MC33368

http://onsemi.com

7

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.

Figure 14. Uncorrected Power Factor Circuit

Rectifiers

Converter

Bulk

Storage

Capacitor

Load

AC

Line

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.

Figure 15. Uncorrected Power Factor Input Waveforms

Rectified

DC

0

Line Sag

AC Line

Voltage

AC Line

Current

0

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 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.

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

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.

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

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.