MC33368 View Datasheet(PDF) - Motorola => Freescale
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Description
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MC33368 Datasheet PDF : 16 Pages
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MC33368
FUNCTIONAL DESCRIPTION
INTRODUCTION
With the goal of exceeding the requirements of legislation
on line current harmonic content, there is an ever increasing
demand for an economical method of obtaining a unity power
factor. This data sheet describes a monolithic control IC that
was specifically designed for power factor control with
minimal external components. It offers the designer a simple
cost effective solution to obtain the benefits of active power
factor correction.
Most electronic ballasts and switching power supplies use
a bridge rectifier and a bulk storage capacitor to derive raw dc
voltage from the utility ac line, Figure 13.
Figure 13. Uncorrected Power Factor Circuit
Rectifiers
Converter
AC
Line
Bulk
Storage
Load
Capacitor
This simple rectifying circuit draws power from the line
when the instantaneous ac voltage exceeds the capacitor
voltage. This occurs near the line voltage peak and results in
a high charge current spike, Figure 14. Since power is only
taken near the line voltage peaks, the resulting spikes of
current are extremely nonsinusoidal with a high content of
harmonics. This results in a poor power factor condition
where the apparent input power is much higher than the real
power. Power factor ratios of 0.5 to 0.7 are common.
Figure 14. Uncorrected Power Factor Input Waveforms
Vpk
Rectified
DC
0
Line Sag
AC Line
Voltage
0
AC Line
Current
Power factor correction can be achieved with the use of
either a passive or active input circuit. Passive circuits
usually contain a combination of large capacitors, inductors,
and rectifiers that operate at the ac line frequency. Active
circuits incorporate some form of a high frequency switching
converter for the power processing with the boost converter
being the most popular topology. Since active input circuits
operate at a frequency much higher than that of the ac line,
they are smaller, lighter in weight, and more efficient than a
passive circuit that yields similar results. With proper control
of the preconverter, almost any complex load can be made to
appear resistive to the ac line, thus significantly reducing the
harmonic current content.
Operating Description
The MC33368 contains many of the building blocks and
protection features that are employed in modern high
performance current mode power supply controllers.
Referring to the block diagram in Figure 15, note that a
multiplier has been added to the current sense loop and that
this device does not contain an oscillator. A description of
each of the functional blocks is given below.
Error Amplifier
An Error Amplifier with access to the inverting input and
output is provided. The amplifier is a transconductance type,
meaning that it has high output impedance with controlled
voltage–to–current gain (gm 50 µmhos). The noninverting
input is internally biased at 5.0 V ±2.0%. The output voltage
of the power factor converter is typically divided down and
monitored by the inverting input. The maximum input bias
current is –1.0 µA which can cause an output voltage error
that is equal to the product of the input bias current and the
value of the upper divider resistor R2. The Error Amplifier
output is internally connected to the Multiplier and is pinned
out (Pin 4) for external loop compensation. Typically, the
bandwidth is set below 20 Hz so that the amplifier’s output
voltage is relatively constant over a given ac line cycle. In
effect, the error amplifier monitors the average output voltage
of the converter over several line cycles resulting in a fixed
Drive Output on–time. The amplifier output stage can sink
and source 11.5 µA of current and is capable of swinging
from 1.7 to 5.0 V, assuring that the Multiplier can be driven
over its entire dynamic range.
Note that by using a transconductance type amplifier, the
input is allowed to move independently with respect to the
output, since the compensation capacitor is connected to
ground. This allows dual usage of the Voltage Feedback pin
by the Error Amplifier and Overvoltage Comparator.
Overvoltage Comparator
An Overvoltage Comparator is incorporated to eliminate
the possibility of runaway output voltage. This condition can
occur during initial startup, sudden load removal, or during
output arcing and is the result of the low bandwidth that must
be used in the Error Amplifier control loop. The Overvoltage
Comparator monitors the peak output voltage of the
converter, and when exceeded, immediately terminates
MOSFET switching. The comparator threshold is internally
set to 1.08 Vref. In order to prevent false tripping during
normal operation, the value of the output filter capacitor C3
must be large enough to keep the peak–to–peak ripple less
than 16% of the average dc output.
MOTOROLA ANALOG IC DEVICE DATA
7
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