Why consider cryogenic operation? What are the major advantages? The most impressive is superconductivity. When electrical current passes through conventional copper wire, it encounters resistance, impeding its flow. Overcoming this resistance requires energy, which is lost as heat. Superconducting wires and cables, on the other hand, offer no resistance to direct currents. Currents flow with zero losses, allowing for astonishing electrical efficiencies in superconducting systems. These cables are potentially much lighter than their copper counterparts.

Superconducting motors, transformers, transmission lines, and circuits are capable of exceptional performance. The illustration below shows a high-temperature superconducting (HTS) motor designed by American Superconductor Corporation. HTS components are typically operated at around 77 K (-196 °C), the temperature of liquid nitrogen. Achieving such low temperatures requires energy. Further losses are incurred when electrical leads and interfaces transition from these low-temperature elements to room-temperature motor controller power circuits. Moreover, the power circuits themselves can be relatively inefficient.

Replacing conventional semiconductors with cryogenic power electronics can greatly reduce these losses. Cryogenic Power Electronics can optimize the advantages of the superconducting elements in a system, and requires little additional hardware to implement, especially when the cryogenic platform is already present for superconducting elements.

In the late 1980’s, around the same time that high-temperature superconductivity was discovered, MTECH staff members were pioneering the field of Cryogenic Power Electronics, or Cryopower.

To illustrate the Cryopower Concept, let’s first look at a conventional power circuit driving a 100-kW motor. At 95 percent efficiency, 5 kilowatts of heat must be dissipated using large heat sinks. In a CryoPower circuit, power is brought to the circuit through zero-loss superconducting cables. The cryogenic semiconductor components dissipate only 200 Watts because of improved performance at low temperatures. This small amount of heat is easily removed by a cooling fluid or refrigerator. The new efficiency is greater than 99.8 percent, quite a significant increase in large power systems.

CryoPower is based on the discovery that the loss-producing on-state resistance of many power semiconductor devices drops drastically when these devices are cooled to 77 degrees Kelvin, the temperature of liquid nitrogen. At room temperature, power semiconductors carrying large currents often reach temperatures up to 400 K, further increasing their resistance. When cooled to 77 K, this resistance is reduced by up to 37 times.

Another key feature of low-temperature operation is the increase in the switching speeds of semiconductor devices. The 1300-V IGBT represented in the following figure switches in 282 ns at 400 K. In liquid nitrogen, the rise time drops to 33 ns, more than eight times faster than at 400 K.

CryoPower devices are also much more reliable than their conventional counterparts. Based on the trend observed at higher temperatures, the mean-time-between-failure (MTBF) is much longer than the age of the observable universe.

In practice, thermal cycling will reduce these lifetimes to some degree, but the potential device lifetimes are breathtaking.

CryoCircuits products are backed by a strong partnership with MTECH Labs, which brings expertise and know-how in designing and implementing Cryogenic Power Electronics into your existing systems.