Hybrid Feedback Oscillation Modulator for Load-Independent Class D Control
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Solution Overview
Problem
Class D amplifiers face challenges such as zero power supply rejection, distortion due to parasitics in power MosFETs, non-linear output filters, and electromagnetic interference (EMI), which complicate achieving robust stability and high audio performance, especially under varying load conditions and power supply ranges.
Innovation Solution
The Hybrid feedback Controlled Oscillation Modulator (HCOM) architecture, which includes a pulse modulator, switching power stage, and a passive filter, employs a first feedback path with a low pass filter and a second feedback path with a phase lead characteristic, forming a closed loop with a forward path to ensure self-oscillating conditions, allowing for enhanced error compensation and stability without excessive feedback differentiation or passive damping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If global loop oscillation modulator architecture is used to maximize loop gain-bandwidth enclosing the output filter, then filter distortion and output impedance are minimized, but oscillation conditions become filter Q dependent introducing load conditioned stability
Solution Approach 1:
The feedback loop is segmented into two separate paths: a local feedback path from the switching stage output and a global feedback path from the filter output. This segmentation allows the local path to provide fast error correction for power stage disturbances while the global path provides overall system regulation, eliminating the dependency between oscillation conditions and filter Q while maintaining stability across different load conditions.
Solution Approach 2:
The local feedback path acts as an intermediary that provides fast error correction for power stage disturbances before they reach the global loop. This intermediary mechanism allows the system to correct power stage errors locally without requiring the global loop to be tightly coupled with the filter, thereby decoupling oscillation conditions from filter Q while maintaining both distortion performance and load-independent stability.
2Stability of the object's composition
If feedback differentiation is applied to improve stability in global loop oscillation modulators, then stability is enhanced, but noise is picked up and fed to the control system and bandwidth is limited
Solution Approach 1:
The feedback function is segmented between local and global paths, allowing the local path to handle fast transient stability without differentiation, while the global path provides overall regulation. This segmentation eliminates the need for feedback differentiation in the noise-sensitive global loop, maintaining stability enhancement without introducing noise pickup or bandwidth limitations.
3Stability of the object's composition
If passive damping is applied to the output filter to ensure stability under all load conditions, then load independent stability is achieved, but efficiency is reduced
Solution Approach 1:
Active feedback control replaces passive damping to achieve load-independent stability. The local feedback path provides fast error correction that stabilizes the system under all load conditions without requiring energy-dissipating passive damping components. This feedback mechanism maintains stability while avoiding the efficiency losses associated with passive damping.
4Power
If classical PWM switching power amplifier is used, then power conversion is achieved, but power supply rejection is zero and distortion from parasitics occurs
Solution Approach 1:
A local feedback path is implemented that senses the switching stage output and provides fast error correction for power stage and power supply disturbances. This local feedback mechanism achieves high power supply rejection and corrects distortion from parasitics in real-time, while maintaining efficient power conversion through the switching architecture.
Solution Approach 2:
The local feedback path performs preliminary error correction for power stage and power supply disturbances before they propagate to the output. By correcting these errors early in the signal path, the system prevents distortion from parasitics and achieves high power supply rejection while maintaining efficient power conversion.
Data Source
Figure 1a~1a
Figure 1b
Figure 2a~2c
AI summary
A Hybrid feedback Controlled Oscillation Modulator (HCOM) has a 1st feedback path from the output voltage of the switching stage Vp and a second feedback path from the filter output Vo, the two feedback paths superposed to provide a weighted state feedback signal Vf. Said state feedback path signal is subtracted from the input signal to form an error signal, said error signal filtered by a forward path compensation block B(s), closing the loop by feeding said pulse modulator, said loop having a transfer function such that self-oscillation can be established in the closed loop system. In a preferred embodiment, the first feedback path has a low pass characteristic and the second feedback path lead characteristic, the first feedback path being weighted by a weighting factor ß. Further embodiments include a pure passive realization without forward path means B(s) and the application of 3rd feedback loops to enhance global amplifier performance.