A dc-to-dc switching power converter of the three-level, pulse-width-modulated, buck type has been designed, built, and verified to operate at temperatures from ambient down to -196 °C (the temperature of liquid nitrogen). Circuits like this one could be useful for supplying electric power to low-temperature circuits in such diverse applications as cryogenic instruments, superconductive magnetic energy-storage systems, magnetic-resonance imaging systems, high-speed computer and communication systems, and high-power motor and generator systems.
The design of a multilevel switching dc-to-dc power converter exploits series connection of power semiconductor switches. The sharing of voltage among the series-connected switches, especially during turn-on and turn-off transients, is a major design issue. The duty factor (switch "on" time to duration of switching cycle) can be chosen to obtain a desired input-to-output voltage ratio. Also, different switches can be turned on and off at different times (equivalently, the switches can be operated at different phase shifts relative to each other and to the overall switching cycle) to minimize the generation of harmonics in the filtered output of the converter.
A three-level converter of the present type (see figure) is a special case of a multilevel switching buck dc-to-dc power converter. In comparison with a standard two-level converter, the three-level converter contains one more switch, one more diode, and one more capacitor. An n-level converter (wheren > 2) offers an advantage over a standard two-level converter; namely, that the voltage ratings applied to the semiconductor devices in the n-level converter are decreased to 1/(n - 1) of those of the two-level converter; the reduction in voltage stresses on semiconductor switches and diodes effects a reduction in switching and conduction losses, and enables the use of semiconductor components with correspondingly lower voltage ratings.
The present three-level converter was designed and constructed using standard, commercially available components, including power metal oxide/semiconductor field-effect transistors (MOSFETs), ultrafast semiconductor power rectifiers, complementary metal oxide/semiconductor integrated circuits for pulse-width modulation and control, metallized-polypropylene-film energy-transfer and output capacitors, and an inductor with a core made of a high-permeability powder. The requirement for low-temperature operation was taken into account in the selection of all components. The design specifications include an input potential of 48±10 V; an output potential of 12 V; an output voltage ripple of 120 mV (1 percent of rated output voltage); minimum and maximum load currents of 1 and 5 A, respectively; maximum output power of 60 W, and a switching frequency of 50 kHz.
The converter was tested in operation at temperatures from 25 down to -195 °C. At room temperature, the converter operated with an efficiency of 89.12 percent. At -195 °C, the measured efficiency was slightly lower; namely, 87.27 percent. Even at -195 °C, the converter was found to be fully functional.
This work was done by Richard L. Patterson of Lewis Research Center and Fausto F. Pérez-Guerrero and Biswajit Ray of the University of Puerto Rico.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Commercial Technology Office
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Refer to LEW-16675.