AN2459 datasheet(29/35 Pages) STMICROELECTRONICS

29 / 35 page

AN2459 - Application note

Components calculation

29/35

∆V

o is usually selected in the range 1 to 5% of the output voltage. Although ESR usually

does not affect the output ripple, it has to be taken into account for power loss calculations.

The total RMS capacitor ripple current, including mains frequency and switching frequency

components, is:

If the application has to guarantee a specified hold-up time, the selection criterion of the

capacitance will change: Co has to deliver the output power for a certain time (tHold) with a

specified maximum dropout voltage:

where Vo_min is the minimum output voltage value (which takes load regulation and output

ripple into account) and Vop_min is the minimum output operating voltage before the 'power

fail' detection from the downstream system supplied by the PFC.

A.3

Boost inductor

Designing the boost inductor involves several parameters and different approaches can be

followed. First, the inductance value must be defined. The inductance (L) is usually

determined so that the minimum switching frequency is greater than the maximum

frequency of the internal starter, to ensure a correct TM operation. Assuming unity PF, it is

possible to write:

Ton being the ON-time and Toff the OFF-time of the power MOSFET, ILpk the maximum peak

inductor current in a line cycle and

θ the instantaneous line phase (θ

∈ (0,π)). Note that the

ON-time is constant over a line cycle.

As previously mentioned, ILpk is twice the line-frequency peak current, which is related to

the input power and the line voltage:

AN2459 Datasheet(PDF) 29 Page - STMicroelectronics