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AN2782 Datasheet(PDF) 13 Page - STMicroelectronics

Part # AN2782
Description  Solution for designing a 400 W fixed-off-time controlled
PDF  39 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2782 Datasheet(HTML) 13 Page - STMicroelectronics

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AN2782
Designing a fixed off-time PFC
Doc ID 14763 Rev 2
13/39
4.3.2
Input capacitor
The input filter capacitor, Cin, is placed across the diode bridge output. This capacitor must
smooth the high-frequency ripple and must sustain the maximum instantaneous input
voltage. In a typical application an EMI filter is placed between the mains and the PFC
circuit. In this application the EMI filter is reinforced by a differential mode Pi-filter after the
bridge to reject the differential noise coming from the whole switching circuit.
The design of the EMI filter (common mode and differential mode) is not described here.
The value of the input filter capacitor can be calculated as follows, simply considering the
output power that the PFC should deliver at full load:
The maximum value of this capacitor is limited to avoid line current distortion. The value
chosen for this demonstration board is 1 µF.
4.3.3
Output capacitor
The output bulk capacitor (Co) selection depends on the DC output voltage (11), the allowed
overvoltage (14), and the converter output power (10).
The 100/120 Hz (twice the mains frequency) voltage ripple (
ΔVout = (Vout = peak-to-peak
ripple value) (15) is a function of the capacitor impedance and the peak capacitor current:
With a low ESR capacitor the capacitive reactance is dominant, therefore:
ΔVout is usually selected in the range of 1.5% of the output voltage.
Although ESR usually does not affect the output ripple, it should be taken into account for
power loss calculations. The total RMS capacitor ripple current, including mains frequency
and switching frequency components, is:
(33)
W
53
.
7
A
25
.
2
V
7
.
0
4
)
A
53
.
3
(
025
.
0
4
P
2
bridge
=
⋅
⋅
+
⋅
Ω
⋅
=
avg
_
in
th
inrms
2
diode
bridge
I
V
4
I
R
4
P
⋅
⋅
+
⋅
⋅
=
(34)
out
3
in
P
10
5
.
2
C
⋅
⋅
=
−
F
1
W
400
10
5
.
2
C
3
in
μ
=
⋅
⋅
=
−
(35)
(36)
(37)
2
2
O
l
out
out
ESR
)
C
f
2
2
(
1
I
2
V
+
⋅
⋅
π
⋅
⋅
=
Δ
out
out
l
out
out
l
out
O
V
V
f
2
P
V
f
2
I
C
Δ
⋅
⋅
⋅
π
=
Δ
⋅
⋅
π
≥
F
338
V
10
V
400
Hz
47
2
W
400
CO
μ
=
⋅
⋅
⋅
π
≥
2
out
rms
2
Crms
I
ID
I
−
=
() (
)
A
36
.
2
A
0
.
1
A
56
.
2
I
2
2
Crms
=
−
=



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