Test results and significant waveforms

AN2485

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2.2

Inductor current in FOT and L6563 THD optimizer

Figure 16, 17, 18, and 19 show the waveforms relevant to the inductor current at different

voltage mains. As shown in Figure 16. and Figure 18., the inductor current waveform over a

line half-period is very similar to that of a CCM PFC. It is also possible to note the transition

angle from DCM to CCM that occurs closer to the zero crossing of the current sine wave at

low mains and move toward the top if the circuit is working at high mains. In Figure 17. and

Figure 19. the magnification of the waveforms at the peak of the sine wave, shows the

different ripple current and Off-times, which is modulated by the input mains voltage.

On both the drain voltage traces reported in Figure 16. and Figure 18., close to the zero

crossing points of the sine wave, it is possible to note the action of the THD optimizer

embedded in the L6563. It is a circuit that minimizes 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.

The result is both a minimization of the time interval where energy transfer is lacking and a

full discharge of 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 the sinusoid. The offset is modulated by the voltage on

Figure 16.

EVAL6563-400W Inductor current

ripple envelope at 115 Vac - 60 Hz -

full load

Figure 17.

EVAL6563-400W Inductor current

ripple (detail) at 115 Vac - 60 Hz -

full load

CH1: Q1/Q2 Drain voltage

CH2: MULT voltage - pin #3

CH4: L4 inductor current ripple envelope

CH1: Q1/Q2 Drain voltage

CH2: MULT voltage - pin #3

CH4: L4 inductor current ripple envelope