Datasheet

L6563 - L6563A Application information
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6.4 THD optimizer circuit
The L6563/A is provided with a special circuit that reduces the conduction dead-angle
occurring to the AC input current near the zero-crossings of the line voltage (crossover
distortion). In this way the THD (Total Harmonic Distortion) of the current is considerably
reduced.
A major cause of this distortion is the inability of the system to transfer energy effectively
when the instantaneous line voltage is very low. This effect is magnified by the high-
frequency filter capacitor placed after the bridge rectifier, which retains some residual
voltage that causes the diodes of the bridge rectifier to be reverse-biased and the input
current flow to temporarily stop.
To overcome this issue the device forces the PFC pre-regulator to process more energy
near the line voltage zero-crossings as compared to that commanded by the control loop.
This will result in both minimizing the time interval where energy transfer is lacking and fully
discharging the high-frequency filter capacitor after the bridge.
Figure 37 shows the internal block diagram of the THD optimizer circuit.
Figure 37. THD optimizer circuit
+
+
MULT
COMP
t
@ Vac1
@ Vac2 > Vac1
t
t
to PWM
comparator
MULTIPLIER
OFFSET
GENERATOR
t
VFF
1 / V
2
t
t
+
+
MULT
COMP
t
@ Vac1
@ Vac2 > Vac1
t
t
to PWM
comparator
MULTIPLIERMULTIPLIER
OFFSET
GENERATOR
OFFSET
GENERATOR
t
VFF
1 / V
2
1 / V
2
t
t