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AN3142 Datasheet(PDF) 14 Page - STMicroelectronics

Part # AN3142
Description  Solution for designing a 400 W fixed-off-time controlled
PDF  44 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3142 Datasheet(HTML) 14 Page - STMicroelectronics

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Designing a fixed-off-time PFC
AN3142
14/44
Doc ID 17005 Rev 3
4.3.3
Output capacitor
The output bulk capacitor (Co) selection depends on the DC output voltage
(4), the allowed
maximum voltage
(7), and the converter output power (3).
The 100/120 Hz (twice the mains frequency) voltage ripple (
∆Vout = peak-to-peak ripple
value)
(8) is a function of the capacitor impedance and the peak capacitor current:
Equation 22
With a low ESR capacitor the capacitive reactance is dominant, therefore:
Equation 23
Vout is usually selected in the range of 1.5% of the output voltage. Although ESR does not
usually 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:
Equation 24
If the PFC stage must guarantee a specified holdup time, the selection criterion of the
capacitance changes. Co has to deliver the output power for a certain time (tHold) with a
specified maximum dropout voltage (Vout min) that is the minimum output voltage value
(which takes load regulation and output ripple into account). Vout min is the minimum output
operating voltage before the 'power fail' detection and consequent stopping by the
downstream system supplied by the PFC.
Equation 25
A 20% tolerance on the electrolytic capacitors must be taken into account for the right
dimensioning.
Following the relationship (
Equation 25), for this application a capacitor Co = 330 µF (450 V)
has been selected in order to maintain a holdup capability for 22 ms. The actual output
voltage ripple with this capacitor is also calculated. In detail:
2
2
O
l
out
out
ESR
)
C
f
2
2
(
1
I
2
V
+
⋅
⋅
π
⋅
⋅
=
∆
out
out
l
out
out
l
out
O
V
V
f
2
P
V
f
2
I
C
∆
⋅
⋅
⋅
π
=
∆
⋅
⋅
π
≥
F
338
V
10
V
400
Hz
47
2
W
400
C
O
µ
=
⋅
⋅
⋅
π
≥
2
out
rms
2
Crms
I
ID
I
−
=
() (
)
A
36
.
2
A
0
.
1
A
56
.
2
I
2
2
Crms
=
−
=
() 2
min
out
2
out
out
Hold
out
O
V
V
V
t
P
2
C
−
∆
−
⋅
⋅
=
() (
)
F
3
.
242
V
300
V
10
V
400
ms
20
W
400
2
C
2
2
O
µ
=
−
−
⋅
⋅
=



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