value) of 100 to 120Hz (twice the mains

frequency) is a function of the capacitor impedance and the peak capacitor current (IC(2f)pk =

Io):

With a low ESR capacitor the capacitive reactance is dominant, therefore:

AN2459 Datasheet(PDF) 28 Page - STMicroelectronics