Hybrid Feedback Oscillation Modulator for Stable Class D Amplifiers
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Solution Overview
Problem
Class D amplifiers face challenges such as zero power supply rejection, distortion due to parasitic effects in power MosFETs, non-linear output filters causing frequency-dependent impedance, and electromagnetic interference (EMI), which complicate achieving robust stability and high audio performance, especially under varying load conditions.
Innovation Solution
The Hybrid feedback Controlled Oscillation Modulator (HCOM) architecture, which includes a pulse modulator, switching power stage, and a passive filter, with specific feedback paths and a forward path to form a closed loop, allowing for self-oscillating conditions that provide wide bandwidth error compensation and stability without excessive feedback differentiation or passive damping, thus eliminating filter and load dependency from the system transfer function.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing 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 patent divides the feedback loop into two separate paths: a local feedback path that provides high-frequency error correction without enclosing the output filter, and a global feedback path that handles low-frequency corrections. This segmentation allows the system to achieve high loop gain-bandwidth for filter distortion compensation without making oscillation conditions dependent on filter Q and load conditions.
Solution Approach 2:
The patent introduces an intermediary local feedback path that acts as a mediator between the switching stage and the global feedback loop. This local path provides high-frequency error correction directly at the switching stage output, enabling the system to minimize filter distortion without requiring the global loop to enclose the output filter, thereby avoiding load conditioned stability issues.
2Stability of the object's composition
If passive filter damping with RC Zoebel networks is used to reduce filter Q in open load situations, then load conditioned stability is improved, but power resistors add complexity to the system and degrade efficiency
Solution Approach 1:
The patent uses active feedback control through the local feedback path to dynamically adjust and dampen filter resonance effects without requiring passive RC Zoebel networks. The feedback mechanism provides load-adaptive damping that maintains stability across different load conditions including open load situations, while avoiding the efficiency degradation and complexity associated with power resistors.
3Stability of the object's composition
If feedback differentiation is used to improve stability, then stability margins are improved, but noise is picked up and fed to the control system and bandwidth is limited
Solution Approach 1:
The patent segments the feedback function into two paths with different frequency characteristics. The local feedback path handles high-frequency error correction without differentiation, avoiding noise pickup. The global feedback path handles low-frequency stability without requiring aggressive differentiation, thus limiting bandwidth less severely than traditional single-loop designs.
4Stability of the object's composition
If excessive loop compensation by feedback path differentiation is used to prevent undesired 2nd state oscillation, then oscillation stability is improved, but the effective loop transfer function gain is reduced
Solution Approach 1:
The patent segments the compensation function between two feedback paths. The local feedback path provides high-frequency error correction with appropriate phase margin without excessive differentiation. The global feedback path provides low-frequency stability compensation. This segmentation allows the system to prevent 2nd state oscillation while maintaining high effective loop transfer function gain for audio bandwidth.
Data Source
AI summary
A Hybrid feedback Controlled Oscillation Modulator (HCOM) is disclosed, having a 1st feedback path from the output voltage of the switching stage and a second feedback path from the filter output, the two feedback paths being superposed to provide a weighted state feedback signal. The state feedback path signal is subtracted from the input signal to form an error signal, the error signal filtered by a forward path compensation block B(s), closing the loop by feeding the pulse modulator, the loop having a transfer function such that self-oscillation can be established in the closed loop system. In an example 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 β. Further embodiments include a pure passive realization without a forward path device and the application of 3rd feedback loops to enhance global amplifier performance.