AN966 Datasheet, PDF (2/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

AN966 APPLICATION NOTE
By using switching techniques, a Power Factor Corrector (PFC) preregulator, located between the recti-
fier bridge and the filter capacitor, allows drawing from the mains a quasi-sinusoidal current, in-phase
with the line voltage. The PF becomes very close to 1 (more than 0.99 is possible) and the aforesaid
drawbacks are eliminated.
Theoretically, any switching topology can be used to achieve a high PF but, in practice, the boost topol-
ogy has become the most popular because of the advantages it offers:
1) mainly, the circuit requires the fewest external parts, thus it is the cheapest. Additionally:
2) the boost inductor located between the bridge and the switch causes the input di/dt to be low, thus
minimizing the noise generated at the input and, therefore, the requirements on the input EMI filter;
3) the switch is source-grounded, therefore is easy to drive.
However, boost topology requires the DC output voltage to be higher than the maximum expected line
peak voltage (400VDC is a typical value for 220V or wide range mains applications). Besides, there is no
isolation between input and output, thus any line voltage surge will be passed on to the output.
Two methods of controlling a PFC preregulator are currently widely used: the fixed frequency average
current mode PWM and the Transition Mode (TM) PWM (fixed ON-time, variable frequency). The first
method needs a complex control that requires a sophisticated controller IC (ST’s L4981A, with the vari-
ant of the frequency modulation offered by the L4981B) and a considerable component count. The sec-
ond one requires a simpler control (implemented by ST’s L6561), much fewer external parts and is
therefore much less expensive.
With the first method the boost inductor works in continuous conduction mode, while TM makes the in-
ductor work on the boundary between continuous and discontinuous mode, by definition. For a given
throughput power, TM operation then involves higher peak currents. This, also consistently with cost
considerations, suggests its use in a lower power range (typically below 150W), while the former is rec-
ommended for higher power levels.
L6561 PFC controller Integrated Circuit
The L6561, whose internal block diagram is shown in fig. 1, is an IC intended to control PFC preregula-
tors by using the Transition Mode technique. The device is available in Minidip and SO8 packages.
The most significant features of the L6561 concern the following points:
— undervoltage lockout with hysteresis;
— true micropower start-up current (50µA typ., 90µA guaranteed) for simple start-up circuits (just one re-
sistor) with very low power dissipation;
— internal reference with 1% precision guaranteed (@ Tj=25 °C);
— disable function to shut down the device and reduce its current consumption;
— two-level overvoltage protection;
— internal starter and Zero Current Detection circuit for TM operation;
— multiplier with extended dynamics for wide range mains applications, with excellent THD;
— on-chip RC filter on the current sense pin;
— high capability totem pole output for MOSFET or IGBT drive.
The IC is optimised for controlling PFC preregulators based on boost topology in electronic lamp bal-
lasts, AC-DC adapters and low power (<150 W) SMPS. However, its excellent performance along with
the extremely reduced external parts count allows also the use in unconventional topologies/applica-
tions. Low power off-line AC-DC converters (using isolated flyback topology) with or without Power Fac-
tor Correction are the most noticeable examples.
Device Blocks Description
SUPPLY BLOCK
As shown in fig. 1, a linear voltage regulator supplied by Vcc generates an internal 7V rail used to supply
the whole integrated circuit, except for the output stage which is supplied directly from Vcc. In addition, a
bandgap circuit generates the precise internal reference (2.5V±1% @ 25°C) used by the control loop to
ensure a good regulation.
In fig.2 is shown the undervoltage lockout (UVLO) comparator with hysteresis used to enable the chip as
long as the Vcc voltage is high enough to ensure a reliable operation.
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