Multilevel Inverter for MRI Gradient Coil Current Control
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Solution Overview
Problem
Current gradient amplifiers in MRI systems face challenges with heat management due to high current and voltage requirements, leading to bulkiness and control complexity, particularly with multilevel inverter topologies that use many switches and capacitors.
Innovation Solution
A multilevel inverter with at least seven potential levels is designed for the MRI system, featuring a cascade of switching arms, diodes forming a full bridge, and a controller generating control signals to manage AC output voltage, reducing the number of switching arms and improving efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If conventional gradient amplifier topology with twelve switches and four capacitors is used, then high current and voltage can be supplied, but the amplifier becomes bulky and difficult to control
Solution Approach 1:
The patent divides the DC voltage source into multiple levels (at least seven potential levels) using a segmented voltage supply structure. Instead of using a single high-voltage source with many switches, the system uses multiple lower-voltage segments that can be combined to achieve the required output voltage levels, thereby reducing the total number of switching components while maintaining power capability
Solution Approach 2:
The patent transitions from a conventional two-level inverter structure to a multilevel inverter structure by adding voltage dimensionality. This creates at least seven distinct voltage levels (including positive and negative potentials), allowing for finer voltage control and reduced switching complexity through the use of diode-based voltage synthesis rather than traditional switch networks
2Productivity
If higher currents and voltages are supplied with shorter rise time, then image sharpness and imaging speed improve, but heat loss in semiconductor components increases significantly
Solution Approach 1:
The patent employs periodic pulse-width modulation (PWM) control to generate the gradient current waveforms. By using periodic switching actions with variable duty cycles, the system achieves precise control over current rise time and amplitude while distributing power delivery over multiple switching cycles, thereby reducing peak current stress and associated I²R losses in semiconductor components
Solution Approach 2:
The patent changes the voltage level parameter by introducing at least seven distinct potential levels in the output waveform. This multilevel voltage structure allows for smoother current transitions and reduced dv/dt stress on components, enabling shorter rise times with lower switching losses by delivering power in stepped increments rather than single high-voltage transitions
3Loss of energy
If multilevel inverter is used to mitigate cooling problems, then heat loss is reduced, but the amplifier becomes bulky and control becomes difficult
Solution Approach 1:
The patent introduces diodes as intermediary components that simplify the multilevel inverter control. Instead of using active switches for all voltage level transitions, passive diodes serve as intermediaries to automatically route current paths and synthesize voltage levels, eliminating the need for complex control logic while maintaining the multilevel voltage structure that reduces switching losses
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables precise, high-precision current pulses with low switching losses, enhancing image sharpness and reducing the complexity and cost of high-fidelity amplifiers, while improving MRI scanning accuracy and image quality.
Implementation Method 1
The multilevel inverter generates an AC output voltage, having at least seven potential levels, from a DC voltage source such that the generated AC voltage produces a current in a gradient coil of a magnetic resonance imaging system
Data Source
AI summary
In one embodiment, a multilevel inverter for generating an AC output voltage, having at least seven potential levels, from a DC voltage source such that the generated AC voltage produces a current in a gradient coil of a magnetic resonance imaging system is provided. The multilevel inverter comprises an input voltage supply device configured for providing a divided DC voltage, at least eight switching arms for deriving the AC output voltages from the divided DC voltages, each of the switching arms comprising an input terminal receiving the divided DC voltage, a switching device controlling the AC output voltage and an output terminal providing the AC output voltage, at least four diodes, each of the four diodes connected to a single pair of switching arms, the four diodes forming a full bridge having positive and negative nodes, one of the nodes connected to the input voltage supply device and another node connected to the input terminals of the switching arms and a controller coupled to the input voltage supply device and the four diodes, the controller configured for producing plurality of control signals for controlling the AC output voltage.