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.

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 7-8.