Achieving high power density, higher efficiency and superior performance for next-generation transportation systems means reducing the volume and weight of power conversion systems. To that end, cryogenic power devices have demonstrated a number of benefits when compared to their room temperature counterparts.
Power electronics is a critical technology for enabling electromechanical drives, transportation, renewable energy systems and power grids. For the electric vehicles, cryogenic power electronics will not fly, but there are applications for large vehicles such as airplanes, ships, and aircraft carriers. The use of superconducting generator with cryogenic power converter for large wind turbine systems could be of significant improvement in improving the power density and efficiency. In addition to efficiency, weight, and size reductions, cryogenic power converters offer other benefits, including improved switching speed and reliability. All of these characteristics make cryogenic power electronics combined with superconducting motors a viable technology for next generation mass power transportation systems, as well as future aircrafts, ships, and for wind energy systems.
A great deal of research has been done to understand how the various components of power electronic systems function at low temperatures and how these components can be combined to attain higher power density, efficiency and performance.
Cryogenic power electronic systems contain a number of components, all of which have been demonstrated to function well at low temperatures, and in some cases better than their room temperature counterparts. For example, operating power semiconductor devices at cryogenic temperatures have shown to improve switching speed and on-state voltage in comparison to room temperature operation because semiconductor materials seem to have demonstrated better electrical and thermal properties at lower temperatures up to about 50 °K. They also have higher carrier mobility and saturation velocity at low temperatures, which results in high-speed operation capability.
Lower temperatures also significantly improve the thermal conductivities of device and substrate materials. This in turns leads to simpler thermal management, lower on-state power loss and higher reliability, while the switching losses of power devices decrease at cryogenic temperatures, leading to increased overall power conversion efficiency. Significant performance improvements have been reported for many power devices when operated at cryogenic temperatures: for a power metal-oxide-semiconductor field-effect transistor (MOSFET), the on-state resistance falls by as much as five times.
Commercially available MOSFETs in plastic or metal packages also have been found to work well if immersed in a bath of liquid nitrogen, even though these devices have not been designed for cryogenic applications. They can be operated at much higher current levels with lower conduction losses; hence high efficiency power converters could be designed. The characteristics of various power devices at low temperatures down to 20 °K have been investigated, and it appears that among the commercially available power devices, silicon n-channel power MOSFETs are the most optimized for cryogenic applications since they can achieve extremely low on-state resistance and reasonable breakdown voltages.
Insulated-gate bipolar transistors (IGBTs) have been found to work more efficiently at low temperatures with the decrease of on-state voltage and turn-off time, despite the decrease of breakdown levels. The reductions in on-state voltage drop were found to be between 20 and 30 percent, and the turn-off time reduction was by a factor of approximately two to three over the temperature range from room temperature down to 50 °K. Most IGBTs exhibited low forward voltage drop at lower temperatures until 100 °K and a slight increase above this temperature due to carrier freeze out. The switching performance of IGBTs is also shown to improve at low temperatures.
Many passive components and off-the shelf integrated circuits have been proven to operate satisfactorily at temperatures down to 50 °K. Low temperature operation of the capacitors depends on the dielectric medium such as polypropylene, polycarbonate, mica, film, and ceramic; they seem to function properly with decreasing leakage current and dissipation factor at cryogenic temperatures. For magnetic components, magnetic losses generally increase with cooling, and the power dissipation is not too much different than at the room temperature. If superconducting windings are substituted with the copper windings, then the loss comparison between core and windings becomes more promising. A coreless design is even better for inductors at cryogenic environment to get maximum efficiency.
More research needs to be done to better understand the longer-term effects and the repeated cyclic operation of passive components at low temperatures.
Research conducted by several universities in partnership with Rolls-Royce to test a DC-DC converter consisting of a number of parallel-connected power MOSFETs at cryogenic temperatures found improved performance in terms of reliability and efficiency.
An advanced radiation-hardened DC-DC converter was found to show good performance in regulation, efficiency, and dynamic characteristics at temperatures down to 73°K, although some instability was observed as the temperature was decreased further. More testing under long-term thermal exposure is needed to fully understand the performance of these converters for potential application in low temperature environments.
The commercial-off-the-shelf integrated circuits such as digital-to-analog and analog-to-digital converters, DC-DC converters, operational amplifiers and oscillators have been investigated for potential use at low temperatures. These components operate well and also maintain stability down to 80 °K.
source: http://powerelectronics.com/power-electronics-systems/cryogenic-power-transportation-heats?page=2
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