Multi-Level Inverter for MRI Gradient Amplifier Harmonic Reduction
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Solution Overview
Problem
Conventional gradient amplifiers used in MRI systems generate rich high frequency harmonics due to their two-level inverter topology, leading to inefficiencies in EMI filtering, instability, and high current ripple in gradient coils.
Innovation Solution
A multi-level inverter is introduced, utilizing H bridge multi-level modules with coupled inductors to provide a lower change in output voltage, reducing high frequency harmonics and improving filter efficiency while minimizing common mode currents and current ripple.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional two-level inverter is used in the gradient amplifier, then the device complexity is low, but rich high frequency harmonics are generated leading to poor EMI filter efficiency
Solution Approach 1:
The patent divides the conventional two-level inverter into multiple multi-level inverter modules (e.g., three-level or five-level inverters). Each module generates a stepped voltage waveform with fewer harmonics, and the modules are connected in series to achieve the required output voltage. This segmentation approach reduces high frequency harmonics while maintaining manufacturing feasibility through modular design.
2Object-generated harmful factors
If large inductance values are used in the EMI filter inductors to attenuate high frequency harmonics, then the filter attenuation performance improves, but the overall system control bandwidth is affected
Solution Approach 1:
The EMI filter is segmented into multiple stages with smaller inductance values distributed across different frequency ranges. This allows effective attenuation of high frequency harmonics while maintaining smaller total inductance, thereby preserving the system control bandwidth.
Solution Approach 2:
The patent changes the inductance parameter from a single large value to multiple smaller values distributed in a multi-stage filter configuration. This parameter transformation maintains the required attenuation performance while reducing the impact on control bandwidth.
3Object-generated harmful factors
If large capacitance value is used in the EMI filter capacitors to attenuate high frequency harmonics, then the filter attenuation performance improves, but the overall system control bandwidth is affected
Solution Approach 1:
The capacitor configuration is segmented into multiple smaller capacitance values distributed across different stages of the filter. This segmentation achieves effective high frequency attenuation while maintaining smaller total capacitance, thus preserving system control bandwidth.
4Object-generated harmful factors
If large resistance values are used in the damping resistors to attenuate high frequency harmonics, then the filter attenuation performance improves, but the filter efficiency becomes poor
Solution Approach 1:
The damping function is segmented across multiple resistors with smaller resistance values distributed in the filter stages. This approach provides effective high frequency attenuation while reducing the total power loss compared to a single large resistance value, thereby improving overall filter efficiency.
Data Source
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AI summary
The invention relates to a multi-level inverter and a method for providing multi-level output voltage by the multi-level inverter, wherein the multi-level inverter comprises at least one cascaded H bridge multi-level module, wherein each H bridge multi-level module comprises a first H bridge multi-level leg and a second H-bridge multi-level leg connected in parallel, wherein each of the first and second H bridge multi-level legs comprises a first inverter leg and a second inverter leg coupled in parallel through a coupled inductor, the first and second inverter legs are coupled respectively to a primary winding and a second winding of the coupled inductor for providing first and second input voltages with multiple voltage levels, the primary and secondary windings of the coupled inductor are coupled in series and a junction node of the primary and secondary windings forms an output terminal of the H bridge multi-level leg, wherein the first input voltage has a predetermined phase shift relative to the second input voltage. The multi-level inverter can have lower change in output voltage and thus eliminates the problems caused by the great change in output voltage.