L6564T Datasheet, PDF (23/33 Pages) STMicroelectronics – 10-pin transition-mode PFC controller

L6564T
Application information
Figure 31. THD optimization: standard TM PFC controller (left side) and L6564T (right side)
Input current
Input current
Rec tified mains voltage
Rectified mains voltage
I^In^mpu^atⁱⁿcu^srrent
MOSFET's draVindvroaltiange
In^Impu^at ⁱⁿcu^srrent
MOSFET's draiVn dvorlataigne
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.
Doc ID 022671 Rev 1
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