Class D Amplifier Current Limiting Without Sense Resistors
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Solution Overview
Problem
Current limiting in voltage-controlled self-oscillating Class D audio amplifiers is challenging due to saturation of the voltage loop during over-current events, leading to audio signal distortion and noisy restarts, and existing current detection methods cause power loss and EMI issues.
Innovation Solution
A self-oscillating current-limiting control scheme, referred to as True Current Limit (TCL), is implemented locally around each switching device, using a comparator circuit to detect over-currents and adjust the current limit independently of other control loops, eliminating the need for external sense devices and optimizing switching loops for reduced power loss and EMI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If standard current programmed control is used in voltage-controlled self-oscillating Class D amplifiers, then over-current detection is achieved, but the voltage loop saturates and switching action stops causing audio signal holes and noisy restarts
Solution Approach 1:
The control system is segmented into independent current limiting control and voltage control loops. The current limiting control operates autonomously at the switching device level while the voltage loop maintains overall system stability, preventing saturation and signal distortion.
Solution Approach 2:
Each switching device has local autonomous current detection and limiting capability that operates independently without requiring intervention from the main control loop, enabling continuous operation during over-current events.
2Measurement precision
If a sense resistor is added in the power path for current measurement, then current detection is achieved, but power loss increases and EMI pollution worsens
Solution Approach 1:
The current sensing function is extracted from the main power path by utilizing the inherent voltage drop across the switching device itself, eliminating the need for separate sense resistors and associated power losses.
Solution Approach 2:
The switching device's on-resistance serves as an intermediary element for current sensing, converting current information into voltage signals without requiring additional power-dissipating components in the power path.
3Loss of energy
If current sense transformers are used to manage higher power levels, then power loss is reduced and switching loop size is reduced, but cost increases and saturation phenomena occur
Solution Approach 1:
The invention uses simple, inexpensive voltage detection circuitry instead of expensive current sense transformers, achieving adequate current limiting functionality without the complexity and cost of magnetic components.
4Measurement precision
If sense resistors are used for current detection, then current measurement is achieved, but the switching loop becomes larger and EMI problems increase
Solution Approach 1:
The current sensing function is merged with the switching device's inherent characteristics by detecting voltage across its terminals, combining measurement and power delivery functions in a single location to minimize loop area and EMI.
Data Source
AI summary
The invention relates to a current limiting method, e. g. for Class D amplifiers comprising a unique detection- and control method. The current detection circuit can be implemented as a voltage measurement where the measured voltage corresponds to the current flowing through the power-switching device. The device can be switched OFF when a set limit is reached. By forcing certain OFF time, the associated control system behaves as a self-oscillating current limiting circuit. This can be implemented locally close to the switching device and be independent of other local or global control systems.