L6563_07 datasheet(21/39 Pages) STMICROELECTRONICS

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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

@ Vac1

@ Vac2 > Vac1

to PWM

comparator

MULTIPLIER

OFFSET

GENERATOR

VFF

1 / V

MULT

COMP

@ Vac1

@ Vac2 > Vac1

to PWM

comparator

MULTIPLIER

OFFSET

GENERATOR

OFFSET

GENERATOR

VFF

1 / V

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