Self-Oscillating Class D Modulator With Coupled Multi-Level Loops

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Solution Overview

Problem

Class D amplifiers using pulse width modulation (PWM) suffer from limited frequency bandwidth, high distortion, and increased costs due to the need for accurate sawtooth signal generation, which affects their power efficiency and silicon surface integration requirements.

Innovation Solution

A self-oscillating modulator circuit comprising at least two class D self-oscillating loop modules with hysteresis comparators, coupling means for synchronization, and summing means to produce a multi-level modulated output signal, offering an alternative to PWM control and enhancing power efficiency and amplitude.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If PWM control is used in class D amplifiers, then frequency bandwidth is limited, but power efficiency is maintained

Engineering Contradiction:

Improvefrequency bandwidthVSAvoidpower efficiency

Core Design Contradiction:

SpeedVSUse of energy by moving object

Solution Approach 1:

The modulator is divided into multiple independent self-oscillating loop modules (first module, second module, etc.), each operating at different frequencies. This segmentation allows the system to overcome the frequency bandwidth limitation of single-loop PWM while maintaining the power efficiency benefits of self-oscillating operation through parallel modular architecture.

Inventive Principle:

Principle #1Segmentation

2Manufacturing precision

If accurate sawtooth signal generation is used to limit PWM distortion, then distortion is reduced, but costs and silicon surface area increase

Engineering Contradiction:

Improvedistortion controlVSAvoidcosts and silicon surface area

Core Design Contradiction:

Manufacturing precisionVSEase of manufacture

Solution Approach 1:

Each module uses an hysteresis comparator that generates its own oscillating signal through feedback from the amplifier output, eliminating the need for external accurate sawtooth signal generation. The system serves itself by using the output signal to drive the oscillation, thereby reducing distortion without increasing costs or silicon surface area.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The hysteresis comparator uses feedback from the amplifier output signal to generate the oscillating reference signal. This feedback mechanism naturally limits distortion by comparing the output against a self-generated reference, avoiding the need for expensive high-precision external sawtooth generators.

Inventive Principle:

Principle #23Feedback

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution provides a multi-level output signal with increased power, reduced jitter sensitivity, and improved efficiency, maintaining large amplitude and frequency synchronization, while reducing distortion and costs.

Implementation Method 1

The use of an hysteresis comparator introduces the delay which is necessary to obtain the self oscillating behaviour.

Methodology Applied

Scientific EffectHysteresis: Hysteresis

Data Source

PatentUS8937507B2Self oscillating modulator

Publication Date: 2015.01.20 TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)

AI summary

There is described a self oscillating modulator circuit comprising at least two coupled self oscillating loop modules, that achieved a good efficiency and a good linearity.