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When stacking MPPTs in series, it is crucial that the total voltage across the whole stack remains around nominal. This becomes a challenge in this configuration because the series configuration forces all MPPT output currents to be equal. In order to maximize total power utilization from each MPPT's input string in situations where the strings are producing different amounts of power, each MPPT is designed to be able to adjust its output voltage depending on its input power such that the total stack's nominal voltage is distributed by the ratios of each MPPT's input power, not just equally.
This begs the question: when one MPPT lowers its output voltage due to a weaker input string, how do other MPPTs in the stack "know" to increase their output voltage, while also ensuring that under normal conditions the stack voltage never exceeds the pack voltage?
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From basic power equations, the MPPT's output current should be a 1/x function of its required output voltage. Physically, the boost converter on the MPPT will try its hardest to fill the output capacitor until the pack draws the same amount of current that the boost converter is able to provide (itself being limited by the input side PV cells), at which point equilibrium is reached.