# A Power Processing Circuit for Indoor Wi-Fi Energy Harvesting for Ultra-Low Power Wireless Sensors
## 5 Simulation Results of the Power Processing Circuit  
There are three main points concerning the proposed power processing circuit that need to be investigated. These points highlight the voltage characteristics of the proposed power processing circuit. The three different points are the voltage across capacitor \(C_{2}\), the differential voltage at the positive terminal of diode \(D_{3}\) and the output voltage of the proposed circuit which is the voltage across \(C_{7}\). The signal characteristics of the three stated points can be found using circuit simulation. LT Spice  
Figure 8: The calculated and measured voltage of the rectification circuit as a function of available power for SMS7621 Schottky diodes at \(f\) = 2.45 GHz. \(R_{g}\) = 50 \(\Omega\). software is utilised for investigation purposes. The spice parameters of the Schottky diode, SMS7621, and the NMOS switch, ALD210800A are obtained from the manufacturer datasheet. In the simulation, the harvesting antenna is assumed as an AC source. The AC source continuously supplies the power processing circuit with a sinusoidal input signal. The push-pull circuit operation is relating to the signals of the voltage across \(V_{C3}\) and \(V_{C4}\) which the driving signal for the switching MOSFET, \(M_{2}\) and \(M_{3}\). The inverting signal is measured at \(V_{C6}\). Figure 9 illustrates the signals for the stated components. As depicted in the figure, the instantaneous signal at \(V_{C6}\) is bigger than the \(V_{AC}\) which is normally less than 10 mV. The higher the AC signal contributes to faster charging time for the storage component. There are four different alternating input voltages were tested as the input signal for the proposed circuit and the values are 2 mVp-p, 6 mVp-p, 20 mVp-p and 35 mVp-p. For all the input voltages, the frequency is at 2.45 GHz. Each value represents an input power of \(-50\) dBm, \(-40\) dBm, \(-30\) dBm and \(-25\) dBm, respectively, being supplied to a 50 Ohm impedance input terminal. As illustrated in Figure 10, there is an initialization time for the power processing circuit to start the circuit boosting operation. The initialization time occurred because the stored energy in \(C_{2}\) is not enough to activate the boosting circuit. However, when the stored energy in \(C_{2}\) is sufficient, the boost circuit will start its function. In practical, it is expected that the initialization time will be longer since this incident signal is low and in burst mode. However, with respect to time, the energy level stored in \(C_{2}\) will reach the required level for boost circuit operational. The boosting signal is represented by the simulation result of \(V_{D3}\). The circuit initialization time becomes shorter when the input voltage is bigger. As can be seen in Figure 10,b, the time needed for the boost circuit to start functioning is approximately 0.8 ms, as the input voltage is increased to 6 mVp-p, and the time required for initialization less, which is approximately 0.25 ms. The initialization became much faster to approximately 0.1 ms when the input voltage is bigger than 20 mVp-p. The reading for \(V_{C2}\) and \(V_{C7}\) is not much difference especially for an input voltage of 2 mVp-p and 6 mVp-p. However, for higher input voltages, 20 mVp-p, and 35 mVp-p, there is a significant result obtained. The power processing circuit boosted the value of \(V_{C2}\) close to 3 times of the input voltage as shown in Figure 10,d. From the simulation, it has been demonstrated that the boost circuit is functioning as expected. For a small input voltage 1 mVp (\(-50\) dBm), the boost circuit of the proposed power processing circuit can generate 6 times bigger than the input voltage of the switching diode \(D_{3}\). The signal boost factor is close to 6.5 times. On average, for all the selected input  
Figure 9: The signals at \(V_{C3}\), \(V_{C4}\) and \(V_{C6}\) for an input signal of \(V_{ac}\) = 3 mV and frequency of 2.45 GHz.