Differential Class D Amplifier Load Measurement Across LC Filters
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Solution Overview
Problem
Existing methods for measuring load in class D amplifiers, particularly in high-frequency applications, face challenges in accurately determining load impedance due to the presence of an LC demodulator filter, which makes it difficult to measure load voltage and current correspondence, especially in fully differential topologies with feedback before the LC filter.
Innovation Solution
A measuring system that senses AC output current magnitude and phase both differentially and in common mode, allowing for effective AC load and LC network impedance measurement, even in scenarios where single-ended driving is not possible, by using a common-mode control circuit and performing ripple current measurements to evaluate inductance before determining load impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If AC current sensing is performed in a class D amplifier with LC demodulator filter, then load diagnostics can be achieved, but measurement precision deteriorates due to difficulty in measuring load voltage and current correspondence
Solution Approach 1:
The patent segments the measurement process into two distinct modes: common-mode measurement for determining amplifier output voltage and differential-mode measurement for determining load current. This segmentation allows each measurement to be performed independently under optimized conditions, resolving the contradiction between achieving load diagnostics and maintaining measurement precision.
Solution Approach 2:
The patent introduces an intermediary measurement approach by using the common-mode signal as a mediator to infer the amplifier output voltage, and the differential-mode signal as a mediator to infer the load current. These intermediary measurements enable load impedance calculation without directly measuring voltage and current at the same point, thus maintaining precision while enabling diagnostics.
2Reliability
If feedback is implemented before the LC demodulator filter in a fully differential topology, then amplifier control is improved, but device complexity increases making single-ended driving impossible
Solution Approach 1:
The patent makes the measurement system universal by enabling it to function in both fully differential topologies with feedback before the LC filter and other configurations. The common-mode and differential-mode sensing approach works regardless of the specific feedback topology, allowing the same measurement methodology to be applied across multiple amplifier designs and configurations.
Solution Approach 2:
Instead of trying to adapt the amplifier topology to accommodate measurement requirements, the patent inverts the approach by adapting the measurement methodology to work with the existing complex fully differential topology. By measuring common-mode and differential-mode signals rather than directly measuring load parameters, the system can handle the complexity of feedback-before-filter configurations.
3Measurement precision
If external shunt resistors or precise LC values are used for measurement, then measurement precision improves, but ease of operation deteriorates due to additional components and calibration requirements
Solution Approach 1:
The patent enables the amplifier system to perform its own load measurements using existing internal signals (common-mode and differential-mode amplifier output signals) without requiring external measurement equipment or additional components like shunt resistors. The system serves itself by utilizing signals already present in the feedback path to determine load impedance.
Solution Approach 2:
The patent changes the measurement parameters from direct voltage and current measurements to common-mode and differential-mode signal measurements. This parameter transformation allows the system to extract load impedance information from signals that are already available in the amplifier's feedback path, eliminating the need for external measurement components and simplifying operation.
Data Source
AI summary
A method can be used to measure a load driven by a switching amplifier having a differential input, an LC output demodulator filter and a feedback network between the amplifier output and the differential input. The amplifier is AC driven in a differential and in a common mode by applying a common. The feedback network provides feedback towards the differential input from downstream the LC demodulator filter by computing the impedance of the load as a function of the differential mode output current and the common mode output current. The feedback network provides feedback towards the differential input from upstream the LC demodulator filter by measuring the impedance value of the inductor of the LC demodulator filter, and computing 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.