Cryogenic MOSFET Rectifier With HTS Filter for Low-Ripple Magnet Current
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Solution Overview
Problem
Existing current sources for superconducting magnets suffer from high AC ripple and inefficient cooling, particularly in conduction-cooled systems, which are unsuitable for high-quality applications like MRI and NMR due to the limitations of traditional current leads and rectifier circuits.
Innovation Solution
A current source design incorporating a DC power source, inverter, step-down transformers, cryogenic synchronous rectifier circuit, and a superconducting AC filter, along with high-temperature superconducting leads, to provide low-voltage, high-current power with reduced AC ripple and improved cooling efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If traditional normal metal current leads are used to deliver current to superconducting magnets, then current can be delivered from room temperature to cryogenic environment, but thermal loss through conduction is high due to Wiedemann-Franz Law
Solution Approach 1:
The patent changes the material parameter from normal metal to high-temperature superconducting material, which fundamentally alters the thermal and electrical conduction properties. HTS materials violate the Wiedemann-Franz law by maintaining low electrical resistance while having reduced thermal conduction, thereby decreasing thermal loss without requiring more complex cooling systems
Solution Approach 2:
The patent employs composite current lead structures combining normal metal and superconducting material sections. The normal metal section handles room temperature current conduction, while the HTS section operates in the cryogenic environment to minimize thermal conduction loss, optimizing the trade-off between electrical conduction and thermal insulation
2Speed
If exciters flux pumps are used to inject current into magnets, then current can be generated through changing magnetic field, but voltage generated is small (tens of millivolt) limiting charging speed
Solution Approach 1:
The patent replaces the mechanical flux pump system with an electronic switching circuit using cryogenic MOSFETs. This substitution eliminates the limitation of small voltage generation by using electronic switches that can handle high voltages and currents, enabling rapid magnet charging while maintaining system reliability
Solution Approach 2:
The patent changes the operating parameters by using MOSFETs with positive temperature coefficient of channel resistance, which exhibit lower resistance at cryogenic temperatures. This parameter change enables efficient high-voltage switching and current injection, dramatically improving magnet charging speed compared to flux pumps
3Loss of energy
If MOSFETs with positive temperature coefficient are used in parallel for cryogenic rectification, then conduction loss decreases, but device complexity increases
Solution Approach 1:
The patent segments the rectifier circuit into multiple parallel MOSFET channels, each handling a portion of the total current. This segmentation allows the use of lower-current-rated MOSFETs in parallel, reducing individual device stress and total conduction loss while distributing the complexity across modular units that are easier to manage and replace
4Ease of manufacture
If conduction-cooled system is used instead of liquid helium-cooled system, then cooling capacity is limited to about 100 W at 60 K, but system becomes more economical
Solution Approach 1:
The patent changes the operating temperature parameter to match the cooling capacity of conduction-cooled systems. By designing the magnet and current leads to operate efficiently at 60 K with a cooling power requirement of approximately 100 W, the system achieves economical operation without requiring expensive liquid helium cooling infrastructure
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 achieves a voltage ripple of less than 10 mV and significantly reduces cooling power requirements, making it suitable for high-quality superconducting magnets.
Implementation Method 1
a superconducting AC filter positioned between the cryogenic synchronous rectifier circuit and the superconducting magnet for reducing AC ripple
Implementation Method 2
a pair of high temperature superconducting leads for electrically connecting said superconducting magnet to said filter
Implementation Method 3
an inverter coupled to the DC power source for generating a high-frequency high-voltage AC signal from the low-current, high-voltage DC signal
Implementation Method 4
a plurality of step-down transformers coupled to the inverter for stepping down the voltage of the AC signal
Implementation Method 5
a cryogenic synchronous rectifier circuit coupled to the transformers for receiving and rectifying the stepped-down AC signal
Data Source
AI summary
A regulated current source that provides high DC current and low voltage ripple to a superconducting electromagnet. The current source is a multi-phase synchronous rectifier, wherein the rectifying elements are cryogenically-cooled MOSFETS coupled with a superconducting filter.