The Electrical Power Subsystem (E.P.S) is one of the fundamental subsystems in our Nano-Satellite. Our subsystem is responsible for a variety of operations which can be categorized in the following sectors: Solar Energy Harvesting, Energy Transformation, Storage and Distribution, General Operations and RF Interference Protection.
Solar Energy Harvesting
With the usage of Triple Junction Gallium Arsenide (GaAs) surface-mount Solar Arrays we are harvesting the solar energy directly from the sun, transforms it to electrical and then use it to power on our Nano Satellite.
These arrays have an efficiency of 29.5%, high radiation resistance, can reach a maximum out power of 25.29W with a nominal voltage of 16.31V and a nominal current of 517mA. Each Solar Array is made of 6 cells with very low mass and is equipped with a temperature and a sun sensor.
Furthermore, these arrays come with a configuration of:
- 3-unit panels on the 3 fully available sides (100 x 300 mm)
- A 1.5/2-unit panel on the side equipped with the patch antenna (100 x 200 mm)
- A 1-unit panel on the -Z face of the CubeSat (100 x 100 mm)
Electrical Energy Storage
While we make use of the electrical energy provided by the sun, while we are in contact, through the panels, in eclipse during each orbit we have a need for a search of an energy source. This is accomplished through a pack of 4 individual rechargeable 18650 Lithium Ion battery cells with a total capacity of 5.2Ah at 38.5Wh. In addition, the configuration of the pack is a layout of two batteries in series and two in parallel with an operational voltage of each battery of 3.7V and a total voltage ranging from 6V to 8.4V. Moreover, the pack is protected from undesired thermal conditions through continuous temperature monitoring and through the use of integrated heaters. To avoid the destruction of the battery pack, we have design an overvoltage and undervoltage protection, an analog voltage-braking system made of comparators.
Battery Pack Charging Circuit
As for the battery pack charger we have chosen to use the IC LT3652, which can be used for both as a charger and for a hardware Maximum Power Point Tracker (MPPT) implementation. Moreover, simultaneous charge and discharge are avoided by the usage of two diodes, one that will allow current flow to both the charging circuit and the rest of the satellite the sunlight period and another that will allow current flow from the battery pack to the subsystems when in eclipse.
Energy Transformation and Distribution
The Energy that has been harnessed is distributed to the rest subsystems through the regulated independent 3.3V, 5V and 12V voltage rails with the usage of the space-proven IC TPS54339 step down converters. For monitoring the voltage, current and total power consumption (and thus calculate the rate of battery discharge) in every subsystem, current sensors are placed at each voltage rail.
For the successful deployment of the antennas after a precise time interval, we have design a high precision timer circuit using digital gates mostly alongside with some 4-bit synchronous binary counters.
Moreover, should a subsystem demand a disproportionate 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 for this detection due to their low power demand (typ. 60uA at 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 due to this interference is the incorrect addition of the interference signal to the DC input voltage of each subsystem which would result an incorrect current increment or a data loss in the IC memory circuits