EPS Walkthrough
MPPT Walkthrough
- Before VMPP and IMPP drop rapidly… PMPP (max power) is drawn from the boards
- We use a buck converter/dcdc… varies the voltage draw of the panels… of the must match the resistance of the source (panel) and the resistance of the load (BMS)
- Provide the battery with as much current as possible - then can draw the most amount of power, voltage stays the same
- range of voltage into the buck is 10 - 17 V because
- Gallium arsenide solar cells (we went with cheaper silicon cells) these are space-ready https://www.azurspace.com/index.php/en/products/products-space/space-solar-cells
- Current-sense —>
- decoupling capacitors C1 - C4, C7 - C10
- inductor sourcing
- ripple —> amount of noise on the steady voltage line… ripple spec (noise to expect) the inductor is recommended on the datasheet
- Load transient analysis —> our application does not require transient response… we cannot allow extra current to be drawn for this,
- only did ripple tests before
- MPPT must be tied to the bucks, bms, system
- E-fuse is for protection purposes, but we can take it out if needed, does not affect functionality
- Buck is tied to the microcontroller, other things tied to the microcontroller
- Potentiometer —> allows the buck to vary the voltages, (buck cannot do this alone, buck can take a single voltage)- feedback for the buck to do switching (changing voltages) to match what the panel is supplying- tied to the microcontroller as well to send commands which send commands to the feedback pin on the buck
- Bucks do not need digital input, if the microcontroller resets, they will still be working
- voltage dividers measure the voltage of the panels
- buck measures the voltage of the batt (next to the potentiometer)
- Define which devices connected to the I2C buses
- Ex. Potentiometer —> A0 and A1, advised to use SPI when you have multiple devices of the same kind, if you have more than 8 devices you have to multiplex…
BMS Walkthrough
- Batteries will be primary source during eclipse, must be able to supply power to the system and charge the battery
- we didn’t use the G22 chip because it was out of stock, then used an I2C to 1-wire bridge
- chose a switch instead of a fuse (mosfet Q1), also because radiation could cause it to flip…
- 2s2p battery configuration - 2 in series, 2 in parallel
- cells in series need to always be balanced, BMS protects the cells and balances the cells, and charges the cells (overcharge = heat, over-discharge = drain the battery life significantly)
- keep in mind the “state of charge algorithm”
- monitors the state of charge of the batteries
- FETS - charge and discharge fets —> if the charge/discharge is at the wrong rate, these close
- deployment switch connections —> only connect the load to the EPS system when deployed, protects EPS system from the load
- R10 - current sense resistor,
- Minco battery heater —> when the batteries get too cold, sends a current through the wire until it is the correct temperature
Solar Panel Walkthrough
- Transimpedance Amplifier
- Sun sensors —> orientation relative to the sun
- we used a photodiode instead of sun sensors, however not very accurate
- converts the voltage to current, then to digital signal, then we can send this to the microcontroller
- ADC talks over SPI
- Temperature sensor is always tracking the temperature
- bypass diodes —> current will still flow through the string one of the cells breaks
- also we will have 7 gallium cells (documentation for this… [NOTION LINK])
- diodes in the front prevent current from flowing the other way into the cells
surplus current shunt
- excess power goes to this resistor, we might not need this if we don’t have excess