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PS18/28/35 Troubleshooting Guide

Theory of Operation

1. Power Supply, Switch-Mode Audio Tracking

1.1 Introduction

There is a growing demand for power in home theater systems. Traditionally a Bose

home theater

system uses an unregulated linear power supply to power multiple linear audio amplifiers. The

power supply is typically based on a line-frequency transformer and the audio amplifiers are either

Class-B or Class-G linear amplifiers. However, there exists a limit to power and size with such a

technology. High-frequency switching technology in the power supply and audio amplifier enables

us to achieve a higher power at a smaller package size. Nevertheless, such an approach is usually

not very cost effective because of the complexity of the circuit and the extra effort needed to

reduce EMI emissions associated with high frequency switching.

A new technology, switch-mode audio tracking power supply, has been developed for Lifestyle

home entertainment systems to achieve higher power from a small package at a reasonable cost.

A tracking power supply delivers power on demand to multiple amplifiers. When the audio signal is

low, the power supply output voltage is low. When the audio signal is high, the power supply output

voltage is high. A rail voltage that tracks audio level reduces the voltage drop at the power ampli-

fier, resulting in lower power loss and hence less heat dissipation at the power amplifier. Conse-

quently, traditional low-cost Class-B amplifiers can be used. While high frequency switching makes

it possible to keep the size small, Class-B amplification makes it possible to keep the cost down.

This document describes the theory of operation of a high-frequency switch-mode audio tracking

power supply, SD 254165, which is used as a power source for multiple audio power amplifiers in

Lifestyle

home entertainment systems. The purpose of this document is to assist in the trouble-

shooting of the switch-mode audio tracking power supply. Brief descriptions of major function

blocks of the power supply is presented first, followed by more detailed discussions on each

function block.

1.2 Functional Block Diagram

Figure 1 shows a block diagram of a switch-mode tracking power supply. Power flows from the AC

line input at left to the DC output at right. Control signal flows from right to left.

Figure 1. Block diagram of a switch-mode tracking power supply