MC33261 データシートの表示（PDF） - Motorola => Freescale

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MC33261

POWER FACTOR CONTROLLERS

Motorola => Freescale 

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(VPin 2 – VFB)VPin 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:

Ipk =

Pin 4 Threshold

R9

With the component values shown in Figure 16, the

Current Sense Comparator threshold, at the peak of the

haversine varies from 1.1 V at 90 Vac to 100 mV at 268 Vac.

The Current Sense Input to Drive Output propagation delay is

typically 200 ns.

6

MOTOROLA ANALOG IC DEVICE DATA

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