L6564T datasheet(22/33 Pages) STMICROELECTRONICS

22 / 33 page

Application information

L6564T

22/33

Doc ID 022671 Rev 1

6.4

THD optimizer circuit

The L6564T 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 results in both minimizing the time interval where energy transfer is lacking and fully

discharging the high-frequency filter capacitor after the bridge.

Figure 30 shows the internal block diagram of the THD optimizer circuit.

Figure 30.

THD optimizer circuit

L6564T Datasheet(PDF) 22 Page - STMicroelectronics