AN3063 Datasheet, PDF (15/33 Pages) STMicroelectronics – 100 W transition-mode PFC pre-regulator with the L6563H

AN3063
Test results and significant waveforms
Figure 16. EVL6563H-100W TM PFC: Vds and Figure 17. EVL6563H-100W TM PFC: Vds and
inductor current at 100 Vac, 50 Hz,
inductor current at 230 Vac, 50 Hz,
full load
full load
CH1: GD - pin #15
CH2: ZCD - pin #13
CH3: CS - pin #4
CH4: L2 inductor current
CH1: GD - pin #15
CH2: ZCD - pin #13
CH3: CS - pin #4
CH4: L2 inductor current
4.3
Voltage feed-forward and brownout function
The power stage gain of PFC pre-regulators varies with the square of the RMS input voltage
as well as the crossover frequency fc of the overall open-loop gain because the gain has a
single pole characteristic. This leads to large trade-offs in the design. For example, setting
the gain of the error amplifier to get fc = 20 Hz at 264 Vac means having f_c ≈ 4 Hz at 88
Vac, resulting in sluggish control dynamics. Additionally, the slow control loop causes large
transient current flow during rapid line or load changes that are limited by the dynamics of
the multiplier output. This limit is considered when selecting the sense resistor to let the full
load power pass under minimum line voltage conditions, with some margin. A fixed current
limit allows excessive power input at high line, whereas a fixed power limit requires the
current limit to vary inversely with the line voltage.
Voltage feed-forward can compensate for the gain variation with the line voltage and allow
overcoming all of the above-mentioned issues. It consists of deriving a voltage proportional
to the input RMS voltage, feeding this voltage into a squarer/divider circuit (1/V² corrector)
and providing the resulting signal to the multiplier that generates the current reference for
the inner current control loop.
In this way, a change of the line voltage causes an inversely proportional change of the half
sine amplitude at the output of the multiplier (if the line voltage doubles, the amplitude of the
multiplier output is halved and vice versa) so that the current reference is adapted to the new
operating conditions with (ideally) no need for invoking the slow dynamics of the error
amplifier. Additionally, the loop gain is constant throughout the input voltage range, which
improves significantly dynamic behavior at low line and simplifies loop design.
Actually, with other PFCs embedding the voltage feed-forward, deriving a voltage
proportional to the RMS line voltage implies a form of integration, which has its own time
constant. If it is too small, the voltage generated will be affected by a considerable amount of
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