Datasheet

LOAD cond _ %
BUS
2
BUS BUS(%) AC
P
C
V V 2 f
´ q
=
´ D ´ ´
BUS
2
90
0.57
0.965
C 628 F
84 0.12 2 50
´
= = m
´ ´ ´
( )
HOLDUP LOAD
BUS
2 2
BUS(min) BUS(min_ reg)
T 2 P
C
V V
´ ´
=
-
( )
( )
BUS
2
2
90
0.003 2
0.965
C 381 F
84 0.95 70
´ ´
= = m
´ -
UCC29910A
www.ti.com SLUSAK8A – MAY 2011– REVISED JUNE 2011
Bulk Capacitor Choice
The value of bulk capacitance required is dictated by requirements on the allowable ripple voltage and hold-up
time. In applications where hold-up time is not the main factor, then the capacitors should be sized for
approximately 12% peak-to-peak ripple as follows:
(4)
For a typical 90-W adapter using buck PFC, the required bus capacitance for ±6% maximum ripple at 90 VAC/50
Hz (12% total ripple) and at 84 VDC bus, assuming 96.5% efficiency of the second stage, would be:
(5)
where:
• P
LOAD
: load power drawn (usually by the second regulation/isolation stage)
• C
BUS
: bus capacitance
• θ
COND_%
: conduction angle at AC line of interest (as decimal percentage of total cycle, e.g. 50% conduction
angle expressed as 0.5)
• f
AC
: AC line frequency
The capacitance required to achieve a specific hold up time may be calculated as follows:
(6)
For example, in order to achieve 3-ms holdup, with nominal bus voltage of 84 VDC, ±5% maximum bus ripple,
and 70-VDC minimum bus regulation level for the second stage, the required bus capacitance would be
calculated as follows for a 90-W load, assuming 96.5% second stage efficiency:
(7)
References
1. 2009/10 Power Supply Design Seminar - SEM1900 Topic 4, Power Factor Correction Using the Buck
Topology – Efficiency Benefits and Practical Design Considerations
REVISION HISTORY
Changes from Original (May 2011) to Revision A Page
• Added New DESCRIPTION .................................................................................................................................................. 1
• Added an updated description for the TST pin. .................................................................................................................... 4
• Added an updated description for the TST pin. .................................................................................................................... 6
• Changed UCC29910A Functional Block Diagram ................................................................................................................ 6
Copyright © 2011, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Link(s): UCC29910A