Class-D Amplifier PWM Short Detection With Dynamic Input Thresholds
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Solution Overview
Problem
Conventional methods for detecting short circuits in class-D amplifiers are complex, power-intensive, and prone to false detection, particularly when the output PWM period varies with input power signals, necessitating a more accurate and less complex solution.
Innovation Solution
A pulse detector and controller system that differentiates between a short circuit and a large digital input signal by monitoring PWM pulse widths and comparing them to expected levels, using a short threshold load lookup table and tracking table to determine the presence of a short circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional short detection methods (serial detection, parallel current detection, or digital detection) are used, then short circuit detection capability is provided, but device complexity and power consumption increase
Solution Approach 1:
The class-D amplifier uses its own existing PWM output signal and internal components to detect short circuits, rather than requiring separate dedicated detection circuits. The output stage itself provides the detection function by monitoring its own PWM signal characteristics, making the system self-diagnostic without additional complexity.
Solution Approach 2:
The existing PWM output stage and control logic are made to serve dual purposes: audio signal amplification and short circuit detection. The same components that generate the PWM signal for audio output also provide the basis for detecting abnormal conditions, eliminating the need for separate detection hardware.
2Reliability
If conventional short detection methods are used, then short circuit detection is provided, but power consumption increases
Solution Approach 1:
The detection mechanism leverages the amplifier's own operating signal and existing components, requiring no additional power for separate detection circuits. The PWM signal already being generated for audio output is reused for detection purposes, and the load impedance measurement is performed using the same power already dissipated during normal operation.
Solution Approach 2:
The load impedance serves as an intermediary parameter that indirectly indicates short circuit conditions. Rather than directly monitoring for shorts, the system measures load impedance as a mediator - under normal conditions the impedance reflects the connected audio device, while under short conditions it reflects the amplifier's output impedance, providing detection without additional power expenditure.
3Device complexity
If digital detection assuming PWM pulse is never skipped is used, then detection simplicity is maintained, but measurement precision deteriorates when output PWM period varies with input power signal
Solution Approach 1:
The detection threshold and expected PWM characteristics are made dynamic rather than fixed. The system adapts its detection criteria based on the actual operating conditions and input signal levels, allowing accurate detection even when the PWM period naturally varies with the audio signal. This dynamic adaptation prevents false positives while maintaining detection sensitivity.
Solution Approach 2:
The system uses feedback from the actual PWM signal characteristics to adjust detection parameters. By continuously monitoring the relationship between input signal levels and PWM output characteristics, the system can distinguish between normal variations and actual short circuit conditions, improving precision without increasing complexity.
Data Source
AI summary
An apparatus detects a short of a class-D amplifier. A pulse detector detects an output PWM pulse exceeds a predetermined width and a controller differentiates whether the pulse width exceeding is caused by a short or by a large digital input signal occurring during normal operation based on an expected level of the digital input signal to a level of the digital input signal when the pulse width exceeds the predetermined width. The expected level is dynamically obtained in response to one or more previous PWM pulses exceeding the predetermined width during the normal operation and may be selected based on an amount the pulse exceeds the predetermined width. A lookup table of predetermined levels, selected based on impedance of a load, provides the expected level if the expected level has not yet been dynamically obtained when the pulse width exceeds.