Full-Bridge Driver Current Sensing With Common-Mode Rejection
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Solution Overview
Problem
Existing Class-D driver stage topologies for audio and haptic devices are sensitive to common-mode leakage current, leading to inaccurate current sensing due to the periodic commutation of switches, which affects the measurement of output current through transducers.
Innovation Solution
A system with a Class-D stage comprising high-side and low-side switches and current sensing circuitry using sense resistors to measure output current, where the measurement circuitry determines the output current based on sense voltages generated across these resistors, reducing common-mode leakage by activating switches in complementary phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single sense resistor is used in the negative polarity node of the Class-D driver stage, then the complete output current through the transducer can be sensed, but the topology becomes highly sensitive to common-mode leakage current due to periodic switch commutation
Solution Approach 1:
The single sense resistor is divided into two separate sense resistors (first sense resistor in the first output leg, second sense resistor in the second output leg). This segmentation allows the measurement circuitry to differentiate between differential current components and common-mode leakage components, enabling accurate output current sensing while rejecting common-mode interference through differential measurement techniques
Solution Approach 2:
The patent introduces measurement circuitry that acts as an intermediary between the sense resistors and the control system. This intermediary processes the sense voltages from both resistors, performing differential measurements that inherently reject common-mode leakage current while preserving the differential output current signal, thus solving the sensitivity problem
2Ease of operation
If switches are periodically commutated to drive the transducer, then the desired output voltage and current can be controlled, but common-mode voltage fluctuates between supply voltage and ground causing measurement errors
Solution Approach 1:
Instead of measuring the sense voltage at the traditional negative polarity node (which experiences large common-mode swings), the patent measures sense voltages at the positive polarity nodes (connected to supply voltage through sense resistors). This inversion of the measurement reference point eliminates the common-mode voltage fluctuation problem while maintaining the ability to sense output current through differential measurement
3Ease of manufacture
If routing resistance mismatches exist in the driver stage, then manufacturing variations are unavoidable, but these mismatches cause errors in current measurement and energy calculation
Solution Approach 1:
The patent employs measurement circuitry that performs differential measurements using sense voltages from both output legs. This feedback-based differential approach automatically compensates for routing resistance mismatches and other systematic errors, as the differential measurement technique rejects common-mode errors including routing resistance variations, thereby maintaining high measurement precision despite manufacturing variations
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 significantly reduces sensitivity to common-mode leakage and routing resistance mismatches, providing accurate and balanced energy measurement across all phases, enhancing the precision of current sensing in Class-D stages.
Implementation Method 1
a first sense voltage proportional to the output current across the first sense resistor when the first high-side switch is activated
Data Source
AI summary
A system may include a Class-D stage comprising a first high-side switch coupled between a supply voltage and a first output terminal of the Class-D stage, a second high-side switch coupled between the supply voltage and a second output terminal of the Class-D stage, a first low-side switch coupled between a ground voltage and the first output terminal, and a second low-side switch coupled between the ground voltage and the second output terminal. The system may also include current sensing circuitry comprising a first sense resistor coupled between the first high-side switch and the supply voltage, such that an output current through a load coupled between the first output terminal and the second output terminal causes a first sense voltage proportional to the output current across the first sense resistor when the first high-side switch is activated. The current sensing circuitry may also include a second sense resistor coupled between the second high-side switch and the supply voltage, such that an output current through the load causes a second sense voltage proportional to the output current across the second sense resistor when the second high-side switch is activated. The system may also include measurement circuitry configured to measure the first sense voltage and the second sense voltage to determine the output current.