DC Power Converter for 200°C Containing SiC Transistors

This is a prototype of power converters for high-temperature applications such as automobile and aircraft engines.

February 1, 2008

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

A DC-to-DC power converter, nominally rated for a power of 180 W, an input potential of 28 V, an output potential of 270 V, and a maximum operating temperature of 200°C has been designed, built, and tested. This power converter serves as a prototype for further development of power converters required to be capable of operating in high-temperature environments in diverse settings, including automobile, aircraft, and spacecraft engines, and oil and gas wells.

The Power Stage of the DC-to-DC power converter contains high-temperature-rated components. The circuit topology is chosen in conjunction with the switching cycle so that transformer leakage inductance energy is fed back to the input capacitor and the maximum voltage stress on S1, S2, RD1, and RD2 is limited to the input voltage.

The power stage of the power converter (see figure) includes a transformer (TX), the DC input to which is switched at a rate of 100 kHz by means of two recently developed high-temperature SiC bipolar junction transistors (S₁ and S₂) that are rated at a maximum potential of 1,000 V, maximum current of 5 A, and maximum operating temperature >300°C. The transformer has a planar core made of a powdered ferrite that has a Curie temperature >300°C. Also included in the power stage are an output filter inductor (L_out) having a core made of the same material as that of the transformer core, two SiC Schottky transformer-reset diodes (RD₁ and RD₂) and rectifier diodes (D₁ and D₂), and input and output capacitors (C_in and C_out, respectively). The capacitors contain ceramic dielectrics that conform to an Electronics Industries of America (EIA) standard, called "X7R," that specifies acceptable ranges of dielectric properties as functions of temperature.

During each switching cycle, the power converter proceeds through three successive modes of operation:

An energy-transfer mode in which S₁ and S₂ are on and D₁ conducts;
A transformer-reset mode in which (1) RD₁ and RD₂ conduct, applying reverse input potential to the primary winding of the transformer and (2) the output inductor current freewheels through D₂; and
A dead-time mode in which S₁ and S₂ are off and RD₁ and RD₂ are not conducting while the output inductor current is freewheeling through D₂.

Because of the manner in which the magnetic flux in the transformer core is reset in this switching cycle, the duty cycle is limited, in principle, to no more than 1⁄2. In the design of this power converter, a practical duty-cycle value of 0.45 is used. One key advantage of the design is that the transformer leakage-inductance energy is fed back to the input capacitor via RD₁ and RD₂. Another advantage is that the maximum voltage stress on S1, S2, RD1, and RD2 is limited to the input voltage, so that primary side snubber circuits are not needed. Yet another advantage is that voltage shootthrough is absent because S₁ and S₂ are turned on and off simultaneously.

In tests, this power converter was found to be capable of operating, as intended, at temperatures up to 200°C. In a contemplated update of the design, the input capacitor would be replaced by one rated for higher temperature in the hope of enabling operation up to at least 250°C. Also under consideration are implementation of a higherpower version using a full bridge topology and the replacement of the SiC bipolar junction transistors with SiC junction fieldeffect transistors or metal oxide/semiconductor field-effect transistors.

This work was done by James D. Scofield and Brett Jordan of the Air Force Research Laboratory, Biswajit Ray of Bloomsburg University of Pennsylvania, and Hiroyuki Kosai of UES, Inc.