STMicroelectronics 19 V - 75 W SMPS using the L6563 and the L6566A EVL6566A-75WES4 EVL6566A-75WES4 Data Sheet
Product codes
EVL6566A-75WES4
Application information
L6563S
28/43
Doc ID 16116 Rev 4
Figure 39.
THD optimization: standard TM PFC controller (left side) and L6563S (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 “Voltage Feedforward” section) 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.
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 “Voltage Feedforward” section) 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
, 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 - thus reducing the effectiveness
of the optimizer circuit.
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 - thus reducing the effectiveness
of the optimizer circuit.
6.5
Tracking boost function
In some applications it may be advantageous to regulate the output voltage of the PFC pre-
regulator so that it tracks the RMS input voltage rather than at a fixed value like in
conventional boost pre-regulators. This is commonly referred to as “tracking boost” or
“follower boost” approach.
regulator so that it tracks the RMS input voltage rather than at a fixed value like in
conventional boost pre-regulators. This is commonly referred to as “tracking boost” or
“follower boost” approach.
With the L6563S this can be realized by connecting a resistor (RT) between the TBO pin
and ground. The TBO pin presents a DC level equal to the peak of the MULT pin voltage and
is then representative of the mains RMS voltage. The resistor defines a current, equal to
and ground. The TBO pin presents a DC level equal to the peak of the MULT pin voltage and
is then representative of the mains RMS voltage. The resistor defines a current, equal to
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