# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 2 Conventional Structure  
The conventional architecture of a combining circuit for a multi-source energy harvester (EH) is depicted in Figure 2. Although Figure 2 shows just two energy harvesters, the number of energy harvesters could be expanded. Each energy harvester converts a certain kind of ambient energy to electrical energy. The maximum power point tracking (MPPT) circuit extracts maximum power from each energy harvester by adapting the impedance conditions. The conventional method by which the energy extracted by each harvester is combined is called the \"power OR-ing method\" [9; 10; 11], and this is the simplest method to combine energy from multiple sources. The interface circuit (such as a dc-dc converter) converts the output voltage to a level appropriate for the load devices. If the energy from each harvester is sufficient, the conventional architecture may be optimal. The unanticipated waste of power that occurs while combining the energy streams can be ignored, and does not affect the result of efficiency gained by combining the energy stream. However, for low power applications such as WBANs, the architecture must be simplified. For this use, the MPPT circuit is not appropriate. It includes a complex algorithm to track and compare the magnitude of harvested power. This may consume more power than is possible to provide in a WBAN. Therefore, eliminating the MPPT circuit may improve the power supplied to a load. In this work, because the application involved micro watt power levels, the MPPT circuit was not taken into account. Next, the OR-ing circuit also needs to be improved. The diode-based power OR-ing circuit has one drawback; there is unwanted power consumption caused by the forward voltage drop on the diode. The forward voltage drop can be minimized with a low forward voltage diode such as Schottky diodes. However, power waste due to the forward voltage drop when using the diode based power OR-ing method is inevitable. To avoid such power waste, a metal-oxide-semiconductor field-effect transistor (MOSFET) switch can be used in place of the diode. The MOSFET based power OR-ing circuit also requires power consumption to drive the circuit. However, as the integration process proceeds, the power consumption of a circuit is reduced continuously. Aside from the unwanted energy loss in a conventional OR-ing stage, both diode based power OR-ing circuits and MOSFET based power OR-ing circuits have one more common drawback in common. When the one energy harvester generates greater energy than the other harvesters, the power OR-ing method causes the energy streams from the other harvesters to be excluded. Thus, low energy harvesters have no chance to contribute their energy to a load device. For example, when two harvesters are photovoltaics (PV) and piezoelectric (PZ) respectively, on a sunny day, the PV transducer generates much more energy than the PZ transducer does. The voltage of energy stored in the PV capacitor is higher than the voltage of the energy stored in PZ capacitor, and only the energy of PV harvester can be transferred to the load through the interface circuit. Until the voltage of the energy stored in the PV capacitor drops below the voltage of the energy stored in the PZ capacitor, the energy harvested in the PZ transducer has no chance to be connected to a load. The energy in the PZ just waits to be connected and the harvested energy may disappear due to the leakage characteristics of its temporary storage. In low power applications like WBANs, all the harvested energy should be saved carefully. It is not desirable to entirely exclude any of the harvesters in any case. To prevent the exclusion of a specific harvester, the magnitude of the energy in all harvesters should be monitored periodically and the energy from each harvester should be moved to storage. Moreover, if the harvester generates more energy than the energy consumed at the load, the voltage of the temporary storage capacitor keeps increasing.