GaN-Based, Ultra-Compact Power Conversion System for the PUFFER Autonomous Mobility Platform

In the pursuit for the development of small rovers for planetary science missions, there is a distinct need for the development of an advanced, autonomously controlled, power subsystem. Existing bus management systems used in large spacecraft missions are not suitable for small spacecraft missions, as they are massive, relatively inefficient, and expensive. For extremely compact rover mission concept, newly developed high-density, high-efficiency, lightweight, and low-cost electronics are required. This paper presents a radiation-hardened power subsystem for the Pop-Up FlatFolding Explorer Robot (PUFFER) mission concept, utilizing GaN-based converters for solar array conversion, battery management, and point of load applications to provide an extremely compact power subsystem. INTRODUCTION The Pop-Up Flat-Folding Explorer Robot (PUFFER) is a small two-wheeled expanding rover designed for swarm surface mobility on planetary bodies throughout the solar system [1]. In a total spacecraft volume of 650 cm3, mass of 1.18 kg, and usable surface area of 130 cm2, it represents one of the most compact mobility platforms ever developed for deep-space planetary surface missions. Because of that extremely compact formfactor, the entire power system, including solar arrays, batteries, and power conversion, is extremely limited in mass, volume, and capacity. These extreme limits require a power subsystem that provides 50 W at greater than 95% efficiency, 2 W/cm3 power density, and 1000 W/kg specific power. In order to meet the efficiency, power density, and specific power targets for the subsystem, a digitally controlled, Gallium Nitride (GaN)-based, buck-boost converter has been developed, along with supporting conversion for point of load and instrument applications. The subsystem consists of a four-switch GaN-based buck-boost converter processing 10 – 16 V photovoltaic (PV) array power to an unregulated 8 – 12 V battery bus at 95% efficiency. Optimal performance on the solar array is achieved using an Adaptive Ripple Correlation Control-based Maximum Power Point Tracking (MPPT), utilizing Ripple Correlation Control to achieve minimal disturbance on the power system, and implemented in a radiation-hardened ARM M0+ Cortex [2], [3]. Down-conversion to 5 V and 3.3 V busses are achieved with integrated radiation-tolerant point of load modules at 93% efficiency. Instrument power conversion is achieved with a GaN-based boost converter to a 28 V ground-penetrating radar system at 97% efficiency. Telemetry for the power subsystem is collected locally and communicated to the spacecraft avionics platform via serial peripheral interface (SPI). The complete subsystem is housed on a rigid-flex PCB with total dimensions 5cm x 3cm x2 cm 0.075 kg. The result of this development and testing is a new ultra-compact power subsystem that will provide PUFFER and other small mobility platforms with a radiation-tolerant, high-density, and high-efficiency solution for their power-conversion needs. Future validation on the Lunar surface will demonstrate the system in environments for the lunar surface and beyond.