Datasheet
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
14
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TYPICAL APPLICATION
I. PFC Boost Power Stage
LBOOST (L1 in Figure 9)
The boost inductor value is determined by the following equations
:
DI +
P
OUT
ǒ
0.25
Ǔ
2
Ǹ
h1 h2
V
IN (min)
,
D + 1 *
V
IN (min)
2
Ǹ
V
BOOST
,
L
BOOST
+
V
IN (min)
2
Ǹ
D
D I f
S
where ∆I, the inductor current ripple was set to approximately 25% of the peak inductor current.
In this design example ∆I is approximately 505 mA. D
represents the duty cycle at the peak of low line voltage,
V
IN(min)
is the minimum RMS input voltage, and V
BOOST
is the controlled output voltage of the PFC stage.
V
BOOST
for this design is selected to be 385 V to ensure the PFC stage regulates for the full input voltage range.
Variable f
S
represent the switching frequency. The switching frequency was selected to be 100 kHz for this
design. The calculated boost inductor required for this design is approximately 1.7 mH.
CBOOST (C2 in Figure 9)
Two main criteria, the capacitance and the voltage rating, dictate the selection of the output capacitor. The value
of capacitance is determined by the holdup time required for supporting the load after the input ac voltage is
removed. Holdup is the amount of time that the output stays in regulation after the input has been removed. For
this circuit, the desired holdup time is approximately 16 ms. Expressing the capacitor value in terms of output
power, output voltage, and holdup time is described in equation (4):
C
BOOST
+
2 P
OUT
D t
ǒ
V
BOOST
Ǔ
2
*
ǒ
V
BOOST (min)
Ǔ
2
In practice, the calculated minimum capacitor value may be inadequate because output ripple voltage
specifications limit the amount of allowable output capacitor ESR. Attaining a sufficiently low value of ESR often
necessitates the use of a much larger capacitor value than calculated. The amount of output capacitor ESR
allowed is determined by dividing the maximum specified output ripple voltage by the capacitor ripple current.
In this design, holdup time is the dominant determining factor and a 100 µF, 450 V aluminum electrolytic
capacitor from Panasonic, part number ECOS2TB101BA, is used. The voltage rating and the low ESR of
0.663 Ω make it an ideal choice for this design.
(1)
(2)
(3)
(4)