AN4149 datasheet(8/43 Pages) STMICROELECTRONICS

8 / 43 page

CCM PFC using FOT control

AN4149

8/43

DocID023523 Rev 2

However, the frequency rise is significantly lower as compared to that of a transition-mode

(TM) operated boost PFC stage, as illustrated in Figure 7 and 8 .

Due to the peak current vs. average current error which becomes much worse at low current

levels (especially in DCM), the current that the boost PFC pre-regulator draws from the

power line will not be exactly sinusoidal but will be affected by a distortion that will be lower

as the current ripple in the boost inductor is smaller as compared to its peak value.

Figure 9 and 10 show some theoretical waveforms, relevant to full load condition, in a line

cycle at different input voltages.

In Figure 9 the line (input) current waveform is shown for different line voltages, while

Figure 10 illustrates the envelope of the inductor current at minimum and maximum line

voltage.

The input current waveform relevant to Vin = 88 Vac shows no visible sign of distortion; the

operation of the boost inductor is CCM throughout the entire line cycle as shown by the

Figure 7. Typical frequency change along

a line half-cycle in a boost PFC operated

in LM FOT

Figure 8. Typical frequency change along

a line half-cycle in a boost PFC operated

in TM

Figure 9. LM FOT controlled boost PFC:

current waveforms (line current)

Figure 10. LM FOT controlled boost PFC:

current waveforms (boost inductor

current envelope)

AM13306v1

0.52

1.05

1.57

2.09

2.62

3.14

0.4

0.6

0.8

1.2

1.4

1.6

Vin=88 Vac

Vin=264 Vac

Vin=230 Vac

Line voltage phase angle (rad)

Vin=115 Vac

AM13307v1

0.52

1.05

1.57

2.09

2.62

3.14

Vin=88 Vac

Vin=264 Vac

Vin=230 Vac

Line voltage phase angle (rad)

Vin=115 Vac

AM13308v1

Line voltage phase angle (rad)

Vin = 88 Vac

Vin = 264 Vac

Vin = 230 Vac

Vin = 190 Vac

Line current

rad)

Vin = 88 Vac

Vin = 264 Vac

Vin = 230 Vac

Vin = 190 Vac

Line current

AM13309v1

Vin = 88 Vac

Vin = 264 Vac

Boost inductor

current envelope

Line voltage phase angle (rad)

Vin = 88 Vac

Vin = 264 Vac

Boost inductor

current envelope

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