# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 7 Conclusions  
In this paper, an effective energy management circuit for combining energy from multi-source harvesters and distributing the harvested energy to dual loads is presented. By combining energy harvested from multi-source harvesters, the small amount of energy harvested in each harvester is overcome. The restrictions in the availability of each energy harvester due to the small amount of energy harvested is alleviated, and the system reliability is improved. By adopting a dual output scheme, a load and a rechargeable battery can be accommodated at the same time, and a wide range of levels of harvested energy is handled. When the input harvesters provide more energy than requiredby the load, the extra energy goes into a rechargeable battery and is thereby conserved safely. The extra energy used to charge the battery can be utilized by a load after a sufficient amount accumulates.  
The proposed energy management circuit is integrated using a 0.13 \(\upmu\)m TSMC CMOS technology. The die size is 960 \(\times\) 910 \(\upmu\)m. The newly developed IC includes analog and digital circuits based on ultra-low-power schemes. With a simple structure, the system monitors the magnitude of harvested energy and effectively combines the energy from each harvester. An evaluation board was designed and tested to obtain the measurement results. Each harvester was externally modeled, and the threshold powers were set at 55 and 275 \(\upmu\)W for low power and high power, respectively. When all the sources were enabled, the operating power consumption of the IC was 1.5 \(\upmu\)W with 1.2 Vdd. The frequency of the internal oscillator was 32 kHz, and the digital control circuit checked the state of the input harvester every 125 \(\upmu\)s. The new technique used a novel algorithm to combine and distribute energy to dual loads. The adopted algorithm decides which output should be connected to a load based on the present and previous state of harvested energy. The proposed algorithm enables all the attached energy harvesters to have the same chance to supply energy to a load. The energy management efficiency is more than 90% over the whole range of input power.  
In WBANs, it is hard to combine energy from multi-source energy harvesters. The conventional diode OR-ing scheme has a forward voltage drop, and the inductor sharing dc-dc converter scheme is hard to employ with micro watt energy harvesters because of the power consumed by the IC. The newly developed IC is suitable for many critical low-power applications with multiple and multi-type energy harvesters. By combining energy streams from very limited power sources with high efficiency, it can provide an effective solution for powering autonomic systems such as wearable devices or WBANs.  
Conceptualization, H.-I.P. and I.G.L.; Methodology, T.K.; Simulations, M.J.P. and J.-J.L.; Measurements, K.-I.O.; Original Draft Preparation and Supervision, S.-E.K.  
This work was supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government (18 ZB1200, Development of Core Technologies for Implantable Active Devices).  
The authors declare no conflict of interest.