# Single-Stage Step-Down Power Factor Corrector without Full-Bridge Rectifier
## 1 Introduction
The studies [19; 30; 31] present buck PFC rectifiers, which can convert a higher input AC voltage into a lower DC voltage. The advantage of this circuit is that the voltage stress of the power switch is the maximum value of the input voltage, and compared with the boost PFC rectifier, the power switch can be selected with a smaller withstand voltage, and thus the cost is cheaper. However, there is no input current when the input voltage is lower than the output voltage, resulting in zero-crossing distortion. Consequently, the input current does not follow the phase of the input voltage at this time, so the power factor is relatively low, as shown in Figure 1. In addition, the switch is floating, which makes it complicated to drive. The studies [32; 33; 34] propose buck-boost rectifiers, which have the advantage of being able to step up and step down the input voltage in CCM, thereby providing a wide output voltage range and being easy to design according to the requirements of the back-end circuits. However, the voltage stress of the switch and output diode is the sum of the input voltage and the output voltage. Consequently, the use of components with higher withstand voltages is required. In addition, the switch is floating, which makes it complicated to drive. In general, traditional active PFC rectifiers are not suitable for high-power applications because when these rectifiers are activated, the full-bridge rectifiers, diodes, and switching elements will have large currents flowing through them, resulting in large conduction losses and switching losses, and hence leading to a reduction in the overall efficiency. In view of  
Figure 1: Input voltage and input current of the buck active PFC rectifier. this, if the loss of the bridge rectifier can be decreased, the overall efficiency of the circuit can be improved. Therefore, bridgeless active PFC rectifiers are presented [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38]. As is generally acknowledged, the PFC control methods can be roughly categorized into the following four types: (1) peak current mode control [7]; (2) average current mode control [8; 39]; (3) critical mode control [9; 40]; (4) voltage-follower control [36]. For the peak current mode control, if the duty cycle is greater than 50%, the current is prone to sub-harmonic oscillations, so if no proper slope compensation is added to this control, it will result in the distortion of the input current. For the average current control, it makes the transient response of the PFC rectifier slow but is suitable for high-power applications. For the critical mode control, it is difficult to design the filter because it is a variable-frequency control. For the voltage-follower control, due to the high peak current in the power switch when the circuit operates in the discontinuous conduction mode (DCM), it is necessary to select power components with a higher current withstand performance, so it is only suitable for low-power applications. Accordingly, as shown in Figure 2a, the proposed single-stage step-down power factor corrector without a full-bridge rectifier is presented, which is modified from the existing bridgeless high-PF buck converter [22], as shown in Figure 2b. From these two figures, it can be seen that the former has a lower number of components than the latter. Firstly, the system configuration is depicted in Section 2. Sequentially, Section 3 describes the operating principle in detail. After this, the design considerations are given in Section 4. Accordingly, in Section 5, some experimental results are presented to validate the theoretical analysis and designed parameters. Eventually, Section 6 presents a conclusion.