# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 3 Energy Management Integrated Circuit (EMIC) Structure
### Energy Management with Multi-Source Harvesters  
The proposed energy management circuit has three input ports and two output ports. Up to three energy harvesters can be attached to the circuit, and two loads can be supported. According to the condition of the harvested energy, sometimes the energy streams from multiple harvesters are combined to one output port by sharing time, and sometimes they are distributed to two output ports. There are four different cases for the conditions under which the energy is harvested.  
The four different cases with input conditions are described in Figure 4. For the case illustrated in Figure 4a, all three energy harvesters generate low power. The three harvesters are connected to the low-power path, and supply energy to a load by sharing time. The difference between this and combining energy using the power OR-ing method is that with the proposed method, all three harvesters are monitored and each is given a chance to be connected to the low-power path. This prevents the energy already harvested from dissipating while in temporary storage.  
For the case illustrated in Figure 4b, the power of the first harvester is high, while the power of the other harvesters remains low. As in the first case, the energy of the first harvester is checked, and then connected to a low-power path. If the power of the first energy harvester is higher than the power consumed by the load, the voltage in Cap_1 remains higher than the threshold voltage, Vth. Then, when the next check is done, the first harvester is switched to a high-power path. The first harvester starts to supply energy to a battery, and the voltage in Cap_1 starts to decrease. At the time of the next check, the first harvester is again connected to a low-power path. The other harvesters still supply energy to the low-power path sequentially as in the first case.  
In the case illustrated in Figure 4c, the second harvester is producing high energy, and only the third harvester is generating low power. The first and second harvesters supply energy to a load and the battery in sequence, while the third harvester supplies energy to a load. In the last case, Figure 4d, all three harvesters produce high energy and all three supply energy to a load and to a battery at the same time. The battery is charged using the energy from all three harvesters. When no harvest-energy stream is passing to the load, the energy in the charged battery can be utilized by the load through a diode or an additional switch.  
The energy output path for a harvester is selected according to the amount of energy harvested. There are three output-path states as classified by the energy threshold levels described in Figure 5. The low energy threshold is determined by the energy consumed in the low-power path, and the high energy threshold is determined by the energy consumed in the high-power path. When the amount of energy harvested is lower than the low energy threshold, the harvester always supplies energy to a low-power path. When the amount of harvested energy is between the low energy threshold and high energy threshold, the harvested energy is supplied to low and high-power path alternately within a given time interval. When the amount of harvester energy is higher than the high energy threshold, a harvester provides energy only to the high-power output path.  
Figure 4: Energy flow according to the magnitude of energy harvested in the proposed energy management circuit. (**a**) 3 low powers; (**b**) 2 low powers and 1 high power; (**c**) 1 low power and 2 high powers; (**d**) 3 high powers.