Test results and significant waveforms

AN2755

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2.2

Inductor current in FOT and L6562A THD optimizer

The following figures show the waveforms relevant to the inductor current at different voltage

mains. As visible in Figure 17 and Figure 19, the inductor current waveform over a line half-

period is very similar to that of a CCM PFC. In Figure 18 and Figure 20 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. We can also note the transition angle from

DCM to CCM that occurs closer to the zero-crossing of the current sine wave at low mains

and moves toward the top if the circuit is working at high mains.

On both the drain voltage traces shown in Figure 17 and Figure 19, close to the zero-

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

embedded in the L6562A. 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

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 the sinusoid.

Figure 17.

EVL6562A-400W inductor current

ripple envelope at 115 Vac - 60 Hz -

full load

Figure 18.

EVL6562A-400W inductor current

ripple (detail) at 115 Vac - 60 Hz -

full load

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CH1: Q1/Q2 drain voltage

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CH2: I L inductor current ripple envelope

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CH3: MULT voltage - pin #3

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CH1: Q1/Q2 drain voltage

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CH2: I L inductor current ripple envelope

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CH3: MULT voltage - pin #3