AN-6026 Datasheet, PDF (6/23 Pages) Fairchild Semiconductor – Design of Power Factor Correction Circuit

AN6026
leading current, which causes phase difference between the
line current and the line voltage. The phase difference
increases as the capacitance of the input capacitor increases.
Therefore, the input capacitor must be smaller than C_in(max)
calculated by Equation 12. C_in(max) is the sum of all the
capacitors connected at the input side.
V_a = V_A = V_{in(peak )} cos(ωt )
(9)
i_a = I_a cos(ωt )
i_A = i_a + i_c = I_a cos(ωt ) − ω ⋅ C_in ⋅V_{in(peak )} sin(ωt ) (10)
APPLICATION NOTE
4) Output Capacitor Design
The output capacitor is selected by the relationship between
the input and output power. As shown in Figure 13, the min-
imum output capacitance is determined by Equation 14.
I_in
I_D
I_O
PFC
+
+
V_in
C_O
V_O
−
−
Figure 12. PFC Configuration
θ
=
tan^−1
⎛
⎜⎜⎝
ω
⋅ C_in
⋅V_{in(peak )}
I_a
⎞
⎟⎟⎠
(11)
( ) ^Cin(max)
=
I_a
ω ⋅V_{in( peak )}
tan
cos^−1(IDF )
( ) ⁼
ω
2 ⋅V_o ⋅ I_o
⋅V_in²⁽_{peak _ max)}
tan
cos^−1(IDF )
(12)
( ) P_in = I_in(rms) ⋅V_in(rms) ⋅ 1− cos(2ωt ) = I_DV_o
( ) I_D
=
I_in(rms) ⋅V_in(rms)
V_o
1− cos(2ωt )
= I_o ⋅ (1− cos(2ωt ))
(13)
I_D(avg) = I_O (1- cos(2ωt))
i_A
+
V_A
−
L_in
i_a
i_{C +}
C_in
V_a
−
Input Filter
Im
i_C
PFC
Circuit
i_A
I_O
ΔV_O
=
I_O
ωC_O
V_O
Figure 13. Diode Current and Output Voltage Waveform
C_o(min)
≥
2π
I_o(max)
⋅ f_ac ⋅ ΔV_o(max)
(14)
θ
i_a
Re
V_A
Figure 11. Input Voltage and Current Displacement Due to
Input Filter Capacitance
5) MOSFET and Diode Selection
The maximum MOSFET RMS current is obtained by Equa-
tion 15 and the conduction loss of the MOSFET is calculated
by Equation 16. When MOSFET turns on, the MOSFET cur-
rent rises from zero, so the turn-on loss is negligible. The
MOSFET turn-off loss and the MOSFET discharge loss are
obtained by Equations 17 and 18, respectively. The switch-
ing frequency of the critical conduction mode boost PFC
converter varies according to the line and load conditions.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.4 • 4/25/09
6
www.fairchildsemi.com