## 1 Introduction  
The amount of power consumption for lighting applications accounts for around 15-20% of the total generated electric power. Therefore, any small increase in lighting efficiency will result in a significant reduction in energy consumption. For improving the illuminous efficiency of lighting sources, vast manpower and resource are engaged to develop light-emitting diodes (LEDs). Compared with traditional lighting sources, LED has the advantages of a small size, high luminous efficiency, and long life span and are the most promising lighting device [1; 2; 3; 4; 5]. Owing to the simple circuit topology and easy control, half-bridge converters are widely used in medium-and low-power applications. Nevertheless, the active switches of the half-bridge converter suffer from hard switching, resulting in high voltage/current stresses and high switching losses. In order to solve the problem of hard switching, an active clamp and snubber circuits are usually used to make the active switches operate at zero-voltage switching on (ZVS), leading to low voltage and/or current stresses [6; 7; 8; 9]. In addition, the circuit efficiency can be effectively improved. However, these techniquesrequire the use of additional auxiliary switches, diodes, and passive components to enable the active switch to operate at ZVS or zero-current switching off (ZCS). It These soft-switching techniques would increase the complexity and cost of the circuit. Besides, current loops in active clamp circuits or snubber circuits can cause additional conduction losses and even more switching losses. Recently, in order to comply with the more stringent regulations such as IEC61000-3-2 class C, an additional alternating current-to-direct current (ac/dc) conversion stage is required to cascade in front of the dc/dc converter to perform the function of the power factor correction (PFC). For the sake of reducing circuit costs, many single-stage circuits have been developed for driving LEDs by integrating a PFC converter into the original LED driver [10; 11; 12; 13; 14; 15]. Although these single-stage approaches can achieve a high power factor and low total current harmonic distortion (THDi), one or more active switches in these single-stage circuits still suffer from the hard switching problem. On the other hand, an LED driver with dimming capability is needed in many applications. Some single-stage approaches are derived by integration of a boost and a half-bridge resonant converter [14; 15]. The half-bridge converters usually use an inductor-inductor-capacitor (LLC) or a capacitor-inductor-capacitor-inductor (CLCL) resonant tank to achieve ZVS. Frequency modulation control is usually used to achieve the dimming operation. Since the boost converter and the resonant converter share the same active switch, the switching frequency of the boost converter also varies to accomplish the dimming operation. Generally, the input power of the boost converter is inversely proportional to the switching frequency. In order to achieve wide dimming operation, the switching frequency should vary in a wide range. The higher the switching frequency, the lower the LED power. Moreover, higher switching frequency would incur more switching losses, leading to low efficiency at low power operation. In single-stage circuits, a dc-link capacitor is usually required to absorb or release power due to the mismatch between input power and output power. The voltage across the dc-link capacitor varies when the LEDs are dimmed. When the dimming range is large, the voltage of the dc-link capacitor changes greatly. Generally, the methods of symmetrical pulse-width modulation (PWM) or asymmetrical pulse-width modulation (ASPWM) are often used to control the active switches to regulate LED power [16; 17; 18; 19]. When both control schemes are compared, the voltage across the dc-link capacitor of the PWM scheme is higher than that of the ASPWM one [18; 19].