AN2761 Datasheet, PDF (22/36 Pages) STMicroelectronics – Solution for designing a transition mode

Designing a TM PFC
AN2761
Figure 11. Multiplier characteristics family
Multiplier characteristic
1.2
1.1 ^{Upper Volt. Clamp}
1.0
5.75 V
0.9
5V
0.8
4.5V
0.7
V COMP (pin2) (V)
4V
3.5V
0.6
0.5
3V
0.4
0.3
0.2
0.1
2.5 V
0.0
-0.1
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
VMULT (pin3) (V)
A complete description is given in Figure 11, which shows the typical multiplier
characteristics family. The linear operation of the multiplier is guaranteed within the range 0
to 3 V of V_MULT and the range 0 to 1.16 V (typ) of V_cs, while the minimum guaranteed value
of the maximum slope of the characteristics family (typ) is:
dV_{CS = 1.1V}
(61)
dV_MULT
V
Taking this into account, the following is the suggested procedure to properly set the
operating point of the multiplier.
First, the maximum peak value for V_MULT, V_MULTmax is selected. This value, which occurs at
maximum mains voltage, should be 3 V or nearly so in wide-range mains and less in case of
single mains. The sense resistor selected is R_s = 0.34 Ω and it is described in the paragraph
concerning pin 4 of this section. The maximum peak value, occurring at maximum mains
voltage is:
VMULT_max
=
IL_pk ⋅R_s
1.1
⋅
VAC_max
VAC_min
VMULT_max
=
2.89A ⋅ 0.34Ω
1.1
⋅
265Vac
85Vac
=
3.06V
(62)
where IL_pk and R_s have been already calculated, and 1.1 V/V is the multiplier maximum
slope reported in the datasheet.
From (62) the maximum required divider ratio is calculated as:
k_p =
V_{MULT max =}
2 ⋅ VAC_max
3.06V _{= 8.16 ⋅10−3}
2 ⋅ 265Vac
(63)
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Doc ID 14690 Rev 2