MC33260
FUNCTIONAL DESCRIPTION
INTRODUCTION
OPERATION DESCRIPTION
The need of meeting the requirements of legislation on
line current harmonic content, results in an increasing
demand for cost effective solutions to comply with the
Power Factor regulations. This data sheet describes a
monolithic controller specially designed for this purpose.
Most off–line appliances use a bridge rectifier associated
to a huge bulk capacitor to derive raw dc voltage from the
utility ac line.
Rectifiers
AC
Line
Converter
+
Bulk
Storage
Capacitor
Load
Figure 19. Typical Circuit Without PFC
This technique results in a high harmonic content and in
poor power factor ratios. In effect, the simple rectification
technique draws power from the mains when the
instantaneous ac voltage exceeds the capacitor voltage. This
occurs near the line voltage peak and results in a high charge
current spike. Consequently, a poor power factor (in the
range of 0.5 – 0.7) is generated, resulting in an apparent input
power that is much higher than the real power.
Vpk
Rectified DC
0
Line Sag
AC Line Voltage
0
AC Line Current
The MC33260 is optimized to just as well drive a free
running as a synchronized discontinuous voltage mode.
It also features valuable protections (overvoltage and
undervoltage protection, overcurrent limitation, ...) that
make the PFC preregulator very safe and reliable while
requiring very few external components. In particular, it is
able to safely face any uncontrolled direct charges of the
output capacitor from the mains which occur when the
output voltage is lower than the input voltage (start–up,
overload, ...).
In addition to the low count of elements, the circuit can
control an innovative mode named “Follower Boost” that
permits to significantly reduce the size of the preconverter
inductor and power MOSFET. With this technique, the
output regulation level is not forced to a constant value, but
can vary according to the a.c. line amplitude and to the
power. The gap between the output voltage and the ac line
is then lowered, what allows the preconverter inductor and
power MOSFET size reduction. Finally, this method brings
a significant cost reduction.
A description of the functional blocks is given below.
REGULATION SECTION
Connecting a resistor between the output voltage to be
regulated and the pin 1, a feedback current is obtained.
Typically, this current is built by connecting a resistor
between the output voltage and the pin 1. Its value is then
given by the following equation:
I
pin1
+
Vo
*
Vpin1
Ro
Figure 20. Line Waveforms Without PFC
Active solutions are the most popular way to meet the
legislation requirements. They consist of inserting a PFC
pre–regulator between the rectifier bridge and the bulk
capacitor. This interface is, in fact, a step–up SMPS that
outputs a constant voltage while drawing a sinusoidal
current from the line.
Rectifiers
AC
Line
PFC Preconverter
High Frequency
Bypass Capacitor
Bulk Storage
Capacitor
Converter
where:
Ro is the feedback resistor,
Vo is the output voltage,
Vpin1 is the pin 1 clamp value.
The feedback current is compared to the reference current
so that the regulation block outputs a signal following the
characteristic depicted in Figure 22. According to the power
and the input voltage, the output voltage regulation level
varies between two values (Vo)regL and (Vo)regH
corresponding to the IregL and IregH levels.
Regulation Block Output
1.5 V
MC33260
Load
+
Io
Ireg–L
(97%Iref)
Ireg–H
(Iref)
Figure 21. PFC Preconverter
Figure 22. Regulation Characteristic
The MC33260 was developed to control an active solution
with the goal of increasing its robustness while lowering its
global cost.
The feedback resistor must be chosen so that the feedback
current should equal the internal current source IregH when
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