Class D Audio Amplifier Overload Sensing With Model Transistor Sampling
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Solution Overview
Problem
Class D audio amplifiers face inefficiencies due to high power consumption of overload protection circuitry, especially during quiescent operation and low audio output levels, which affects battery life and heat dissipation in portable devices.
Innovation Solution
A class D audio amplifier design with a model transistor-based overload protection circuit that determines a drain-source reference voltage to detect overloads, using a digital or analog controller to generate an overload signal, and operates in an intermittent scheme to minimize power consumption by switching between tracking and holding phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If continuous monitoring of drain-source voltage is implemented to detect overload conditions, then protection reliability is improved, but power consumption increases significantly during quiescent operation and low output levels
Solution Approach 1:
The patent implements periodic sampling of the drain-source voltage at specific intervals during the switching cycle rather than continuous monitoring. The controller samples the voltage at predetermined times when overload conditions are most likely to occur, thereby reducing the average power consumption of the protection circuitry while maintaining adequate protection reliability.
Solution Approach 2:
The patent establishes predetermined voltage thresholds and sampling intervals in advance based on expected operating conditions. The controller is pre-programmed with these parameters so that no real-time calculation or complex decision-making is required during operation, minimizing the computational overhead and power consumption of the protection circuit.
2Measurement precision
If a model transistor is used to determine drain-source reference voltage for overload detection, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent uses a model transistor that replicates the electrical characteristics of the power transistor under test. By measuring the drain-source voltage of this simplified model transistor under controlled conditions, the system obtains precise reference values for overload detection without needing to directly measure the complex voltage waveform of the actual power transistor during switching.
Solution Approach 2:
The model transistor acts as an intermediary between the complex power transistor and the measurement circuitry. It translates the difficult-to-measure voltage characteristics of the power transistor into a simplified reference voltage that can be easily compared against actual measurements, reducing the complexity of the overall measurement system.
Data Source
AI summary
A class D audio amplifier with an output stage is provided. The class D audio amplifier includes a plurality of power transistors coupled in cascade between a first DC supply voltage and a second DC supply voltage. The class D audio amplifier also includes a plurality of gate drivers having respective inputs coupled to a modulated audio signal and configured to generate respective modulated gate drive signals to the plurality of power transistors. A first overload protection circuit includes a model transistor possessing electric characteristics representative of a first power transistor of the output stage for determining a drain-source reference voltage of the model-transistor and this drain-source reference voltage may be utilized to indicate an overload event or condition of the first power transistor.