# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 5 Measurement Results
The next experiment was done to show the output path transition from a low-power path to a high-power path as the magnitude of input harvested energy increased using multi-source harvesters. The experiment was conducted with two energy harvesters because of the limitation of channel number in the oscilloscope. The waveforms with multi-source energy harvesters are depicted in Figure 10b-d. If the first and the second energy harvesters generate less power than that consumed by the load in the low-power path (55 \(\upmu\)W); both energy harvesters supply energy to the load in the low-power path at the same time by allocating time to each harvester as shown in Figure 10b. If two energy harvesters supply energy at the same time, the time to supply power to the load in the low-power path becomes twice the time for a single energy harvester. This means that the energy supplied is doubled with two energy harvesters. If the power of the second energy harvester increases to more than the power consumed at the load on the low-power path (55 \(\upmu\)W), the first energy harvester still supplies power to the load on the low-power path and the second energy harvester sometimes goes to the high-power path. Then the second harvester supplies power to a battery in a high-power path as shown in Figure 10c. If the power of the second energy harvester is higher than the power consumed in a high-power path, the second harvester maintains the energy supply to the battery in a high-power path. However, in this experiment, the harvested power in the second harvester was not higher than the power consumption from the battery, and the voltage of the second capacitor dropped. Therefore, next time, the second harvester supplied power to the load on the low-power path again. If the powers of the first and the second energy harvester increase upper than the power consumed at the load on the low-power path, the first and the second energy harvesters supplied power to the load in the low-power path and a battery in high-power path at the same time as shown in Figure 10d. The graphs for the last experiment are depicted in Figure 11. When total harvested power from three harvesters is under 55 \(\upmu\)W, all the energy is supplied through the low-power path. As the harvested power increases, some portion of the harvested energy starts to be supplied to a battery on the high-power path. The power supplied to a battery on the high-power path continues to increase as the harvested power increases. When the harvested power is 350 \(\upmu\)W, the power supplied to both of output paths is 330 \(\upmu\)W; whereas, 55 \(\upmu\)W is supplied to a load on the low-power path, and 275 \(\upmu\)W is supplied to a battery on the high-power path. If the harvested power is more than 350 \(\upmu\)W, the energy supplied through the low-power path is stopped, and all the energy is supplied to a battery on the high-power path, and all input energies are stored into the battery. The battery can supply the energy required at the load on the low-power path. When the harvested energy is excessively higher than the energy required at the load, the voltage of temporary storage may rise rapidly. To keep the IC safe, the energy stored in the temporary storage should be transferred as soon as possible. With a higher energy consumption on the high-power path, input harvested energies are assigned only to the high-power path at all times. The efficiency is derived from the ratio between the total power supplied to a load and total power harvested by the three harvesters. Generally, with the constant power consumption of the IC, as the harvested power increases, the efficiency improves. However, if the harvested power becomes excessively high, the resistance of the path switch may affect the efficiency, and the overall efficiency might decrease. During measurement, in the harvest-energy range 50-480 \(\upmu\)W, the efficiency was sustained at over 90%, and continuously increased.