Development and testing of the 3U+ CubeSat PCDU for SOURCE

SOURCE (Stuttgart Operated University CubeSat for Evaluation and Education) is a 3U+ re-search CubeSat that is being developed by students at the University of Stuttgart in coopera-tion with the Institute for Space Systems and the Small Satellite Student Society KSat e.V.. The objectives include technology demonstrations, atmospheric research and the investigation of satellite demise while also serving as an educational program. SOURCE was selected by ESA's "Fly your Satellite" program and is currently in Phase D. The electrical power supply system combines commercial off-the-shelf parts with self-devel-oped units to meet the requirements of the payloads. The solar array configuration and Power Conditioning and Distribution Unit (PCDU) are self-developed, while the battery is a commer-cial product. A total of 56 solar cells provides up to 32W under ideal conditions, which can be stored in a 75Wh space-qualified lithium-ion battery. To maximise the power output of the solar cells, maximum power point tracking is performed by the PCDU. This is controlled by a radiation hardened microcontroller. The PCDU provides regulated 3.3V, 5V and unregulated battery voltage to the subsystems with 32 switchable outputs, 27 of which are latch-up current protected. The microcontroller controls these individual output channels and the switching between the various CubeSat modes as commanded by the on-board computer. Additionally, every output channel power consumption is monitored for overcurrents. The PCDU functions as a watchdog by checking the health of the on-board computer, rebooting it in case of a failure. High priority commands can be sent directly to the PCDU from the ground via the communication system, bypassing the on-board computer. These can reset either the communication subsystem, the on-board computer or the entire satellite. Four hybrid inhibits, using a combination of mechanical switches and FETs are integrated in the PCDU, replacing the usual fully mechanical design. Three are used to deactivate the satellite in the deployer configuration and the fourth is a remove-before-flight inhibit. An engineering model was manufactured during phase C and is being tested functionally, en-vironmentally and for performance. This paper presents the detailed design of the PCDU, the acquired test results and outlines issues encountered during the tests