H-Bridge Power Converter Bias Current Injection for MRI Gradient Control
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Solution Overview
Problem
Existing power converters for MRI systems face challenges in accurately controlling high voltage and current for gradient coils due to blanking-time errors, which result in poor output quality and difficulty in tracking reference signals, especially at low load currents.
Innovation Solution
The proposed electric power converter includes a set of four controllable power switches in an H-bridge configuration with a controller and bias current injection circuits at the central points of each switching leg, eliminating the need for measurements of parasitic component parameters and ensuring zero dead-band at low load currents by forcing a known current direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a blanking time is introduced to prevent shoot-through in H-bridge power converters, then switching safety is improved, but output signal quality deteriorates due to dead-band effects at low load currents
Solution Approach 1:
The patent applies preliminary anti-action by introducing a compensating current that acts in advance to counteract the harmful effect of blanking time. The compensating current is injected during the blanking period to maintain the magnetic field continuity, preventing the dead-band effect from degrading output signal quality while preserving the necessary blanking time for switching safety.
Solution Approach 2:
The patent uses an intermediary approach by introducing a compensating current as a mediator between the switching leg and the gradient coil. This compensating current, injected through an additional switching leg, serves as an intermediate element that maintains magnetic field continuity during the blanking period, thereby eliminating dead-band effects without compromising the primary switching leg's safety blanking time.
2Power
If the number of H-bridge cells is increased to provide higher output power, then power capability is improved, but blanking time errors are amplified affecting control precision
Solution Approach 1:
The patent applies segmentation by dividing the compensation function into separate compensating switching legs, each associated with a corresponding H-bridge cell. This allows independent compensation for each cell's blanking time error, enabling the system to maintain high output power with multiple cells while achieving precise control by compensating for each cell's individual blanking time effects.
3Measurement precision
If model-based compensation methods are used to correct blanking time errors, then control accuracy is improved, but system complexity increases due to requirement of parasitic parameter measurements
Solution Approach 1:
The patent applies self-service by designing a compensation system that does not require external measurements of parasitic parameters. The compensating switching legs automatically generate the necessary compensating current based on the switching control signals, enabling the system to correct blanking time errors without needing to measure or know the specific parasitic inductance values, thereby reducing system complexity.
Data Source
AI summary
The present specification relates to an electric power converter, comprising at least a set of four controllable power switches, arranged in an H-bridge or a functionally equivalent circuit comprising two switching legs of two series switches connected to a voltage source, each power switch comprising an antiparallel diode, a controller configured for controlling the switches with a blanking time, a feedback loop for the load current, characterized by a first bias current injection circuit, coupled to the central point of the first leg of the H-bridge and a second bias current injection circuit, coupled to the central point of the second leg of the H-bridge. The specification further relates to a MRI scanner, provided with an electric power converter according to any of the preceding claims, for driving the gradient coils.