AN966 Datasheet, PDF (1/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

AN966
®
APPLICATION NOTE
L6561, ENHANCED TRANSITION MODE
POWER FACTOR CORRECTOR
by Claudio Adragna
The TM (Transition Mode) technique is widely used for Power Factor Correction in low power appli-
cations, such as lamp ballasts or low-end monitors. The L6561 is the latest ST’s proposal for this
market and the emerging ones that are supposed to require a low-cost Power Factor Correction.
Based on a well-established architecture, the L6561 offers excellent performance that enlarges its
field of application considerably.
Introduction
The front-end stage of conventional off-line converters, typically made up of a full wave rectifier bridge
with a capacitor filter, gets an unregulated DC bus from the AC mains. The filter capacitor must be large
enough to have a relatively low ripple superimposed on the DC level. This means that the instantaneous
line voltage is below the voltage on the capacitor most of the time, thus the rectifiers conduct only for a
small portion of each line half-cycle. The current drawn from the mains is then a series of narrow pulses
whose amplitude is 5-10 times higher than the resulting DC value.
Lots of drawbacks result from that: much higher peak and RMS current drawn from the line, distortion of
the AC line voltage, overcurrents in the neutral line of the three-phase systems and, after all, a poor utili-
sation of the power system’s energy capability.
This can be measured in terms of either harmonic contents, as norms EN61000-3-2 envisage, or Power
Factor (PF), intended as the ratio between the real power (the one transferred to the output) and the ap-
parent power (RMS line voltage times RMS line current) drawn from the mains, which is more immedi-
ate. A traditional input stage with capacitive filter has a low PF (0.5-0.7) and high harmonic contents.
Figure 1. Internal Block Diagram of the L6561.
1
INV
-
2.5V
+
COMP
2
MULT
3
MULTIPLIER
CS
4
40K
VOLTAGE
REGULATOR
OVER-VOLTAGE
DETECTION
+-
5pF
8
V_CC
20V
R2
INTERNAL
SUPPLY 7V
R1
+
-
V_REF2
2.1V
1.6V
UVLO
RQ
S
DRIVER
ZERO CURRENT
+ ^DETECTOR
-
STARTER
V_CC
7
GD
6
GND
5
ZCD
DISABLE
D97IN547D
March 2003
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