Class D Amplifier Load Impedance Measurement via Common-Mode Sensing
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Solution Overview
Problem
Existing methods for measuring AC load and passive filter network impedance in class D amplifiers are limited, especially when single-ended driving is not possible and feedback is applied before the LC demodulator filter, leading to inaccurate load evaluation due to load-dependent frequency response and variable LC filter components.
Innovation Solution
A measuring system that senses AC output current magnitude and phase both differentially and in common mode, allowing for impedance measurement before the LC demodulator filter, and uses common-mode control to evaluate load current and impedance, even in fully differential system configurations with feedback from upstream of the LC filter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If single-ended driving is not possible and feedback is applied before the LC demodulator filter, then the amplifier can operate in fully differential configurations, but load evaluation becomes inaccurate due to load-dependent frequency response and variable LC filter components
Solution Approach 1:
The patent segments the measurement process into two distinct modes: common-mode measurement for accurate load evaluation and differential-mode operation for normal amplification. This segmentation allows the system to obtain accurate load information without compromising the fully differential operational capability, resolving the contradiction between configuration flexibility and measurement accuracy.
Solution Approach 2:
The patent performs preliminary common-mode measurements to determine load impedance characteristics before proceeding with differential-mode amplification. By预先 obtaining accurate load evaluation data through common-mode excitation, the system can then operate in fully differential mode with confidence in load conditions, eliminating the measurement inaccuracy that would otherwise prevent such operation.
2Measurement precision
If common-mode measurement is used to evaluate load current and impedance, then accurate load diagnostics are achieved in fully differential configurations, but the system complexity increases due to additional measurement circuitry
Solution Approach 1:
The patent implements a universal measurement system that can operate in both common-mode and differential-mode using the same basic circuitry. The measurement apparatus serves multiple functions: it can perform accurate load evaluation through common-mode excitation while also supporting normal differential amplification operations, thereby reducing overall system complexity despite the added capability.
Solution Approach 2:
The measurement system uses the amplifier's own output stages and internal circuitry to perform self-diagnosis and load evaluation. By leveraging existing components rather than requiring entirely separate measurement equipment, the system achieves accurate load measurement while minimizing the increase in overall system complexity.
3Reliability
If AC output current is sensed both differentially and in common mode, then frequency response dependence on load impedance is reduced, but the sensing system complexity increases
Solution Approach 1:
The patent merges the differential and common-mode sensing functions into a unified measurement system. By combining both sensing approaches, the system achieves frequency response stability that is independent of load impedance variations. The merged system processes both signal components to provide comprehensive load evaluation while avoiding the need for entirely separate sensing circuits.
Data Source
Figure 1~2
Figure 3a~5b
Figure 6~8b
AI summary
A load driven by a switching amplifier, such as a Class D audio amplifier having a differential input, a LC output demodulator filter and a feedback network between the amplifier output and the differential input is measured by: - AC driving (1000) the amplifier in a differential mode by applying a differential AC signal to the differential input and sensing the output current from the amplifier while driving the amplifier in the differential mode, - AC driving (1002) the amplifier in a common mode by applying a common mode AC signal to the differential input and sensing the AC output current from the amplifier while driving the amplifier in the common mode, and a) with the feedback network providing feedback towards said differential input from downstream the LC demodulator filter, computing (1006) the impedance of the load as a function of the differential mode output current and the common mode output current, b) with the feedback network providing feedback towards the differential input from upstream the LC demodulator filter, measuring (1008a) the impedance value of the inductor of the LC demodulator filter, and computing (1008b) the impedance of the load as a function of the differential mode output current, the common mode output current and the impedance value of the inductor of the LC demodulator filter.