Datasheet

A
DODET LOSS
B
AC _ DO
R
V 1 V
R
V
2
æ ö
+ +
ç ÷
è ø
=
A
DOCLR LOSS
B
DO _ CLR
R
V 1 V
R
V
2
æ ö
+ +
ç ÷
è ø
=
UCC28063
www.ti.com SLUSAO7 –SEPTEMBER 2011
Dropout Detection
It is often the case that the ac-line voltage momentarily drops to zero or nearly zero, due to transient abnormal
events affecting the local ac power distribution network. Referred to as ac-line dropouts (or sometimes as
line-dips) the duration of such events usually extends to only 1 or 2 line cycles. During a dropout, the
down-stream power conversion stages depend on sufficient energy storage in the PFC output capacitance, which
is sized to provide the ride-through energy for a specified hold-up time. Typically while the PFC output voltage is
falling, the voltage-loop error amplifier output rises in an attempt to maintain regulation. As a consequence,
excess duty-cycle is commanded when the ac-line voltage returns and high peak current surges may saturate
the boost inductors with possible overstress and audible noise.
The UCC28063 incorporates a dropout detection feature which suspends the action of the error amplifier for the
duration of the dropout. If the VINAC voltage falls below 0.35 V for longer than 5 ms, a dropout condition is
detected and the error amplifier output is turned off. In addition, a 4-μA pull-down current is applied to COMP to
gently discharge the compensation network capacitors. In this way, when the ac-line voltage returns, the COMP
voltage (and corresponding duty-cycle setting) remains very near or even slightly below the level it was before
the dropout occurred. Current surges due to excess duty-cycle, and their undesired attendant effects, are
avoided. The dropout condition is cancelled and the error amplifier resumes normal operation when VINAC rises
above 0.71 V.
Based on the VINAC divider-resistor values calculated for brownout in the previous section, the input RMS
voltage thresholds for dropout detection V
AC_DO
and dropout clearing V
DO_CLR
can be determined using
Equation 12 and Equation 13, below.
(12)
(13)
Avoid excessive filtering of the VINAC signal, or dropout detection may be delayed or defeated. An RC
time-constant of ≤ 100-μs should provide good performance. An example of the timing for the dropout function is
illustrated in Figure 32.
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