The Nomura MPPTs evaluated by the team have a fairly unique design compared to most other solar car MPPTs. Instead of having each MPPT designed to output the full battery pack voltage, Nomura MPPTs are designed to only output up to 37 V and must be stacked in series on their output sides in order to charge the battery. This configuration is advantageous because it enables each MPPT to have a smaller string of PV cells on its input side, making the entire array significantly more robust against uneven lighting. Moreover, due to the large surface area of the SunPower cells being used on MSXII, the total stackable cell voltage for the entire car is quite low and would only support around 2 strings for traditional MPPTs.

However, this also posed some interesting and potentially confusing questions about how MPPTs in a "stack" coordinate together to output the correct pack voltage. After some testing with 3 Nomura MPPTs, this article has been written to document our best understanding of how they work to hopefully reduce confusion about these devices moving forward.

Considerations for MPPT stacking

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?

Modes of Operation

To answer this question, it is important to first understand the three operating regions of the Nomura MPPT. First, consider a simplified design of the MPPT that does not need to take into account pack overvoltage, and can operate at an unlimited boost ratio (output/input ratio). Furthermore, assume there is only one MPPT in the series stack (the MPPT directly connects to the battery). For a given input power, the MPPT should try to output as close to this input power as possible. The output voltage is constrained by the state of charge of the battery.

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. We assume the MPPT is never capable of supplying enough current to cause overcurrent.

This, of course, results in issues when the battery pack is nearing full charge and is drawing a low current from the MPPT. If the MPPT were to continue to try to maintain the same output power, its output voltage would increase without bound and eventually cause a battery overvoltage. The Nomura MPPT thus implements a voltage limiting function, tuned via potentiometers, which prevents the boost converter from continuing to charge up the output capacitor once it has reached a preset limit. From a power perspective, voltage-limit mode is inefficient because it does not use the full power potential of the input PV string.

Additionally, because the MPPT is only capable of boosting, not stepping down the voltage of the input side PV string, there exists a feed-through mode where the MPPT will directly connect the input string to the output in situations where the output voltage is less than the minimum boost voltage, 1.1 x Vin. This bypasses the boost circuitry entirely and results in no power point tracking, but elegantly handles a short-circuit and can still deliver some power in certain cases.

Outside of the voltage-limit and feed-through modes, the MPPT is in constant power mode, where its output current is directly determined by the output voltage and input power. All three regions are shown in the drawing below. Note that the IV curve can be idealized as a doubly-truncated 1/x curve, where the untruncated region corresponds to the constant-power region.

The MPPT also has a fourth mode, not shown in the above drawing, called pass-through mode, which occurs if the input voltage drops below the minimum required voltage for the boost converter. In this case, the output side is shorted so as to prevent the entire MPPT stack from going open-circuit, and the MPPT itself delivers no power.

MPPT Stack Dynamics

When operating in the constant-power region, the MPPTs in a string will naturally converge to the correct voltage distribution based on their input power. As a simple example, consider the case of a stack of two MPPTs connected to a battery. If one MPPT suddenly drops its output voltage, the second MPPT will see a larger portion of the total pack voltage. Initially, this will result in no current being delivered, since the MPPT sees a load with a higher potential than it. The MPPT's built-in diode prevents the battery from driving current into it.

As the output current of the MPPT starts to decrease towards zero, the boost converter will naturally start to charge up its output capacitor until the voltage once again reaches (then slightly exceeds) the new load voltage, at which point current will start to be delivered to the load, acting to discharge the output capacitor. Because the boost converter is now stepping the input side voltage by a larger ratio, it has a decreased output current ability and thus the MPPT reaches a new equilibrium with a reduced current, but the same power.

This balancing only works when all MPPTs in the stack are operating in the constant-power region. If one MPPT saturates and enters voltage-limit mode, it will prevent itself from further boosting its output voltage and thus the voltage of the entire stack may decrease. The same effect will result from an MPPT entering pass-through mode (a large part of its input PV string becomes shaded or damaged).