The Electrical Power Systems (EPS) board is responsible for converting the power input from the satellite’s solar panels into usable power for the rest of the satellite, and distributing this power to the various loads and the battery.
The EPS has 3 main sections: MPPT, buck converters, and load switches.
Maximum Power Point Tracking (MPPT) is a term to represent a variety of techniques for optimizing the power output from solar panels.
The graph above is a sample IV curve for a solar panel. This plots the current outputted by the solar panel against its voltage. You can see that the solar panel acts pretty much like a constant current source up to a certain cutoff voltage, at which point it quickly spikes down. The goal of MPPT is to find the voltage right around the cutoff where power is optimized (the maximum of the blue curve). Our chosen MPPT algorithm is incremental conductance.
This is the input from the solar panels. There is a Schottky diode to prevent current from flowing backwards into the solar panels in the case of some fault with the MPPT circuit (short caused by soldering, reverse polarity during testing, etc). The 10mR shunt resistor generates a potential difference that is filtered by R5, R6, C5 into the current sense amplifier below.
The INA230AIRGT current sense amplifier amplifies the voltage difference across the shunt resistor to measure the MPPT input current. A0, A1 are addressing pins. The chip outputs over I2C. ALERT has a variety of possible functions.
The MPPT system uses a buck converter to effectively match the load’s input impedance to the output impedance of the solar panels, optimizing power output. The buck converter steps down the input voltage from the solar panel to 8.4V (battery voltage), but since the battery voltage is effectively fixed, the buck converter actually controls the solar panel voltage, allowing us to move along the curve above and find the MPP. The buck converter IC is the RT8297BZQW. There are a variety of input and output capacitors for stability.
The MCP4562-103E/MS is an I2C-controlled potentiometer that lets us change the voltage at the feedback node, indicating to the buck converter to either increase or decrease its duty cycle, modifying the solar panel input voltage.
The following page has more detailed information on how the MPPT circuit actually works:
MPPT System High-Level Description
The EPS board uses buck converters to step down the 8.4V battery voltage to 3V3 and 5V rails which are used by the other electrical systems in the satellite. A buck converter is a type of switching converter, which is much more efficient than something like a LDO. We are using the RT8297BZQW buck converter IC for all buck converters.