Class-D Amplifier Current Sensing via H-Bridge VDS Averaging
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Solution Overview
Problem
Class-D audio amplifiers face challenges in accurately reading load current due to limitations in current sensing methods, including errors from sampling instants and saturation issues, especially at high signal levels, which affect the accuracy and reliability of current measurement.
Innovation Solution
The proposed solution involves a sensing circuit that computes the difference between drain-to-source voltages from both half-bridges and performs averaging using a differential amplifier and low-pass filter to obtain a sense voltage for analog-to-digital conversion, eliminating the need for external circuits and reducing ripple contributions, and includes a sensing network with an auxiliary transistor for improved accuracy and protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sampling is performed at a specific instant to eliminate current ripple, then measurement precision is improved, but device complexity increases due to required sampling circuits and timing control
Solution Approach 1:
The patent extracts only the essential information needed for current measurement by reading the drain-to-source voltage during the ON period of low-side transistors, eliminating the need for complex sampling circuits that were previously required to capture ripple-free current values at specific instants
Solution Approach 2:
The sensing circuit utilizes the existing ON periods of the low-side transistors in the H-bridge to perform current measurement, turning the natural operation of the power transistors into a self-service mechanism that eliminates the need for external sampling control circuits
2Measurement precision
If external current sensing circuits are used, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent merges the current sensing function with the existing H-bridge power stage by utilizing the drain-to-source voltage of the low-side transistors, which naturally reflects the load current during their ON periods, thereby eliminating the need for separate external current sensing circuits
Solution Approach 2:
The low-side power transistors in the H-bridge serve dual functions: they perform their primary power switching role while simultaneously acting as current sensors through their drain-to-source voltage measurement during ON periods, eliminating the need for dedicated sensing components
3Ease of operation
If drain-to-source voltage is measured during transistor ON period, then ease of operation is improved, but measurement precision deteriorates due to current ripple from LC filter
Solution Approach 1:
The patent performs continuous measurement during the entire ON period of the low-side transistors rather than attempting to capture a single ripple-free instant, using the accumulated measurement data to represent the load current while accepting and averaging out the ripple contributions
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
This approach enhances current sensing accuracy by eliminating ripple contributions and saturation issues, allowing for continuous-time reading of load current with improved reliability and simplicity, even in the absence of LC filters, and automatically protects the electronics from high voltages.
Implementation Method 1
performing an averaging operation on said difference to obtain a sense voltage value to be supplied to an analog-to-digital converter
Data Source
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AI summary
A class-D audio-amplifier apparatus including a circuit (50) for reading a load current supplied by said amplifier to a load, said amplifier comprising a final stage, which includes a full-H bridge comprising a first half-bridge and a second half-bridge; said circuit (50) being configured for estimating said load current through reading of a current (IOUTP, IOUTM) supplied by at least one power transistor (13b, 23b), by measuring a drain-to-source voltage (VDSLP) thereof during an ON period. According to the invention, said apparatus comprises a sensing circuit (50) connected for detecting a first drain-to-source voltage (VSP) from a transistor (13b) of the first half-bridge (12) and a second drain-to-source voltage (VSM) from a corresponding transistor (23b) of the second half-bridge (22), said sensing circuit (50) comprising: a module (51) for computing a difference between said first detected drain-to-source voltage (VSP) and said second detected drain-to-source voltage (VSM); and a module (53) for performing an averaging operation on said difference to obtain a sense voltage value (VSENSE) to be supplied to an analog-to-digital converter (54).