MC34261 Datasheet, PDF (6/13 Pages) Motorola, Inc – POWER FACTOR CONTROLLERS

MC34261, MC33261
FUNCTIONAL DESCRIPTION
Introduction
Most electronic ballasts and switching power supplies use
a bridge rectifier and a filter capacitor to derive raw dc
voltage from the utility ac line. This simple rectifying circuit
draws power from the line when the instantaneous ac voltage
exceeds the capacitor’s voltage. This occurs near the line
voltage peak and results in a high charge current spike. 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.
The MC34261, MC33261 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. 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 MC34261, MC33261 contains many of the building
blocks and protection features that are employed in modern
high performance current mode power supply controllers.
There are, however, two areas where there is a major
difference when compared to popular devices such as the
UC3842 series. 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
A fully compensated Error Amplifier with access to the
inverting input and output is provided. It features a typical
dc voltage gain of 85 dB, and a unity gain bandwidth of 1.0
MHz with 58° of phase margin (Figure 4). The noninverting
input is internally biased at 2.5 V ±2.0% and is not pinned
out. 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 Amp Output is internally connected
to the Multiplier and is pinned out (Pin 2) for external loop
compensation. Typically, the bandwidth is set below 20 Hz,
so that the Error Amp output voltage is relatively constant
over a given ac line cycle. The output stage consists of a 500
μA current source pull−up with a Darlington transistor
pull−down. It is capable of swinging from 2.1 V to 5.7 V,
assuring that the Multiplier can be driven over its entire
dynamic range.
Multiplier
A single quadrant, two input multiplier is the critical
element that enables this device to control power factor. The
ac haversines are monitored at Pin 3 with respect to ground
while the Error Amp output at Pin 2 is monitored with
respect to the Voltage Feedback Input threshold. A graph of
the Multiplier transfer curve is shown in Figure 1. Note that
both inputs are extremely linear over a wide dynamic range,
0 V to 3.2 V for the Multiplier input (Pin 3), and 2.5 V to 4.0
V for the Error Amp output (Pin 2). The Multiplier output
controls the Current Sense Comparator threshold (Pin 4) as
the ac voltage traverses sinusoidally from zero to peak line.
This has the effect of forcing the MOSFET peak current to
track the input line voltage, thus making the preconverter
load appear to be resistive.
Pin 4 Threshold ≈ 0.62(V_{Pin 2} − V_FB)V_{Pin 3}
Zero Current Detector
The MC34261 operates as a critical conduction current
mode controller, whereby output switch conduction is
initiated by the Zero Current Detector and terminated when
the peak inductor current reaches the threshold level
established by the Multiplier output. The Zero Current
Detector initiates the next on−time by setting the RS Latch
at the instant the inductor current reaches zero. This critical
conduction mode of operation has two significant benefits.
First, since the MOSFET cannot turn on until the inductor
current reaches zero, the output rectifier’s reverse recovery
time becomes less critical allowing the use of an inexpensive
rectifier. Second, since there are no deadtime gaps between
cycles, the ac line current is continuous thus limiting the
peak switch to twice the average input current.
The Zero Current Detector indirectly senses the inductor
current by monitoring when the auxiliary winding voltage
falls below 1.6 V. To prevent false tripping, 110 mV of
hysteresis is provided. The Zero Current Detector input is
internally protected by two clamps. The upper 6.7 V clamp
prevents input overvoltage breakdown while the lower 0.7
V clamp prevents substrate injection. Device destruction
can result if this input is shorted to ground. An external
resistor must be used in series with the auxiliary winding to
limit the current through the clamps.
Current Sense Comparator and RS Latch
The Current Sense Comparator RS Latch configuration
ensures that only a single pulse appears at the Drive Output
during a given cycle. The inductor current is converted to a
voltage by inserting a ground referenced sense resistor R₉ in
series with the source of output switch Q1. This voltage is
monitored by the Current Sense Input and compared to the
Multiplier output voltage. The peak inductor current is
controlled by the threshold voltage of Pin 4 where:
I_pk =
Pin 4 Threshold
R₉
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