AN4149 datasheet(9/43 Pages) STMICROELECTRONICS

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DocID023523 Rev 2

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AN4149

CCM PFC using FOT control

inductor current envelope. The brown waveform is relevant to Vin=190 Vac, which is the

condition where CCM operation no longer occurs at zero-crossings (this voltage value, for a

given power level, depends on the inductance value of the boost inductor); a certain degree

of distortion is already visible. The waveform relevant to Vin = 264 Vac shows the highest

degree of distortion and the largest portion of the line cycle where the boost inductor

operates in discontinuous mode (DCM). However, its harmonic content, shown in Figure 9

and 10, is still so low that is not an issue for EMC compliance. Almost all the distortion is

concentrated in the third harmonic, whose amplitude is 17% of the fundamental one, while

the THD is 17.7%.

Figure 11. Line-modulated, FOT controlled boost PFC: input current harmonic

contents

The compliance with conducted EMI emission regulations is also facilitated by the voltage

ripple appearing across the output capacitor CO , at twice the line frequency fL, which has

peak amplitude

ΔVout proportional to the output current IOUT. As a consequence, fsw is not

constant but is modulated at 2fL, spreading the spectrum of the electrical noise injected

back into the power line. The relative frequency change due to the output voltage ripple is:

Equation 11

where

ΔVout can be found with the following expression:

Equation 12

AM13310v1

Vin = 264Vac

THD = 17.7%

Harmonic order (n)

Vin = 264Vac

THD = 17.7%

Harmonic order (n)

Vout

Cout

Iout

Vout

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