Theory Of Operation - Bose Lifestyle PS 28 Troubleshooting Manual

PS18/28/35 Troubleshooting Guide

Theory of Operation

1. Power Supply, Switch-Mode Audio Tracking
1.1 Introduction
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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
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