## 3 Circuit Architecture for Multi-MFC PMS
### Overview of Energy Harvesting for MFC  
Due to the technology's nature, the MFC output voltage is very low and non-regulated [28], which represents a major challenge for efficient power extraction. A parallel configuration of MFCs can be used to increase the output current with the disadvantage that the output voltage will remain at low levels. Another drawback of the parallel connection of MFCs is that if their output voltage deviates from each other, this effect leads to a detrimental charge transfer among cells in the array, also known as voltage reversal, preventing the stack of MFCs to fully deliver its charges to a load [26]. This results in power losses that lead to a low overall power efficiency of the array.  
On the other hand, a serial connection of MFCs provides higher output voltages based on the sum of individual \(V_{MFC}\), with a common current that flows throughout the MFC array. In [33] a voltage balancer is used along with a DC-DC converter for an array of serially connected MFCs. By doing so, the output voltage, now higher than that of a single MFC, is efficiently harvested by providing an active switching capacitor method to balance the system. However, the system can also face multiple drawbacks, including power extraction interruption, and even system failure if a single MFC in the stack is damaged or physically disconnected, due to a contact deterioration, for example. This can potentially disable the operation of the DC-DC converter block, leading to a loss of power available from the remaining properly-functioning MFCs.  
Previous works in PMS have mainly focused on power extraction from a single MFC [27; 42]. This approach is limited to the amount of power that a single MFC can generate, which can be depleted quickly, resulting in the interruption of the power supply at the load. In addition, if the system does not integrate an automatic MPPT algorithm, achieving maximum power harvesting can become cumbersome and time-consuming if conducted manually [23; 27; 43; 44; 45; 33]. Even more important, power extraction may not be efficient, since the MPP can shift during the lifetime of an MFC. For this reason, the integration of an MPPT is essential for the DC-DC converter to dynamically adjust to the electrical characteristics across multiple MFCs.  
In this work, the system proposed is intended to harvest the energy from an array of MFCs in a time interleaved fashion, allowing the inactive MFCs to recharge its internal capacitances (\(C_{MFC}\)) when they are not being harvested. The system also integrates an MFC-failure protection mechanism, which automatically isolates one or multiple MFCs from the system if its available power is below a minimum threshold. The MFC-failure protection feature is intended to isolate an MFC when it has been depleted, damaged or it has been physically disconnected.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Specification** & **Value** \\ \hline Number of MFCs & 4 \\ Average Vin & 350 mV \\ Vout & 3.3 V \\ Output supercapacitor & 5 F \\ \hline \hline \end{tabular}
\end{table}
Table 2: Specifications of the MFC-powered Internet of Things (IoT) system.