## 5 Experimental Results  
A prototype circuit was built and tested. The circuit parameters are shown in Table 2, and the prototype circuit is shown in Figure 11. The experimental apparatuses consisted of an oscilloscope (Lecroy HDO4034, New York, NY, USA), voltage probes (Cybertek DP6150A, Shenzhen, China), current probes (Cybertek CP8030A, Shenzhen, China), and a power quality analyzer (Precision International Corp. PA-750B, New Taipei City, Taiwan). Figure 12 shows the measured voltage waveform on the primary winding and the self-excited windings of the center-tapped transformer. It can be seen that the self-excited voltage \(v_{gs1}\) and \(v_{gs2}\) are in phase with the primary voltage \(v_{p}\). Obviously, the driving voltages \(v_{gs1}\) and \(v_{gs2}\) are symmetrically displaced by 180\({}^{\circ}\). Hence, the switching frequency of the power switch could be the same as the resonant frequency of the resonant converter. Figure 13 shows the input voltage of the resonant converter \(v_{AB}\) and the resonant current \(i_{r}\). It demonstrates that \(i_{r}\) is in phase with \(v_{AB}\). This marks the equality of the resonant  
Figure 14 shows the waveforms of the input line voltage and the PFC inductor current \(i_{LPFC}\). It reveals that the envelope of \(i_{LPFC}\) is in phase with the low-frequency input line voltage. Figure 15 shows the waveforms of \(i_{LPFC}\) and the voltage across the power switch \(S_{2}\). It indicates that the PFC converter operates at BCM when the input line voltage is at the peak value, while it operates at DCM elsewhere. The waveforms of the input line voltage and current are shown in Figure 16. It can be seen that the input current is in phase with the line voltage. The measured unity power factor and total current harmonic distortion (THDi) is 0.98 and 10.2%, respectively. It is seen that the input current is close to a triangle waveform. In this proposed circuit, the power switches are operated at a fixed switching frequency and fixed duty ratio (\(D\) = 0.5). Since the boost-typed PFC operates at DCM, the peak value of the inductor current is proportional to the rectified input voltage, i.e., the envelope of the inductor current would be a sinusoidal waveform. Theoretically, the average value of the rising portion of the inductor current within a high-frequency cycle is also sinusoidal. Nevertheless, the duration of the inductor current dropping from peak to zero is not constant. The higher the input voltage, the longer the declining duration. When the PFC operates at the time when the input voltage is at the peak of a sinusoidal waveform, the average value of the inductor current would be much higher than the peak. This is why the current waveform looks triangular. Figure 12: Voltage waveforms of the center-tapped transform. (\(v_{p}\): 100 V/div, \(z_{gs1}\), \(v_{gs2}\): 20 V/div, time: 10 \(\mu\)s/div). Figure 13: Voltage and current waveforms of the resonant converter. (\(v_{AB}\): 100 V/div, \(i_{r}\): 0.2 A/div, time: 10 \(\mu\)s/div). Figure 14: Waveforms of the input voltage and the PFC inductor current. (\(v_{in}\): 100 V/div, \(i_{LPFC}\): 1.0 A/div, time: 5 ms/div). Figure 15: Waveforms of the PFC inductor current and the voltage across \(S_{2}\) at (**a**) the peak value, and (**b**) near 135 degrees of the input line voltage. (\(v_{DS2}\): 100 V/div, \(i_{LPFC}\): 0.5 A/div). Figure 17 shows the voltage and current waveforms of the power switches. Since the switching frequency is identical to the resonant frequency, \(S_{1}\) can be turned off at zero current. Besides, as mentioned in operation Mode II, the PFC inductor current diverts from \(S_{2}\) to \(D_{S1}\) as soon as \(S_{2}\) is turned off. In this way, the voltage across \(S_{1}\) is clamped to near zero volts. It helps the power switch \(S_{1}\) to be turned on at zero voltage. Theoretically, the power switch \(S_{2}\) should be turned on at zero voltage when the resonant circuit presents a resistive characteristic.