AN966 Datasheet, PDF (7/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

Figure 8. Output Driver.
VCC
8
DRIVER
AN966 APPLICATION NOTE
7 GD
Q
UVLO
6
GND
D97IN677
TM PFC Operation (Boost Topology)
The operation of the PFC Transition Mode controlled boost converter, can be summarized in the follow-
ing description.
The AC mains voltage is rectified by a diode bridge and the rectified voltage delivered to the boost con-
verter. This, using a switching technique, boosts the rectified input voltage to a regulated DC output volt-
age (Vo).
The boost converter consists of a boost inductor (L), a controlled power switch (Q), a catch diode (D), an
output capacitor (Co) and, obviously, a control circuitry (see fig. 9).
The goal is to shape the input current in a sinusoidal fashion, in-phase with the input sinusoidal voltage.
To do this the L6561 uses the so-called Transition Mode technique.
Figure 9. Boost Converter Circuit.
L
D
~
I_L
I_Q
CONTROLLER
I_D
I_C
Q
I_O
C_O
LOAD
~
C_in
D94IN119
The error amplifier compares a partition of the output voltage of the boost converter with an internal ref-
erence, generating a signal error proportional to the difference between them. If the bandwidth of the er-
ror amplifier is narrow enough (say, below 20 Hz), the error signal is a DC value over a given half-cycle.
The error signal is fed into the multiplier block and multiplied by a partition of the rectified mains voltage.
The result will be a rectified sinusoid whose peak amplitude depends on the mains peak voltage and the
value of the error signal.
The output of the multiplier is in turn fed into the (+) input of the current comparator, thus it represents a
sinusoidal reference for PWM. In fact, as the voltage on the current sense pin (istantaneous inductor
current times the sense resistor) equals the value on the (+) of the current comparator, the conduction of
the external MOSFET is terminated. As a consequence, the peak inductor current will be enveloped by a
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