# An AC/DC LED Driver with Unity Power Factor and Soft Switching
## 1 Introduction  
Nowadays, high-brightness light-emitting diodes (LEDs) are popularly used to replace incandescent and fluorescent lamps in many applications owing to their advantages of small size, high luminous efficiency, long lifespan, high reliability, and environmental friendliness [1; 2; 3]. On the purpose of increasing the utilization factor of the power electric equipment and reduce the electromagnetic interference (EMI), some regulations such as IEC 61000-3-2 and IEEE 519, are enacted to restrict the power factor (PF) and total current harmonic distortion (THDi) of the alternating current-to-direct current (AC/DC) LED drivers to a reasonable range. In order to meet these regulations, an additional converter serving as a power-factor correction (PFC) stage is added to a DC/DC converter of which the output voltage is adjusted to drive LEDs. It results in a LED driver with two power-process stages. Although the two-stage approaches have satisfactory performance, they are not cost-effective products since two converters and two corresponding control circuits are required. In addition, the two-stage approaches take two energy-conversion processes, and hence produce more losses including switching loss and conduction loss. Aiming to solve the shortcomingsof the two-stage approaches, many single-stage LED drivers have been proposed by integrating the PFC stage and the DC/DC stage [4; 5; 6; 7; 8; 9; 10].  
Among the single-stage approaches, one power switch is commonly shared by the PFC stage and the DC/DC stage. Hence, they only use one control circuit and own the benefits of less component count and simply circuit topology. Nevertheless, the shared power switch should handle the current in both power-conversion stages, leading to more conducting losses. Moreover, the power switches of the single-stage approaches usually operate at hard switching and suffer the problem of high switching losses and high voltage and current stresses.  
To further improve the circuit efficiency, literature proposed single-stage converters of which the power switches can fulfill soft switching to effectively reduce the switching losses [11; 12; 13]. Also, high PF and low THDi can be achieved. Unfortunately, there are still some defects required to be improved. In Reference [11], a coupled inductor is used in the buck-boost converter and its leakage inductance would incur high spike voltage across the power switch. In Reference [12], a boost converter serves as the PFC converter. Figure 1 shows the circuit topology that was proposed in Reference [12]. It integrates a boost converter and a buck converter. The boost converter serves as a PFC stage while the buck converter outputs a stable voltage to drive the load. High PF can be achieved, provided that the boost converter is operated at discontinuous current mode (DCM). However, a boost-typed PFC converter needs a high DC-link voltage to ensure DCM operation. It causes the circuit to require the use of high voltage rated components. For pursuing a better solution, this paper proposes a novel AC/DC converter to drive high power LEDs. The proposed circuit mainly consists of a buck-boost converter and a buck converter. Different from the circuit of Reference [12], when the boost-buck converter operates at DCM, it can obtain high power factor without needing a high DC-link voltage. A prototype circuit of 60-W was built and tested to verify the analytical predictions. Satisfactory performance is obtained from the experimental results.