MC33260
http://onsemi.com
9
FUNCTIONAL DESCRIPTION
Pin Numbers are Relevant to the PDIP- -8 Version
INTRODUCTION
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 appliancesuse a bridge rectifier associated
to a huge bulk capacitor to derive raw dc voltage from the
utility ac line.
Figure 22. Typical Circuit Without PFC
Load
Converter
Rectifiers
Bulk
Storage
Capacitor
+
AC
Line
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
instantaneousacvoltageexceedsthecapacitorvoltage.This
occursnearthelinevoltagepeakandresultsinahighcharge
current spike. Consequently, a poor power factor (in the
rangeof0.5- -0.7)isgenerated,resultinginanapparentinput
Figure 23. Line Waveforms Without PFC
Line Sag
Rectified DC
AC Line Voltage
AC Line Current
0
0
V
pk
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.
Figure 24. PFC Preconverter
Converter
Rectifiers
+
AC
Line
PFC Preconverter
TheMC33260wasdevelopedtocontrolanactivesolution
withthe goalof increasingitsrobustnesswhile loweringits
global cost.
OPERATION DESCRIPTION
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 (startup,
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 isnot 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
powerMOSFETsizereduction.Finally,thismethodbrings
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
=
V
o
V
pin1
R
o
where:
R
o
is the feedback resistor,
V
o
is the output voltage,
V
pin1
is the Pin 1 clamp value.
Thefeedbackcurrentiscomparedtothereferencecurrent
so that the regulation block outputs a signal following the
characteristicdepictedinFigure 25.Accordingtothepower
and the input voltage, the output voltage regulation level
varies  between  two  values  (V
o
)
regL
and  (V
o
)
regH
corresponding to the I
regL
and I
regH
levels.
Figure 25. Regulation Characteristic
1.5 V
Regulation Block Output
I
o
I
regL
(97%I
ref
)
I
regH
(I
ref
)
Thefeedbackresistormustbechosensothatthefeedback
current should equal the internal current source I
regH
when
the output voltage exceeds the chosen upper regulation
voltage [(V
o
)
regH
].