Class-D Full-Bridge Current Sensing With Common-Mode Rejection
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing Class-D driver stage topologies for personal audio devices are highly 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 and method utilizing a Class-D stage with high-side and low-side switches, coupled with current sensing circuitry that includes sense resistors between the low-side switches and ground, allowing measurement of sense voltages proportional to the output current, and a calibration scheme to adjust for mismatches in switch durations, reducing common-mode leakage and improving measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single sense resistor is used in the traditional Class-D driver topology, then the complete output current 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 segmented into two separate sense resistors (46A and 46B), each associated with a specific low-side switch. This segmentation isolates the sensing paths, allowing differential measurement that cancels out common-mode leakage currents while maintaining complete output current sensing capability.
Solution Approach 2:
A differential amplifier is introduced as an intermediary between the sense resistors and the measurement system. This intermediary circuitry processes the differential voltages from the two sense resistors, rejecting common-mode signals (including leakage currents) while amplifying the differential signal that represents the true output current.
2Object-affected harmful factors
If sense resistors are placed between low-side switches and ground, then common-mode leakage is reduced, but measurement accuracy requires calibration to account for switch duration mismatches
Solution Approach 1:
A calibration process is performed before normal operation to determine calibration factors that compensate for switch duration mismatches. This preliminary action stores correction data in memory, which is then applied during operation to maintain measurement accuracy despite variations in switch timing.
Solution Approach 2:
The system incorporates feedback through the calibration process, where measurement data is used to adjust and store calibration factors. These feedback-derived factors continuously correct for switch timing variations, ensuring sustained measurement precision throughout device operation.
3Device complexity
If traditional current sensing topology is used, then circuit simplicity is maintained, but routing resistance mismatches significantly affect measurement reliability
Solution Approach 1:
The differential amplifier acts as an intermediary that inherently rejects common-mode voltage variations caused by routing resistance mismatches. By measuring the difference between two symmetrically arranged sense resistor voltages, the system cancels out the effect of unequal routing resistances, improving reliability without adding complex compensation circuitry.
Solution Approach 2:
The system changes the measurement parameter from absolute voltage levels to differential voltage differences. This parameter transformation makes the measurement immune to common-mode variations caused by routing resistance mismatches, thereby improving reliability while maintaining circuit simplicity.
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 proposed solution significantly reduces common-mode leakage and insensitivity to routing resistance mismatches, providing accurate and precise measurement of output current, thereby enhancing the reliability of current sensing in Class-D driver stages.
Implementation Method 1
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
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 low-side switch and the ground 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 low-side switch is activated. The current sensing circuitry may also include a second sense resistor coupled between the second low-side switch and the ground 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 low-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.