AN3142 datasheet(16/44 Pages) STMICROELECTRONICS

16 / 44 page

Designing a fixed-off-time PFC

AN3142

16/44

Doc ID 17005 Rev 3

Figure 8.

Switching frequency fixing the line voltage

Figure 9.

The effect of fixing off-time - boundary between DCM and CCM

The effect of fixing the off-time is generating a continuous conduction mode in the center

region of the line half-cycle between the two transition angles. Close to the zero-crossing,

the system works in discontinuous conduction mode and in transition mode at the boundary.

The inductor core size is determined assuming a peak flux density Bx ~0.25 T (depending

on the ferrite grade selected and relevant specific losses) and calculating the maximum

current according to

Equation 15 as a function of the maximum current sense pin clamping

voltage and sense resistor value.

DC and AC copper losses and ferrite losses must also be calculated to determine the

maximum temperature rise of the inductor.

Frequency modulation with the Line half period

100

1000

0.4

0.8

1.2

1.6

2.4

2.8

÷ [Linehalf period]

Switching Freq.@VacMin

Switching Freq.@VacMax

CCM

DCM

θ1

θ2

DCM

CCM

TOF F

θ1

Half Line Cycle

DCM

CCM

TOF F

θ1

Half Line Cycle

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