AN966 datasheet(9/21 Pages) STMICROELECTRONICS

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quency inductor current ripple (twice the average line current, see fig. 9). The worst conditions will 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):

Cin

I rms

π ⋅ ƒsw ⋅ r ⋅ Virms (min)

High values of Cin alleviate the burden to the EMI filter but cause the power factor and the harmonic con-

tents of the mains current to worsen, especially at high line and light load. On the other hand, low values of

Cin 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.

Output Capacitor

The output bulk capacitor (Co) selection depends on the DC output voltage, the admitted overvoltage,

the output power and the desired voltage ripple.

The 100 to 120Hz (twice the mains frequency) voltage ripple (

∆VO= 1/2 ripple peak-to-peak value) is a

function of the capacitor impedance and the peak capacitor current (IC(2f)pk = Io):

∆VO = IO ⋅

√

(2π ⋅ 2f ⋅ CO)

2 +

ESR

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

≥

π ⋅ f ⋅ ∆VO

π ⋅ f ⋅ VO ⋅ ∆VO

∆Vo is usually selected in the range of 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 losses

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

components, is:

ICrms

√

√2

⋅ I rms

⋅

Virms

− I o

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:

⋅ PO ⋅ tHold

V o_min

− V op_min

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.

Boost Inductor

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

First, the inductance value must be defined. The inductance (L) is usually determined so that the mini-

mum switching frequency is greater than the maximum frequency of the internal starter, to ensure a cor-

rect TM operation. Assuming unity PF, it is possible to write:

Ton

⋅ ILpk ⋅ sin(θ)

√2 ⋅ Virms ⋅ sin(θ)

⋅ ILpk

√2 ⋅ Virms

Toff

⋅ ILpk ⋅ sin(θ)

− √2 ⋅ Virms ⋅ sin(θ)

AN966 APPLICATION NOTE

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