The Electrical Power System of FINCH is in charge of Solar Power generation, conversion, storage, and distributing this power to the entire satellite.
Our aim is to provide the voltage and current required to power the satellite's payload sensor, the ADCS sensors, the on-board data processing chip, and the communication (RF) system that will allow us to transmit data to our ground station.
An EPS System usually consists of a combination of solar panels, Microcontrollers, sensors, and power converters. Our system will contain:
- PV cells (InGaP/GaAs/Ge on Ge substrate triple junction solar cells) with a 30% efficiency rating (30% of solar energy harvested is converted to electrical energy)
- (1) battery pack (4 cylindrical 21700 Li-ion cells connected in series),
- (1) PWM converter with Maximum Power Point Tracking (insert mppt TIDA-010042 page confl link here) (an algorithm used for optimal energy harvesting from a variable power source, i.e. a solar panel) to condition the power coming straight out of our solar panels,
- (2) Solar charge controllers (used to charge the batteries and prevent over-charging, prolongs their life!),
- (2) Battery Monitoring chips (used to track the temperature and State of Charge of the battery pack),
- Over-current and over-voltage protection circuitry (i.e. E-fuse, current and voltage sensors, emergency off-switches),
- (4) load switch(es) used to modulate power send to the subsystems, controlled by
- (A) Microcontroller to receive information from the sensor chips and control the power converters' duty cycles accordingly (i.e., regulate the voltage level),
- switching regulator(s) and/or Low Dropout Regulator(s) to step-down the main voltage bus to get 5V, 3.3V lines to power the components within the EPS itself,
- and a Deployment switch to kick-start the system once we're in space!
A preliminary list of system components and their datasheets can be found on the System Components List page. All our current projects are in Phoenix Projects.
NOTE: MPPT and Solar charge controllers usually come as one chip, but we might be separating the two given stock shortages.
Block Diagram - v3.0
Version 3 if the system overview has changed the voltage regulation to be centralized in EPS and separate levels distributed to each subsystem post-downregulation. Updated protection circuitry elements and positioning and removed dedicated power supply to EPS (unnecessary, EPS chips automatically powered by panels and battery). New dep switch location. New legend position.
Block Diagram - v2
Version 2 of the system overview includes a more detailed description of the Power Control Unit, including a separate power bus to supply the EPS system with a dedicated voltage line and increase reliability of the system ten-fold- the power regulation to the EPS unit is now completely independent of the rest of the satellite and thus is not reliant on any MCU, which could be potentially prone to errors or failures that would cause the whole system to malfunction. We now also have specified singal lines to and from the MCU, so we know it can communicate with voltage regulators and load switches reset power to the satellite or certain parts of the satellite, can control cell balancing, can control when batteries are charged, can receive battery health information as well as current and voltage readings from sensors now specified across the system - and thus modulate the gate drivers for the main switching power converters accordingly.
To come in v3: regen braking, external testing power supply connections, bypass diodes for the PV cells.
Block Diagram - v1.2
The block diagram now shows the path that the power takes from generation to distribution. It includes updated connections based on the various chips' real ports and functionality. It also shows the 2 basic modes of operation, dubbed "Day" and "Night".
Block Diagram - v1
The block diagram shows the connections between each of the parts of the EPS architecture for the first iteration of design. This diagram was last updated on August 24th, 2021. The main components of the system are the Solar Panels, Maximum Power Point Tracker (MPPT), Battery Management System (BMS), Batteries, Microcontroller (EPS Core), Protection Circuitry, and Buck Converter.
Microcontroller (EPS Core)
The microcontroller (MCU) being used for EPS is the STM32G431RBT6. (NOTE: EPS Core processing might be done on the OBC MCU - merge decisions in progress).
Solar Panels
The solar panels are located on the external faces of the satellite and are each made up of 8 solar cells. The solar panels are our source of power generation by using the photovoltaic effect of converting the solar energy from the Sun to electrical power. We are planning to use GaAs solar cells from AzureSpace. The solar panel will be connected to the MPPT module to supply the satellite with power.
Figure 1: GaAs solar cell from AzureSpace.
Maximum Power Point Tracker/Charge Controller IC
The purpose of MPPT is to ensure we are delivering the maximum amount of power from the solar panels to the downstream loads (the batteries and onboard systems). It is connected to the solar panels and to the main voltage bus. The reason an MPPT module is needed is because the photovoltaic cells vary in voltage depending on the amount of solar radiation it is exposed to and the temperature (this is known as the cell's I-V curve). The MPPT module is able to adjust its own impedance to track the maximum power point of the I-V curve and maintain the best energy conversion efficiency from the solar cells. The MPPT IC we are using uses the Constant Voltage (CV) algorithm.
This chip also performs charge controlling - it charges the battery when the solar panels are producing energy, and allows the battery to discharge (power the system) when the solar panels are no longer generating power (i.e., when the satellite is in eclipse or not facing the sun).
Battery Monitoring System
The battery management system is directly connected to the cells of our battery pack from the main voltage bus. The purpose of the management system is to ensure safe charge and discharge of our batteries and to extend their lifetime through cell balancing. The IC we are using allows for both autonomous and host-controlled passive cell balancing, as well as a long list of protection measures for each of our cells including and not limited to cell under-voltage/over-voltage protection, overcurrent charge/discharge protection, and under-temperature/over-temperature charge/discharge protection. It also provides a State of Charge readouts, calculated using the Coulomb Counting (CC) method.
Buck Converter
The buck converter steps down the voltage from the main voltage bus of the satellite to the required voltage for the EPS MCU and the satellite's subsystems. Due to EPS' distributed architecture (more in Power System Architecture page), each buck converter will be implemented on the subsystem's PCB. A 5V and 3.3V converter have been designed but will be the responsibility of each subsystem to implement. Therefore, EPS will be connecting to each subsystem delivering the same power line that will interface with the system's unique buck converter.
E-Fuse and Load Switch
The E-Fuse and Load Switch are hardware protection circuitry measures that will help EPS control the power being distributed to each system. There will be one e-fuse for the main voltage bus that has overvoltage protection, undervoltage lockout, short circuit protection, overcurrent protection, etc. The load switch acts as an electronic switch that we can control to turn off and on the power for each downstream system. There will be a load switch for each subsystem, and all load-switches will be on the main EPS PCB.