UCC28180
SLUSBQ5A –NOVEMBER 2013–REVISED NOVEMBER 2013
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Bridge Rectifier
The input bridge rectifier must have an average current capability that exceeds the input average current.
Assuming a forward voltage drop, VF_BRIDGE, of 1 V across the rectifier diodes, BR1, the power loss in the input
bridge, PBRIDGE, can be calculated:
P
= 2VF _ BRIDGEIIN_ AVG(max)
BRIDGE
(14)
(15)
P
= 2´1V ´ 4.097 A = 8.195W
BRIDGE
Heat sinking will be required to maintain operation within the bridge rectifier’s safe operating area.
Inductor Ripple Current
The UCC28180 is a Continuous Conduction Mode (CCM) controller but if the chosen inductor allows relatively
high-ripple current, the converter will be forced to operate in Discontinuous Mode (DCM) at light loads and at the
higher input voltage range. High-inductor ripple current has an impact on the CCM/DCM boundary and results in
higher light-load THD, and also affects the choices for the input capacitor, RSENSE and CICOMP values. Allowing an
inductor ripple current, ΔIRIPPLE, of 20% or less will result in CCM operation over the majority of the operating
range but requires a boost inductor that has a higher inductance value and the inductor itself will be physically
large. As with all converter designs, decisions must be made at the onset in order to optimize performance with
size and cost. In this design example, the inductor is sized in such a way as to allow a greater amount of ripple
current in order to minimize space with the understanding that the converter operates in DCM at the higher input
voltages and at light loads but optimized for a nominal input voltage of 115 VAC at full load. Although specifically
defined as a CCM controller, the UCC28180 is shown in this application to meet the overall performance goals
while transitioning into DCM at high-line voltage, at a higher load level.
Input Capacitor
The input capacitor must be selected based upon the input ripple current and an acceptable high frequency input
voltage ripple. Allowing an inductor ripple current, ΔIRIPPLE, of 40% and a high frequency voltage ripple factor,
ΔVRIPPLE_IN, of 7%, the maximum input capacitor value, CIN, is calculated by first determining the input ripple
current, IRIPPLE, and the input voltage ripple, VIN_RIPPLE
:
I
RIPPLE= DIRIPPLEIIN(max)
(16)
(17)
(18)
(19)
(20)
DIRIPPLE = 0.4
RIPPLE= 0.4 ´ 6.436 A = 2.575 A
= DVRIPPLE _IN
I
V
V
IN_ RECTIFIED(min)
IN_ RIPPLE
DVRIPPLE _IN = 0.07
V
=
2V
IN
IN_ RECTIFIED
(21)
V
=
2 ´85 V = 120 V
IN_ RECTIFIED
(22)
(23)
V
= 0.07 ´120 V = 8.415 V
IN_ RIPPLE
The recommended value for the input x-capacitor can now be calculated:
I
RIPPLE
CIN
=
8fSW
V
IN_ RIPPLE
(24)
(25)
2.575 A
CIN
=
= 0.324mF
A standard value 0.33-µF Y2/X2 film capacitor is used.
26
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