Design and Implementation of a COTS, GaN-based Power Converter for Spacecraft Applications

There is increasing interest in the use of Commercial Off-the-shelf (COTS) components in space, especially for relatively short (less than ten years) low earth orbit (LEO) missions. Using of COTS components can drastically reduce the lead time and cost. These devices however are not characterised for the space environment and how these components respond to the radiation environment encountered must be understood and appropriate mitigations must be taken during circuit design. A spacecraft power system will typically involve many stages of voltage conversion, and this is often most efficiently carried out using field-effect transistors (FETs). Of special concern is the destructive failure modes of silicon Metal Oxide Semiconductor Field Effect Transistors (MOSFETS) under heavy ion bombardment caused by rupture of the isolated gate. Due to the lack of manufacturer's data the susceptibility of COTS parts cannot predicted with any degree of accuracy. Expensive and time consuming testing is required to have confidence that these devices can be used, and the manufacturer of COTS components can make minor changes to the die that do not materially impact performance but lead the part to have a different response to radiation. An alternative to the use of silicon MOSFETs is to use gallium nitride (GaN) FETs. Whilst not completely immune to radiation damage they are much less susceptible to catastrophic failure. When coupled with tested drivers and control circuits, they can be employed in radiation tolerant power converters with efficiencies at least as good as their silicon counterparts. At SSTL, GaN based power converters ranging from 1W to 300W design are implemented. In this paper, the implementation of a 100W converter used to supply an isolated, regulated 20.3V to a transmitter will be described. The design has a conversion efficiency of up to 94% and occupies the space previously taken up by a much more expensive bought in `brick' converter.