AN3027 datasheet(11/41 Pages) STMICROELECTRONICS

11 / 41 page

AN3027

Designing a TM PFC

Doc ID 16134 Rev 4

11/41

3.3.2

Input capacitor

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

high-frequency inductor current ripple (twice the average line current,

Figure 3).

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

high-frequency voltage ripple across Cin is usually imposed between 5% and 20% of the

minimum rated input voltage. This is expressed by a coefficient r (= 0.05, 0.2) as an input

design parameter:

Equation 12

In real conditions the input capacitance is designed to take the EMI filter into account and to

have a tolerance on the component of about 5% -10% (typ. for polyester capacitors).

A commercial capacitor of Cin = 0.47 µF has been selected. Of course a bigger capacitor

benefits the EMI but hurts the THD, especially at high mains. Therefore a compromise must

be found between these two parameters. A good quality film capacitor for this component

must be selected in order to have an effective filter.

3.3.3

Output capacitor

The selection of the output bulk capacitor (Co) depends on the DC output voltage

(4), the

allowed maximum output voltage

(7) and the converter output power (3).

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

∆Vout = peak-to-peak ripple

value) is a function of the capacitor impedance and the peak capacitor current:

Equation 13

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

Equation 14

∆Vout is usually selected in the range of 1.5% of the output voltage. Although ESR usually

does not affect the output ripple, it should be taken into account for calculating the power

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

frequency components, is:

Equation 15

●

Ripple voltage coefficient (%):

(13)

min

VAC

⋅

359

Vac

kHz

⋅

out

ESR

)

(

⋅

∆

out

MAINS

out

∆

⋅

∆

⋅

≥

400

100

⋅

≥

out

rms

Crms

−

() (

)

Crms

−

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