# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 6 Discussion  
There is now great interest in autonomous WBANs with renewable energy sources. Autonomous WBANs could get energy from their surrounding environment and use it to charge a battery. Autonomous WBANs are a very important technology, especially for use in implantable devices for which it is hard to maintain a battery. However, the energy obtained from most harvesters is limited to micro watt scale. To obtain significant amounts of energy from harvesters, technology for combining the energy from multiple harvesters is essential. In recent research on WSNs, a multi-source energy harvesting platform or modular unit with a micro-processor was proposed [17; 18; 19]. In this work, an energy management circuit that includes analog and digital parts was fabricated using a 0.13 \(\upmu\)m TSMC CMOS technology. In WBANs, the IC solution may be a major feature when considering the size, weight, and lifetime of wearable devices. With the newly developed IC, power consumption was minimized to 1.5 \(\upmu\)W with 1.2 Vdd when combining the energy streams from three harvesters with more than 90% efficiency. With the power consumption that occurs when combining energy, it is advantageous to regulate and supply energy to a load after combining the energy from multi-source energy harvesters. Figure 11: Measured graphs for the newly developed IC. (**a**) measured-output flow as the magnitude of power; (**b**) operating efficiency as the magnitude of power. In applications with multi-source energy harvesters, various kinds of energy harvesters might be adopted, and the energy streams generated from various kinds of harvesters should be combined. Every kind of harvested energy has a different form and standards do not yet exist. According to the type of transducers, DC or AC power can be generated. The harvested energy in PV cells is DC type, and that in PZs cells is AC type. The proposed IC only manages DC power. When AC power is generated from an ambient energy harvester, a rectifier circuit is required in front of the IC as derived in [20; 21; 22]. Much research has been done to improve the efficiency of rectifier and multiplier circuits, and in [23], a CMOS solution in a rectifier with a 0.18 \(\upmu\)m technology was also implemented. In low-power applications such as WBANs, if possible, the energies from multi-harvesters have to be combined effectively prior to doing other work. When the magnitude of energy becomes sufficient to be utilized by the electronic systems, the output voltage should be converted to a regulated voltage level suitable for a load device by an additional interface circuit. This is because it is not easy to change the output voltage with very small amounts of energy. By combining the low energy in harvesters, this study shows the potential for utilizing low-power harvesters. Energy sources that were thought useless previously can now be utilized in electronic devices and can be commercialized. The power consumption in electronic devices has been decreased continuously. There are now many applications that can operate with low power consumption such as mesh networks [24], sensor and control systems, RF identification (RFID) devices, and MEMS [25]. As an example of wearable applications, several types of energy harvesters can be attached to the clothes. PZs utilize vibrations from body movement and TEGs utilize temperature difference between body and air. All of these attached transducers concurrently generate electrical energy from every day behavior and could provide energy to a wearable sensor network. By adopting multiple inputs and multiple load schemes, the harvested energy could allow network modules to communicate. The system can only be made autonomous with multiple low-power energy harvesters. If the energy harvested on clothes is not sufficient to operate the network as a primary energy source, it could still play a role as a secondary energy source with a battery. The battery would be the main energy supply for the network, while the harvested energy would extend the battery life.