High-Temperature Silicon Carbide Power Module for Military Hybrid Electric Vehicles

High-performance SiC power modules such as these will be essential for future military electric vehicle platforms.

February 1, 2012

Army RDECOM-TARDEC, Warren, Michigan

The ever-increasing electrical power, power density, and cooling requirements of present and future military platforms are pushing silicon-based power electronics systems to their operational limits. To address future needs, wide bandgap semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) were developed for electronic applications. Using these semiconductor materials, a new generation of wide bandgap power devices is being developed.

The HT-2000 Series **Power Module** (left); module, gate driver, and busbar assembly (center); and the complete module assembly (right).

These new power devices offer characteristics and capabilities that are orders of magnitude improvements over their silicon counterparts, including 10x voltage blocking capability, 10-100x switching speed capabilities, 1/10th energy losses, inherent rad-hard operation, and theoretical junction temperature operation up to 600 °C. This power device technology could deliver power systems with power densities as high as 10x of the present Si-based system in addition to minimum cooling needs, improved efficiency, as well as improved reliability.

Significant material and device advancements have been made over the past decade and power devices such as SiC Schottky diodes, SiC JFETs, SiC MOSFETs, and SiC BJTs are now commercially available. However, a new power module technology that maximizes the performance of these devices is needed. Such a new power module technology must be capable of delivering ultra-low parasitics that will enable high-speed switching. Second, it must be capable of reliable high-temperature operation (250 °C and beyond). Third, it must be capable of achieving ultra-high reliability. Lastly, to fully capitalize on the advantage of this new power module technology, a new gate drive technology as well as system level design techniques are imperative.

A new line of high-performance SiC power modules, HT-2000, was developed. The HT-2000 series of modules is rated to 1200V, are operational to greater than 100A, can perform at temperatures in excess of 250 °C, and can be constructed with SiC MOSFETs, JFETs, or BJTs.

The newly developed module implements a novel ultra-low parasitic packaging approach that enables high switching frequencies in excess of 100 kHz, and weighs in at just over 100 grams (offering >4x mass reduction in comparison with industry standard power brick packaging technology). The HT-2000 line is deviceneutral, meaning that the internal interconnection and layouts are compatible (or adaptable) with most currently, or soon to be available, SiC devices.

A second key design feature of the module is that it employs a novel interconnection scheme that allows it to be configured as either a half- or full-bridge configuration through external bussing. There are six external power connectors, two each for V+, V-, and output. For both configurations, the V+ nodes are shorted and the V- nodes are shorted. The difference is how the outputs are connected. Full-bridge operation keeps the outputs separate, while half-bridge connects them together. Independent gate and source Kelvin signal pins allow for independent control of each quadrant.

A third key aspect in the design of the module is the manner in which parasitic impedances are handled. To start, the magnitude of the undesired path inductances are reduced through a number of techniques. While effective, parasitic impedances may never be eliminated, the layouts of these modules utilize a technique that “matches” effective current paths to devices in parallel (including the path for the power and the gate drive signals). This reduces the effect of non-simultaneous switching events, where switches in parallel turn on in a cascading fashion, resulting in uneven current sharing among paralleled devices during highspeed transition.

The benefits of matched power device layouts were demonstrated in a study comparing different meth ods to arrange six devices in parallel. These methods included a linear arrangement and a more symmetrical “forked” arrangement with pairs of matched current lengths. By forking the layout, the current path length to the furthest device is halved, and the burden is shared by two switches. This reduces the current peaks by nearly half, as well as halving the duration — resulting in a considerable reduction in switching loss.

The new module applies a novel layout that places devices in a complete parallel arrangement, allowing for simultaneous switching of greater than eight devices in parallel (per switch position), with all devices, in theory, sharing current equally under transient and dc conditions.

This new module series takes advantage of the unique benefits of SiC devices, including high current density, high junction temperatures, fast switching speeds, and reduced losses. Two variants of the modules were also developed — each built with either JFET or MOSFET SiC devices (and associated SiC diodes). Ultra-low switching losses and extremely fast switching speeds (<50 ns) were measured and demonstrated.

This work was done by R. M. Schupbach, B. McPherson, T. McNutt, and A. B. Lostetter of Arkansas Power Electronics International; and John P. Kajs and Scott G. Castagno of Science Applications International Corporation for the Army RDECOM-TARDEC. ARL-0139