Characteristics of p-i-n power diodes for power electronics operated at cryogenic temperatures

The operation of switching power converters at cryogenic temperatures has received increasing attention as key a subsystem in the implementation of electrical generation, storage and transmission systems which are being developed to take advantage of favorable dielectric and electrical conductivity properties which occur at these temperatures. Systems which have been considered and/or are currently under development include hyperconducting and superconducting alternators, superconducting power transmission, superconducting magnetic energy storage and cryogenic capacitive direct electric energy storage. The advent of high temperature superconductors has increasingly focused attention on 77 K as the operating temperature but, for certain aerospace applications, around 150 K is considered to be optimum from an overall system viewpoint. All studies performed to date indicate that cryogenic switching power converters will utilize essentially the same topologies, modulation methods, filtering and controls found in ambient or elevated temperature versions. The critical need is for switching devices capable of ultra-low loss, high switching speed operation and it has already been established that majority carrier devices in the power MOSFET category operate successfully down to at least 20 K and that, for the higher temperature (150 K condition, other devices, such as IGBTs, can be utilized. On the other hand, the all-important power diode has received much less attention although its bi-polar character has to raise concerns about its ability to meet operating requirements, especially at 77 K and below. This paper reports the results of a study of p-i-n power diodes, including experimentally obtained static and dynamic characteristics at ambient, dry ice and liquid nitrogen temperatures. These are used to identify critical parameters and their range of values appropriate to successful power diode operation down to 77K. From these results, a preliminary approach to diode optimization is presented.