Cascaded Switching Amplifier Control for Constant Common-Mode Voltage
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Solution Overview
Problem
In MRI systems, gradient drivers using pulse width modulation (PWM) generate high-frequency harmonic contents, leading to significant degradation of MR image quality due to common mode (CM) electromagnetic noise, which contaminates RF signals.
Innovation Solution
A switching amplifier with cascade elements and a control device configured for space vector modulation (SVM) to maintain a constant common mode voltage, reducing electromagnetic interference (EMI) by controlling switches to keep the common mode voltage within a predetermined range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If pulse width modulation (PWM) method is used to control power semiconductor devices, then electrical power transfer control is achieved, but high frequency harmonic contents are generated which contaminate RF signals and degrade MR image quality
Solution Approach 1:
The patent divides the conventional single-stage inverter into a cascaded multi-stage structure, where each stage processes a portion of the output voltage requirement. This segmentation allows each stage to operate at lower switching frequencies and reduces the amplitude of voltage switches, thereby minimizing high-frequency harmonic generation while maintaining effective power control capability
Solution Approach 2:
The patent transitions from conventional PWM control in the time domain to space vector modulation (SVM) control that operates in the complex voltage space domain. This dimensional transformation enables more efficient voltage vector synthesis, reduces voltage switching transitions, and minimizes high-frequency harmonics while maintaining precise power control
2Ease of operation
If conventional PWM control is used, then simple control implementation is achieved, but common mode electromagnetic noise is generated which significantly degrades MR image quality
Solution Approach 1:
The patent implements dynamic control of the cascaded inverter stages using space vector modulation, where the switching patterns of individual stages are continuously adjusted based on the required output voltage vector. This dynamic coordination between stages ensures that common-mode voltage components are minimized throughout the switching cycle, reducing electromagnetic noise while maintaining control flexibility
Solution Approach 2:
The patent introduces an intermediate control layer that coordinates the switching operations of multiple cascaded stages. This intermediary control mechanism synchronizes the switching patterns across stages to minimize common-mode voltage generation, acting as a mediator between the individual stage operations and the overall output voltage requirement
Data Source
AI summary
A method is used for operating a switching amplifier, the switching amplifier includes a plurality of cascade elements. The method includes: coupling the cascade elements in series between two terminals of a load; providing two leg circuits each comprised of switches in each of the cascade elements; and controlling all of the switches comprised in the switching amplifier using space vector modulation (SVM), such that a change of a common mode (CM) voltage generated by the switching amplifier is in a predetermined range.