## 1 Introduction  
A light-emitting diode (LED) has the advantages of small size, high energy efficiency, long service life, fast response time, good color rendering, environmental friendliness, etc. Nowadays, LEDs have gradually replaced fluorescent lamps and high-intensity gas discharge lamps, and have been widely used in various lighting systems [1; 2; 3; 4]. When the input voltage is an ac line voltage source, a large capacitor filter needs to be installed after the rectifier circuit to reduce the ripple of the rectified dc voltage. Since the diode of the rectifier circuit only conducts when the ac voltage is higher than the dc voltage. The conduction time is very short, and the input current is a large pulse inrush current. This results in a low power factor and high total harmonic distortion of current (THDi) [5]. High-order harmonic currents can interfere with control circuits or communication systems and cause malfunctions in measuring instruments and electrical protection equipment. Therefore, advanced countries have formulated strict specifications, such as IEC 61000-3-2 [6] and IEEE 519 [7] to regulate the power factor and THDi of electrical products [8; 9]. In order to meet the specifications of power factor and THDi, a power factor corrector (PFC) should be added in front of the original ac/dc converter, so that the LED drivers have a two-stage circuit architecture [10; 11]. In order to improve the shortcomings of using more components in a two-stage converter and to reduce the number of energy transfer times in the converter and thereby improve the circuit efficiency, researchers have developed a number of single-stage LED drivers in which the active switches and control circuit are shared by a PFC converter and a dc/dc converter [12; 13; 14; 15; 16; 17]. However, in order to serve as switches for both the PFC converter and the DC/DC converter, some shared switches operate in hard switching. Hard switching means that the switch cannot operate at zero-voltage switching on (ZVS) or zero-current switching off (ZCS). For active switches, if they cannot operate at ZVS, their parasitic capacitance will generate huge spike currents due to an instant short circuit when they are turned on, and the energy originally stored in the parasitic capacitance will be dissipated inside the switches. For diodes, if they cannot operate at ZCS, a reverse recovery current will be generated when the diodes are turned off. In other words, in addition to huge switching losses, hard switching is also accompanied by high current and high voltage stress. Therefore, hard switching circuits must use switches with high voltage/current specifications to reduce their failure rate. Without the use of higher specification components, the LED drivers with hard switching can only be used in low-power products. The literature [18; 19; 20] proposed LED drivers with single-stage topologies. Although they can achieve a high power factor, their energy conversion efficiency is not high because the active switches cannot meet ZVS operation, so they are only suitable for low-power products (9-50 W). In order to solve the hard-switching problem, pulse-width modulation (PWM) converters usually use active clamping or snubber circuits to implement ZVS and/or ZCS [21; 22; 23; 24]. However, these soft-switching techniques require the use of additional auxiliary switches, diodes, inductors, and capacitors, which increases circuit costs. In addition, current loops in active clamping or snubber circuits generate additional conduction losses, thereby reducing circuit efficiency. In the literature [25; 26; 27; 28], a single-stage topology is obtained by integrating a PWM converter and a resonant converter, where the PWM converter is used as the PFC circuit and the full-bridge or half-bridge resonant converter is used as the DC/DC converter. The resonant converter not only has the advantage of galvanic isolation.