UCC28180
SLUSBQ5A –NOVEMBER 2013–REVISED NOVEMBER 2013
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Output Capacitor
The output capacitor, COUT, is sized to meet holdup requirements of the converter. Assuming the downstream
converters require the output of the PFC stage to never fall below 300 V, VOUT_HOLDUP(min), during one line cycle,
tHOLDUP = 1/fLINE(min), the minimum calculated value for the capacitor is:
2POUT(max)tHOLDUP
VO2UT - VO2UT _ HOLDUP(min)
COUT(min)
³
(56)
(57)
2´360W ´ 21.28ms
COUT(min)
³
³ 247mF
It is advisable to de-rate this capacitor value by 10%; the actual capacitor used is 270 µF.
Verifying that the maximum peak-to-peak output ripple voltage will be less than 5% of the output voltage ensures
that the ripple voltage will not trigger the output over-voltage or output under-voltage protection features of the
controller. If the output ripple voltage is greater than 5% of the regulated output voltage, a larger output capacitor
is required. The maximum peak-to-peak ripple voltage, occurring at twice the line frequency, and the ripple
current of the output capacitor is calculated:
VOUT _ RIPPLE(pp) < 0.05 VOUT
(58)
VOUT _ RIPPLE(pp) < 0.05 ´390 V = 19.5 VPP
(59)
IOUT
VOUT _ RIPPLE(pp)
=
2p(2fLINE(min))COUT
(60)
(61)
0.923A
VOUT _ RIPPLE(pp)
=
= 5.789 V
2p(2´ 47Hz)´ 270mF
The required ripple current rating at twice the line frequency is equal to:
IOUT(max)
ICOUT _ 2fline
=
2
0.923 A
2
(62)
(63)
ICOUT _ 2fline
=
= 0.653 A
There is a high frequency ripple current through the output capacitor:
16 VOUT
ICOUT _ HF = IOUT(max)
-1.5
3pV
IN_ RECTIFIED(min)
(64)
(65)
16 ´390 V
3p ´120 V
ICOUT _ HF = 0.923 A
-1.5 = 1.848 A
The total ripple current in the output capacitor is the combination of both and the output capacitor must be
selected accordingly:
I
COUT _ RMS(total)= IC2OUT _ 2fline + IC2OUT _ HF
(66)
(67)
I COUT _ RMS(total)
=
0.653 A2 +1.848 A2 = 1.96 A
30
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