Differential Amplifier Common-Mode Control at Low PWM Duty Cycle
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Solution Overview
Problem
Class-D amplifiers experience distortion and efficiency loss due to one bridge-half stopping switching, particularly in low duty-cycle PWM operation, which is not effectively addressed by existing feedback mechanisms.
Innovation Solution
A differential amplifier circuit with a pulse width modulation modulator, differential and common mode current digital-to-analog converters, and loop integrators, employing feedforward and feedback control to manage common-mode and differential-mode signals independently, reducing ripple currents and switching activity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If low duty-cycle PWM operation is used to reduce ripple currents and conduction loss, then efficiency is improved, but distortion occurs when one bridge-half stops switching
Solution Approach 1:
The patent segments the control of common-mode and differential-mode signals by introducing a dedicated common-mode feedback loop that operates independently from the differential-mode signal path. This allows the common-mode voltage to be regulated separately, preventing distortion while maintaining the efficiency benefits of low duty-cycle operation.
Solution Approach 2:
The patent implements a common-mode feedback mechanism that monitors and adjusts the common-mode voltage level. This feedback loop detects when one bridge-half stops switching and compensates by adjusting the common-mode reference voltage, thereby preventing distortion while allowing low duty-cycle operation to continue.
2Loss of energy
If low duty-cycle PWM operation is used to align common-mode voltage with feedback loop reference, then efficiency is improved, but common-mode control becomes challenging
Solution Approach 1:
The patent separates common-mode control from differential-mode control by implementing an independent common-mode feedback loop. This segmentation allows each control function to be optimized independently, reducing the overall control complexity while maintaining efficiency benefits.
Solution Approach 2:
The patent introduces a common-mode reference voltage as an intermediary element that mediates between the differential-mode PWM operation and the feedback loop. This intermediary allows the system to maintain proper common-mode levels without directly complicating the differential-mode control path.
3Loss of energy
If one bridge-half stops switching to reduce transition losses, then efficiency is improved, but distortion and supply rejection deteriorate
Solution Approach 1:
The common-mode feedback loop continuously monitors the common-mode voltage and adjusts it to maintain proper operating levels even when one bridge-half stops switching. This feedback mechanism ensures that supply rejection is maintained despite the asymmetric switching conditions that occur in low duty-cycle operation.
Solution Approach 2:
The patent dynamically adjusts the common-mode reference voltage parameter based on the operating conditions. When one bridge-half stops switching, the common-mode feedback loop modifies the common-mode voltage level to compensate, thereby maintaining supply rejection performance without requiring symmetric switching from both bridge-halves.
Data Source
AI summary
A differential amplifier circuit is provided. The circuit includes a PWM modulator for generating a PWM signal representative of a difference between the first and the second staircase-like reference signals and the digital input signal, a DM-IDAC for receiving the PWM signal and providing a first and second differential mode current, a CM-IDAC for receiving the PWM signal and providing a common mode current, first and second loop integrators, and first and second comparators; each loop integrator comprising virtual ground node terminal for receiving the differential mode current, the common mode current, and a feedback signal from an output stage of the differential amplifier circuit via a feedback loop, and integrator output terminal for providing loop integrator output signal proportional to an integral of the signals received at the virtual ground node terminal, the comparators receiving the loop integrator output signal, and triangular reference signal.