L6564T

Application information

Doc ID 022671 Rev 1

23/33

Figure 31.

THD optimization: standard TM PFC controller (left side) and L6564T (right side)

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. Furthermore, the offset is

modulated by the voltage on the VFF pin (see

Section 6.3) so as to 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.

The effect of the circuit is shown in

Figure 31, where the key waveforms of a standard TM

PFC controller are compared to those of this chip.

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

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 -

even with an ideal energy transfer by the PFC pre-regulator - therefore reducing the

effectiveness of the optimizer circuit.

6.5

Inductor saturation detection

The boost inductor's hard saturation may be a fatal event for a PFC pre-regulator: the

current up-slope becomes so large (50-100 times steeper, see

Figure 32) that during the

current sense propagation delay the current may reach abnormally high values. The voltage

drop caused by this abnormal current on the sense resistor reduces the gate-to-source

voltage, so that the MOSFET may work in the active region and dissipate a huge amount of

power, which leads to a catastrophic failure after few switching cycles.

Imains

Vdrain

Imains

Vdrain

Input current

Input current

MOSFET's drain voltage

MOSFET's drain voltage

Rectified mains voltage

Rectified mains voltage

Input current

Input current