Gradient Amplifier Drive Circuit for Low Current Ripple
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Solution Overview
Problem
Current gradient amplifiers in MRI systems face challenges in achieving optimal rise time, intensity, linearity, and stability of gradient magnetic fields, which affect imaging speed and image quality.
Innovation Solution
The proposed solution involves a gradient amplifier with multiple working and freewheeling half-bridge groups, where each group includes switches configured to form current paths through a gradient coil, with inductors for noise filtering and phase-shifted drive signals to reduce current ripple, and a drive circuit with phase shifters and frequency dividers to optimize power distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple working half-bridge groups are used to supply power to the gradient coil, then the current ripple is reduced and imaging performance is improved, but the device complexity increases
Solution Approach 1:
The gradient amplifier is divided into multiple working half-bridge groups (first, second, third, and fourth groups), each independently supplying power to the gradient coil. This segmentation allows parallel current paths that reduce current ripple and improve gradient magnetic field stability while distributing the power delivery load across multiple modular units.
2Reliability
If inductors are added for noise filtering in each half-bridge group, then the current ripple and noise are reduced, but the device complexity and component count increase
Solution Approach 1:
Inductors are strategically placed at specific locations within each half-bridge group circuit - specifically in the current paths of the first, second, third, and fourth working half-bridge groups. This local placement of filtering components targets the exact points where current ripple generation occurs, providing effective noise reduction while minimizing unnecessary component addition.
3Power
If phase-shifted drive signals are used to control the switches, then the power distribution is optimized and current ripple is reduced, but the control circuit complexity increases
Solution Approach 1:
Phase-shifted periodic drive signals are applied to control the switches in different half-bridge groups. The drive signals have specific phase relationships (e.g., 180 degrees between complementary switches within a group, and staggered phases between different groups) that create optimized power delivery patterns and reduce current ripple through constructive and destructive interference of the current waveforms.
4Manufacturing precision
If multiple current paths are formed through the gradient coil, then the gradient magnetic field intensity and linearity are improved, but the voltage and current stress on individual IGBTs increases
Solution Approach 1:
The power delivery function is segmented across multiple working half-bridge groups, with each group contributing a portion of the total current to the gradient coil. This segmentation distributes the voltage and current stress across multiple IGBTs within each group, preventing any single device from bearing excessive load while collectively achieving the required gradient magnetic field intensity and linearity.
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
This configuration reduces power density, voltage, and current stress on IGBTs, significantly lowering current ripple and improving the accuracy and reliability of the gradient amplifier, leading to enhanced imaging performance.
Implementation Method 1
The first inductors of adjacent working half-bridge groups of the working half-bridge groups can be configured to couple with each other to obtain first common-mode inductor sets and first differential-mode inductor sets, and the second inductors of the adjacent working half-bridge groups can be configured to couple with each other to obtain second common-mode inductor sets and second differential-mode inductor sets. Each of the common-mode inductor sets is configured to filter out common-mode noise, and each of the differential-mode inductor sets is configured to filter out differential-mode noise.
Data Source
AI summary
A gradient amplifier includes N working half-bridge groups. In each of the working half-bridge groups, a first working half-bridge includes a first switch and a second switch, and a second working half-bridge includes a third switch and a fourth switch. An emitter of the first switch is coupled with a collector of the second switch at a first coupling point, and an emitter of the third switch is coupled with a collector of the fourth switch at a second coupling point. A gradient coil is coupled between the first coupling point and the second coupling point in each of the working half-bridge groups, and a current flowing through the gradient coil is a sum of currents flowing through the N working half-bridge groups.