useful source - implementation of EPS from another university https://github.com/spacelab-ufsc/eps2
List of changes made from the CSDC-6 EPS board design - Winter 2025 (Lewis Stone)
- fixed PGOOD diode configuration on 5V and 3V3 buck converters
- replaced BOOT/SW caps
- For MPPT Breakout REV 2 I reconfigured MPPT buck converter:
- switched to consistently using the B model of the buck
- re-designed the feedback resistor network to get the proper range of output voltages with the digital pot
- On MPPT breakout board, design originally used a 8.06kR resistor in the feedback network but I changed it out for a 5.76kR resistor based on my new understanding of how the MPPT control loop will actually work hardware-wise (see below)
- Removed one of the 22uF caps on input and output since we had 3 of each (a little overkill)
- Considered switching buck to buck-boost - realized this was due to my lack of knowledge on how solar cells work.
- Replaced the current sense amplifier with the I2C-output one used for the load switches.
- Fixed addressing on the MPPT digital pots by tying them high/low directly rather than driving with MCU
- Decided not to add bootstrap diode since no reliable 5V supply on this board and very unlikely for duty cycle to exceed 66%
EPS MPPT Buck Calculations
Using site https://www.monolithicpower.com/learning/resources/how-to-calculate-a-buck-converters-inductance
- Output voltage of 8.4V (battery voltage). With an input voltage of 12V-17V that gives a duty cycle range of 50% to 70%
- 800kHz oscillation frequency on the RT8297B model - Tswitch = 1.25 us
- Picked ripple factor of 20% (bit lower than 30% which is what I found online)
- max: L = ( (17-8.4) * 625ns * 0.50 ) / 0.3A = 9uH
- min: L = ( (12-8.4) * 875ns * 0.70 ) / 0.3A = 7.4uH
- Currently using 18uH inductor due to incorrect calculations - this may work since it’ll just bring down the ripple more, should probably switch to an 8uH inductor for next rev if needed
Current selection is SRP1050WA-180M (18uH +/- 20%)