MC33368_11 Datasheet, PDF (7/16 Pages) ON Semiconductor – High Voltage GreenLine Power Factor Controller

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 14.
Rectifiers
Converter
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 16, 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.
AC
Line
Bulk
Storage
Load
Capacitor
Figure 14. Uncorrected Power Factor Circuit
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 15. 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.
V_pk
Rectified
DC
0
Line Sag
AC Line
Voltage
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 (g_m  50 mmhos). 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 mA 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 mA 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.
0
AC Line
Current
Figure 15. Uncorrected Power Factor Input Waveforms
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
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 V_ref. 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.
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