MC34262_11 Datasheet, PDF (6/19 Pages) ON Semiconductor – Power Factor Controllers

MC34262, MC33262
16
14
13
12
Startup Threshold
12
(V_CC Increasing)
11
8.0
V_FB = 0 V
10
Current Sense = 0 V
4.0
Multiplier = 0 V
C_L = 1.0 nF
9.0
Minimum Operating Threshold
(V_CC Decreasing)
f = 50 kHz
8.0
T_A = 25°C
0
7.0
0
10
20
30
40
- 55 - 25
0
25
50
75 100 125
V_CC, SUPPLY VOLTAGE (V)
T_A, AMBIENT TEMPERATURE (°C)
Figure 14. Supply Current
versus Supply Voltage
Figure 15. Undervoltage Lockout Thresholds
versus Temperature
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 16.
frequency switching converter for the power processing,
with the boost converter being the most popular topology,
Figure 18. 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.
V_pk
Rectifiers
Converter
Rectified
DC
AC
Line
0
+
Bulk
Storage
Load
Capacitor
AC Line
Voltage
Line Sag
Figure 16. 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 17. 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.
Power factor correction can be achieved with the use of
either a passive or an 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
0
AC Line
Current
Figure 17. Uncorrected Power Factor
Input Waveforms
The MC34262, MC33262 are high performance, critical
conduction, current−mode power factor controllers
specifically designed for use in off−line active
preconverters. These devices provide the necessary
features required to significantly enhance poor power
factor loads by keeping the ac line current sinusoidal and
in phase with the line voltage.
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