TID Testing of COTS-based, Two-Phase, Point-of-Load Converters for Aerospace Applications

Abstract

The use of commercial off-the-shelf (COTS) parts in space applications has elicited increased interest, especially in the pursuit of higher-performance satellite hardware for missions that can accept higher risk. This hardware includes DC-DC point-of-load (PoL) converters; this category of power electronics performs the critical function of adjusting the voltage and current levels provided by a mission's power distribution infrastructure in order to appropriately feed its loads, which are often computational in nature. The COTS-equivalent parts available for PoL converters enable significantly higher efficiencies, increased current output, reduced volume and mass, improved EMI characteristics, and lower costs. Additionally, the growing availability of COTS switching devices based in gallium nitride (GaN), which is a wide bandgap semiconductor, offers fast switching with reliable radiation performance in a small physical footprint, among other advantages. To effectively integrate potential COTS components into aerospace designs that feature high-power processors, FPGAs, and memories as loads, it is necessary to ascertain the total ionizing dose (TID) tolerance of the COTS control circuitry and power switches. As a result, multiple high-power-density, two-phase synchronous buck converters were developed utilizing various COTS control chips and GaN high electron mobility transistors (HEMTs). GaN devices were used due to their resistance to TID, with several devices having been tested up to 1Mrad. Additionally, multiphase buck converters are a favorite for generating high current power rails that are needed for computational loads like FPGAs. A variety of COTS controllers and GaN HEMTs were selected as candidates for future mission applications based on current and voltage ratings. For controllers, the LTC7802, ADP1850, and LTC3861 from Analog Devices were selected. These controllers were used with the EPC EPC2015C, EPC EPC2001C, and GaN Systems GS61008P GaN HEMTs. A comparison between the designed PoL modules is presented with both simulation and hardware results. To see how the controllers perform in an extended radiation environment, the various modules were stressed in 5krad doses up to 10krad through enhanced low dose rate sensitivity (ELDRS) testing. The modules were then stressed up to 100krad in varying increments at a high dose rate. Converters were tested and measured against their baseline performance after each application of radiation. The electrical design and characteristics before, throughout, and after radiation testing for the converter modules are presented. Comparisons between simulation and hardware performance, in addition to measured efficiency data for each converter module, are also presented.