PWM Audio Amplifier Output Slope Control
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Solution Overview
Problem
Pulse-width modulated (PWM) switching power amplifiers, or class-D amplifiers, face issues with uncontrolled output slope during non-overlapping transitions, leading to electromagnetic interference (EMI) and distortion due to body diode conduction, which degrades efficiency and increases failure rates.
Innovation Solution
A switching power amplifier with a half-bridge transistor pair and a switching control circuit that uses a current source to maintain output current continuity during non-overlap periods, controlling the output voltage slope and preventing body diode conduction, thereby reducing EMI and distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If half-bridge transistors are operated in a non-overlapping fashion to avoid conduction through both transistors, then efficiency is improved and failure rate is reduced, but body diode conduction causes ringing and electromagnetic interference (EMI)
Solution Approach 1:
The patent applies preliminary action by proactively controlling the output slope during the non-overlap interval before body diode conduction can occur. The slope control circuit pre-regulates the voltage transition rate, preventing the conditions that lead to body diode turn-on and subsequent ringing/EMI, rather than reacting after the problem occurs.
Solution Approach 2:
The patent changes the parameter of output voltage slope (dV/dt) during the non-overlap interval. By actively controlling and limiting the slope rate, the patent modifies the electrical parameters to prevent body diode conduction while maintaining efficient switching operation, thus resolving the contradiction between efficiency and EMI.
2Loss of energy
If half-bridge transistors are operated in a non-overlapping fashion, then shoot-through current is reduced, but uncontrolled output slope increases distortion
Solution Approach 1:
The patent controls the output voltage slope parameter during the non-overlap interval to remain within predetermined limits. This parameter control ensures that distortion remains below a predetermined threshold while maintaining the non-overlapping operation that prevents shoot-through current.
Solution Approach 2:
The patent employs feedback through slope control circuitry that monitors and regulates the output voltage transition rate. This feedback mechanism ensures the slope remains controlled during switching transitions, preventing excessive distortion while maintaining efficient non-overlapping transistor operation.
3Speed
If switching transitions are performed quickly to improve response time, then bandwidth is improved, but output slope control becomes difficult and EMI increases
Solution Approach 1:
The patent applies dynamics by making the output slope control adaptive and variable rather than fixed. The slope control circuit dynamically adjusts the voltage transition rate based on operating conditions, allowing fast switching when appropriate while preventing EMI-generating slopes, thus resolving the contradiction between speed and EMI.
Data Source
AI summary
An audio switching power amplifier having an output with controlled-slope transitions maintains efficiency while avoiding uncontrolled non-overlap intervals during switching transitions. A pair of transistors forming a half-bridge that supplies an output signal at an output terminal of the amplifier are operated so that neither transistor is fully on during an overlap time period. A current source provides an output current to the output terminal during the non-overlap time period to control the output voltage while changing the transistor that conducts the output current from a first one of the pair of transistors to a second one of the pair of transistors. The current source may be provided by operation of one of the transistors in a current source configuration. The voltage of a gate of one of the transistors can be compared with a threshold to provide an indication of the current.