STCMB1 datasheet(24/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

24/59

DocID028958 Rev 4

7.8

PFC: CTON calculation

Figure 11 shows how the PFC on-time is defined by the PWM comparator, as explained in

Section 7.7: PFC: error amplifier, regulation loop and VFF.

Figure 11. PFC on-time definition

In detail, CTON must be larger than the maximum on-time TON_MAX required to carry the

maximum power demanded by the load of the PFC stage with the minimum specified ac

voltage. The TON_MAX is a known parameter once the power stage has been designed, in

fact:

Equation 11

The maximum programmable on-time is achieved when the output of the error amplifier is

saturated high. This must occur even when the saturation voltage spreads at its minimum

value (VCOMPSAT_MIN = 4.2 V for a consumer temperature range design, 4 V for an

extended temperature range design).

Therefore, since during the on-time (GD_PFC high) the current generator ITON raises the

voltage V(TON) across the capacitor CTON, it is possible to find the minimum CTON value

according to the following relationship:

Equation 12

Since this relationship is referred to the minimum input voltage, the appropriate value of

ITON_MAX must be used: ITON = 200 µA (ITON_MAX = 220 µA) in US, Japan, or wide-range

mains applications, ITON = 800 µA (ITON_MAX = 960 µA) in European mains applications

(these value are reported in Table 4 on page 11).

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