IEEE Electrification Magazine - December 2013 - 67

Table 1. The summarized ﬁndings of the on-state behavior for all of the measured devices.
On-State Behavior
Temperature
Range
Si n-channel MOSFETs and
superjunction MOSFETs

20 K

50 K

100 K

Little degradation in the on state

Optimum range

Non-ohmic behavior and negative temperature
dependence
Si p-channel MOSFETs

Negative temperature dependence

Optimum range

Non-ohmic behavior
SiC MOSFETs

Positive temperature dependence

Negative temperature dependence

No improvements compared to higher temperatures
GaN HEMTs

Almost temperature independent

Small positive temperature dependence

Si/SiC Schottky diodes

No improvements when compared to higher temperatures

GaAs Schottky diodes

Improvements at high current levels

(With permission from K.K. Leong, 2011.)

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. in
these studies, it is shown that these components operate
well and maintain stability in temperatures as low as 80 K.
ray et al. evaluated a 175-W buck dc-
dc converter operating at 50 kHz. this
converter exhibited full-load efficiency of 97% at liquid-nitrogen temperature compared to 95.8% at room
temperature. a similar test on a 60-W
dc-dc buck converter showed full
functionality at temperatures as low
as 77 K with a slight efficiency degradation. another comparative study by
chunjiang reported the testing of dc-
dc converters, such as synchronous
rectifiers, zero-voltage switching
(ZVs), and multilevel topologies, operating from 120 to 500 V in temperatures as low as 20 K. among these
converters, ZVs has been suggested as
the most efficient option; its overall
losses were 18% fewer than the room temperature losses.
Gardiner et al. evaluated a 50-kW three-phase inverter
with soft switching at 77 K and observed that the total
inverter loss was about 1% of the input power.

large electric machines. a depiction of such a machine is
shown in Figure 1. the high-temperature superconductors
enable practical operation at temperatures well above liquid nitrogen. naturally, a higher operating temperature
reduces the cooling cost while maintaining the superconductivity of the coils. also, the reduced ohmic losses in Hts
motors yield significant annual savings
in electricity consumption by a factor
of 50%. since the Hts wires are capable of carrying a significantly higher
current than copper and create a very
strong magnetic field in the air gap, a
lighter superconducting motor with a
smaller form factor could be used to
produce the same amount of torque as
that produced by a traditional motor.
the size and weight reduction may
reduce the assembling time as well as
the manufacturing material, transportation, and labor costs. the elimination
of iron teeth in the armature (stator
windings) of Hts synchronous motors
not only results in smaller-size and
lighter-weight designs but also
decreases the motor noise caused by rotor and stator interaction. superconducting motors operate more stably during
transients than conventional motors due to their small load
angles (15° versus 70° for a conventional motor) and yield
up to three times higher peak torque. as a result, the motor
can operate even under large transients without losing synchronism, which is essential for transportation systems in
which the motor operates with a varying duty cycle. in

The advent of
high-temperature
superconductors
created an
important
opportunity for the
commercialization
of large electric
machines.

Superconducting Motors
the advent of high-temperature superconductors created
an important opportunity for the commercialization of

IEEE Electrific ation Magazine / d ec em be r 2 0 1 3

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013