AN3065

Test results and significant waveforms

Doc ID 16279 Rev 2

13/33

is in phase with the input AC voltage, giving low distortion of the current waveform and high

power factor. On both the drain voltage traces, close to the zero-crossing points of the sine

wave, it is possible to note the action of the THD optimizer embedded in the L6563S. It is a

circuit that minimizes the conduction dead-angle of 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. Essentially, the circuit artificially increases the ON-

time of the power switch with a positive offset added to the output of the multiplier in the

proximity of the line voltage zero-crossings. This offset is reduced as the instantaneous line

voltage increases, so that it becomes negligible as the line voltage moves toward the top of

have little offset at low line, where energy transfer at zero-crossings is typically quite good,

and a larger offset at high line where the energy transfer gets worse.

To obtain maximum benefit from the THD optimizer circuit, the high-frequency filter

capacitors after the bridge rectifier should be minimized, compatibly with EMI filtering needs.

A large capacitance, in fact, introduces a conduction dead-angle of the AC input current in

itself, thus reducing the effectiveness of the optimizer circuit.

In

Figure 13 and 15 the detail of the waveforms at switching frequency allows measuring the

operating frequency and the current peak at top of the input sine wave during operation at

100 Vac and 230 Vac. The multiplier waveform has been captured as a reference.

Figure 12.

EVL6563H 100W TM PFC: Vds and

inductor current at 100 Vac, 50 Hz,

full load

Figure 13.

EVL6563H 100W TM PFC: Vds and

inductor current at 100 Vac, 50 Hz,

full load (detail)

CH1: Q1 drain voltage

CH2: MULT voltage - Pin #3

CH4: L2 inductor current

CH4: Q1 drain voltage

CH2: MULT voltage - pin #3

CH1: L2 inductor current