The Electrical Power Subsystem (EPS) is responsible for collecting and providing sufficient energy for the satellite, both while under sunlight and during the eclipse. It is also responsible for rebooting subsystems, in case such action is required, and filtering possible noise and interference caused in the process.
Power is harnessed directly from the solar panels. The raw voltage is stepped down and regulated using the space-proven TPS54339 converters. The step down converters are used in two configurations, offering stable 3.3 and 5V rails. All MCUs used require 3.3V, while various components (such as the magnetorquers used by the ADCS) function with 5V. Current sensors are placed at each voltage rail, to monitor the total current and power consumption (and thus calculate the rate of battery discharge), as well as at the input of every subsystem, so as to monitor each subsystem separately. Should a subsystem demand a disproportional amount of power that would endanger the mission or would cause the satellite to enter safe mode, the respective subsystem will be reset or temporarily shut down via a MOSFET switch controlled by the EPS MCU. We have chosen the INA139 current sensor modules due to their low power demand (typ. 60uAat 3.3V) and the fact they have also been flight proven.
RF interference protection
Since solar panels function as receiving antennas at RF frequencies, it is essential to filter the input voltage in order to avoid any possible interference with the Communications subsystem. Another possible issue would be the interference signal loading on the DC component of a subsystem’s input power voltage and raising wrong overcurrent flags or damaging data on the memory ICs.
The EPS board shall charge the battery module when sunlight suffices. The EPS Engineering Model makes use of the linear TP4056 li-ion charging module and will implement Maximum Power Point Tracking (MPTT) through the MCU. The MCU can perform MPPT by varying the equivalent resistance the “PROG’ pin of the charging IC responds to, and thus increasing/decreasing the charging current at will. The EPS flight model will make use of the LT3652 IC that offers hardware MPTT implementation for maximum efficiency. The battery cells are to be charged separately and thus we have a balanced charger to maximize individual cell lifetime.
The battery module shall be comprised of 3 high-capacity 18650 cells connected in series, offering an output voltage of 10.8V. This is particularly important, as it allows us to avoid using step-up converters that would reduce efficiency by approx. 20%.
Our CubeSat shall make use of surface-mounted Gallium Arsenide (GaAs) solar cells in the following configuration:
•3-unit panels on the 3 fully available sides (100x300mm)
•A 1.5/2-unit panel on the side equipped with the patch antenna (100x200mm)
•A 1-unit panel on the -Z face of the CubeSat (100x100mm)
with each 3-unit panel being capable of producing approximately 8W under direct sunlight.