MPPT System High-Level Description

Design V2 with Differential Op Amp

Design uses a differential op amp amplifier between the (stable) battery voltage and the MPPT input voltage (solar panel voltage), in order to ‘flip’ the input voltage’s contribution to the feedback. This is so that upward fluctuations in input voltage will actually cause the duty cycle to increase rather than decrease as would be intended if the FB pin were used for output voltage as normal.
Differential op amp was used because the relative changes in the input voltage need to be inverted to get the feedback pin to adjust the duty cycle accordingly. This is because normally for a buck converter, increased voltage at the feedback pin means the output is too high, meaning the duty cycle should decrease. However, since the buck converter here is regulating the input voltage, an increased voltage at the feedback pin means the input is too high, so the duty cycle actually has to increase to force the input voltage closer to the 8.4V output. A simpler inverting amplifier could also work here with some extra resistors (ie. resistor divider between FB pin, op amp output and Vbat) but diff op amp was used in this case.

Calculator for how resistance should scale to get correct FB voltage in LTspice simulation

Latest LTspice simulation version (06/21/2025)

Previous LTspice simulation version (06/01/2025)

Op amp-buck converter feedback loop stability investigation