8.3.12 Line 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 UCC28064A 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 17 and Equation 18, below.

Equation 17.

Equation 18.

Avoid excessive filtering of the VINAC signal, or dropout detection may be delayed or defeated. An RC time-constant of ≤ 100 s. should provide good performance. Figure 29 shows an example of the timing for the dropout function.

Figure 28. AC-Line Brownout Timing and System Behavior

Figure 29. AC-Line Dropout Timing With Illustrative System Behavior