# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 3 Energy Management Integrated Circuit (EMIC) Structure  
### Energy Management with a Single Harvester  
The proposed architecture for the energy management circuit is shown in Figure 3. This architecture is simplified to the case of single energy harvester to help with comprehension of the operating algorithm of the proposed energy management circuit. Because the amount of energy harvested changes according to environmental conditions, the energy management circuit adopts a dual load scheme to deal with a wide range of energy levels. A dual load scheme has two output paths: a low-power path and a high-power path. According to the magnitude of the energy harvested, the energy can be supplied through either the low-power path or high-power path. As shown in Figure 3, the architecture provides a load in the low-power path, and a storage element such as a rechargeable battery in the high-power path. Normally, harvested energy is supplied to a load directly. If the energy in a harvester exceeds the energy consumed by a load, the extra energy is stored in a storage element. By storing the extra power in a storage element, the excess power can be utilized in the load later. Figure 3: Architecture for a single-source energy harvester with the proposed energy management circuit. Figure 2: Conventional architecture of a multi-source energy harvester. With this architecture, the capacitance provided is an important factor and determines the operating clock frequency. This affects the power consumption of the circuit. Because a circuit with high operating clock frequency consumes more power, the capacitance should be considered carefully. The equation for sizing capacitance is derived below. \[\mathrm{Q}=\mathrm{C}\cdot\mathrm{V} \tag{1}\]  
\[\frac{d\mathrm{Q}}{dt}\ (i)=\mathrm{C}\cdot\frac{d\mathrm{V}}{dt} \tag{2}\]  
In Equation (1), Q is the quantity of charge, C is the size of capacitance, and V is the voltage of the capacitor. When Q is differentiated with time, the current can be described as the product of the capacitance size and the voltage change, as shown in Equation (2). The current into the capacitor is proportional to the speed of the voltage change in the capacitor. When current is harvested in the input stage, the voltage of the capacitor rises. The speed of voltage change in the capacitor is determined by the capacitance. If the voltage in the capacitor changes rapidly, the integrated circuit (IC) should check the voltage more frequently. Therefore, the relation between capacitance and operating frequency of the IC is very close. Equation (2) also shows the relation between capacitance and the current consumed at the load stage. When power is consumed at the load stage, the voltage of the capacitor starts to fall. To prevent the voltage of the capacitor from falling below the threshold voltage, the size of the capacitor and the operating frequency of the IC should be controlled appropriately. To slow the operating clock frequency, large capacitance is preferred. The operation starts with the energy harvested from an energy source. When the energy is harvested, it is stored in a temporary storage capacitor, C, which rises the voltage of C. The voltage of C is periodically checked by internal comparators whether the voltage of C is higher than threshold voltage, Vth, or not. If the voltage of C is higher than Vth, the switch in the low-power path turns on, and the stored energy in C is supplied to a load through the low-power path. If the harvested power is less than the power consumed by the load, the voltage of C will decrease, and the switch on the low-power path turns off. If not, the voltage of C keeps increasing. Then, the switch on the low-power path turns off, and the switch on the high-power path turns on. The energy stored energy in C is connected to the battery through the high-power path instead of to the load in the low-power path. Thus, when the harvested power exceeds the load power, the harvester power is used to charge a battery.