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.
  • Need to adjust shunt resistance to probably 0.5mOhms → https://www.digikey.ca/en/products/detail/stackpole-electronics-inc/CSSH0805FTL500/13414827
  • The load switches themselves also need to have the current sense amplifier updated on the EPS board (see Load Switch board)
• 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 10V-13V that gives a duty cycle range of 65% to 85%
• Max input current of 18m * 5 = 90mA → max output current of 90m/65% ~ 140mA
• 800kHz oscillation frequency on the RT8297B model - Tswitch = 1.25 us
  • t on → 0.813us to 1.063us
• Picked ripple factor of 40% (bit higher than 30% which is what I found online)
  • ΔiL = 0.40(140m) ~ 60mA
• max: L = ( (13-8.4) * 0.813us) / 60mA = 62uH
• min: L = ( (10-8.4) * 1.063us) / 60mA = 28uH
• According to simulation, a 30uH inductor would probably be optimal (any higher resulted in large Vin ripple at Vin ~ 10V) and acceptable current ripple in simulation at this value. Will stick with the 18uH inductor for the first MPPT breakout board and validate simulation results.