Silicon Carbide Junction Field-Effect Transistor Devices for Scalable Solid-State Circuit Breakers

Semiconductor devices used as contactors help prevent power electronic converter damage.

February 1, 2010

Army Research Laboratory, Adelphi, Maryland

Power electronic converters functioning as components in high-power systems, such as those of hybrid military ground vehicles, require fast fault isolation, and in most cases, benefit additionally from bidirectional fault isolation. To prevent converter damage or failure, fault current interrupt speeds in the hundreds of microseconds to few millisecond range are necessary. Presently used mechanical contactors do not provide adequate actuation speeds, and suffer severe degradation during repeated fault isolation. Instead, it is desired to use a large array of semiconductor devices having a collectively low conduction loss to provide large current-handling capability and fast transition speed for current interruption.

The back-to-back VJFET Test Configuration.The bidirectional snubber was connectedacross the parts. An isolated DC power supplywas used as a source, and a 50-ohm load wasconnected in series with the VJFETs.

A solid-state contactor for protection of a non-isolated DC-DC converter was developed. A solid-state contactor was designed with silicon carbide (SiC) metal oxide semiconductor field-effect transistors (MOSFETs) placed in parallel. A large number of 20A rated SiC MOSFET die were placed in parallel to handle in excess of 500A of current with the desired 1V conduction drop across the part. To behave like a mechanical contactor by stopping current flow in both directions, the MOSFET-based contactor would require an additional set of paralleled MOSFETs placed in series with, but in the opposite direction of, the original set (back-to-back MOSFETS) with the source terminal of the MOSFETs connected together. This added set of MOSFETs would effectively double both the number of devices needed, and the amount of contactor loss compared to original estimates.

The SiC Junction Field Effect Transistor (JFET) was considered as a possible candidate for use instead of a MOSFET in the design of a solid-state contactor. In this application, a normally-on JFET would provide operation as a circuit breaker, interrupting current when actuated, much like fault protection provided by circuit breakers in utility power systems. Due to the lack of a parasitic body diode between the drain and source terminals of the JFET, it was suggested that a single device may be capable of bidirectional current conduction and bidirectional voltage blocking. A bidirectional snubber was designed to reduce solid-state breaker voltage stress during fault isolation. Simulations were conducted to determine suitable component ratings based on estimated worst-case line inductances.

Northrop Grumman Corporation (NGC) developed SiC normally-on vertical JFETs (VJFETs) having current ratings of 20A and 50A with respective blocking voltages as high as 1900V and 1200V. Smaller 8A rated sample VJFET parts were obtained for scaled-down application testing. An isolated driver was designed and built to provide an adjustable gate-to-source (VGS) bias of nominally +2V for VJFET conduction, and an adjustable VGS bias down to –24V for blocking conditions. The driver was tested on a small capacitive gate load.

After the VJFET asymmetries were characterized, the parts offered advantages over MOSFETs in a back-to-back configuration. Compared to MOSFETs, VJFETs offer higher operating temperatures due to the absence of an oxide layer. Present VJFETs also have lower on-state resistances for a given chip area. Finally, VJFETs have significantly lower gate capacitances resulting in faster transition speeds.

Evaluation of the parts was done in a simple test setup. Two 8A VJFET parts were connected in a back-to-back configuration with common sources. The bidirectional snubber was connected across the parts. An isolated DC power supply was used as a source, and a 50-ohm load was connected in series with the VJFETs. The setup allowed the connections of the VJFET pair to be easily reversed for bidirectional testing. The driver was connected with the gate line to the gates of both VJFETs through individual 5-ohm resistors, and with the source line connected to the common source of both parts.

For the back-to-back VJFET configuration to function properly in this application, either a reduced VGS (for at least the reverse conducting device) is required, or enough pairs of devices must be connected in parallel to keep to the voltage drop between the source and drain of the reverse conducting device below 1V. In either case, these actions would effectively de-rate the devices by nearly a factor of two, causing almost twice as many back-toback device pairs to be required for a given current level. To apply a reduced on-state VGS to only the reverse conducting device would also increase gate driver complexity, in addition to the sensing and conditioning needed to determine current direction for sending proper gating delays.

This work was done by Damian P. Urciuoli of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. ARL-0092