## 1 Introduction
Power quality and performance are required to maintain the THD value at the standard values, such as IEEE 519-20142 [17] and IEC 61000-3-2 [18], which suggests that to improve the circuit PF, by reducing the current stresses and power losses in the different parts of the system, THD should be kept at the lowest value within the given standard requirements [17, 18]. The current THD in the digitally controlled PFC boost converters can be reduced using different techniques, such as passive harmonic filters connected in parallel with the input side. This technique can be used to reduce the THD for low switching frequency PFC converters; however, for high-frequency PFC converters where the electro-magnetic interface (EMI) filters connect on the input side of the converters, the reliability of the passive filters to decrease the THD is low due to the difficulty of tuning the filter's resonance frequency in the presence of the EMI filters [19]. Digital current filters, such as finite response impulse (FIR) and infinite impulse response (IIR) filters, can be optimally designed and used to moderate the feedback digital current signals for the inner current-control  
Figure 1: Block diagram of the two stages AC-DC telecom power supply. loops to reduce the current error and thereby reduce the current distortion. Using the adaptive FIR filter to reduce the THD of the current conduction mode (CCM)-bridgeless PFC converter was proposed in [20]. Another widely used technique to reduce the THD in PFC converters is the variable online (VOT) switching control technique by optimizing the duty cycle of the PFC converter to decrease the zero-crossing distortion (ZCD) period [20; 21; 22; 23]. The VOT switching control technique can reliably reduce the THD in PFC converters, but uses complex mathematical analysis of the converter operation around the zero-crossing point to optimize the exact dead time around the zero-crossing point for exact on-time switching prediction [24]. This paper presents the small-signal stability modeling of the conventional PFC boost converter. Based on the signal stability model, the optimal integral-proportional (IP) current controller consists of a double-loop control strategy, where the integral gain feed-forward returns the inductor current to the reference set point, and the proportional gain implemented in the feedback path increases the controller response. The IP controller is usually used in the control circuits of DC/DC converters and DC motor drive systems [25; 26; 27]. Compared with the PI controller, the IP controller has less overshoot and a fast-dynamic response [27]. So, the IP current controller with fast dynamic response was designed for the inductor current to track the reference current, removing the total distortion that occurs around the zero-crossing point and increasing the power factor. Two different current techniques based on the conventional PI controller and the proposed IP controller are presented in this paper; a comparative analysis was performed based on system stability and controller reliability to reduce the zero-crossing current distortion. For the economic design of the digitally controlled PFC converter, the isolated voltage sensors with inexpensive components for interfacing with the MCU analog-digital converter (ADC) was completely designed and is proposed here. The remainder of this paper is organized as follows: Section 2 briefly explains the problem of high THD in the current zero-crossing point of PFC converters and the impact of this problem on the circuit PF. Section 3 outlines the operation principle and the complete design of the PFC boost converter employed. Section 4 presents the design technique for the proposed controlling circuits for the CCM-PFC converter based on small-signal stability modeling. Section 5 introduces the proposed isolated AC and DC voltage sensors for the economic and reliable operation of digitally controlled PFC converters. Section 6 provides the simulation of the complete designed converter with different control techniques.