I. INTRODUCTION

　　THE RAPID development of LED over the last few years has opened up new opportunities in the general illumination market, thanks to its distinct advantages such as high efficacy, long lifetime, environmental friendliness, and small size over incandescent and fluorescent lamps [1]–[4]. The power supply for LED lighting is an ac/dc converter, which converts a regular ac voltage to a low dc voltage for an LED driver. The input power factor is an important requirement of the ac/dc converter. It needs to be higher than 0.9 for most commercial luminaries [5]. Thus, the ac/dc converter must typically have the function of power factor correction (PFC). In a PFC converter, the input current is forced to be in phase with the input voltage, leading to a pulsating input power, while the output power is constant. To achieve this, a storage capacitor with large capacitance is required for balancing the instantaneous power difference. Due to the high capacity required for capacitance, an electrolytic capacitor is often used as the storage capacitor

　　However, it is well known that because of its liquid electrolyte, the lifetime of an electrolytic capacitor is quite limited with only several thousand hours under rated operating conditions. Even with a conservative design, the theoretical lifetime of electrolytic capacitors is only about 30 000 h (at a high operating LED’s temperature) [6]. This is much shorter than the potential lifetime of LEDs (50 000 h). Thus, the electrolytic capacitor is an obstacle to the overall long-term reliability of the LED and its power supply.

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Fig. 1. Schematic diagram of a two-stage PFC converter

Fig. 2. Key waveforms of the converter when the input power factor is unity