Solid-state lighting (SSL) has become to change from being a niche market to penetrating residential markets. And compact fluorescent lamp (CFL) retro-fit is one of the potential market.

The cost, size and reliability are important for CFL retro-fit applications. One could use either passive PFC (power factor correction) or active PFC to achieve a high power factor.

Passive PFC requires large passive components, so it difficult to achieve a small size. Traditional active PFC circuits require a two-stage topology, which entails a boost stage for PFC, then buck or flyback for current regulation of the LEDs. The cost of the two stages is high.

In this application note, a non-isolated, soft-switched, single-stage high power factor offline LED driver is introduced. Because its simplicity and low cost, the buck-boost converter is chosen. The converter operates with constant peak current for constant power control, and in transition mode (boundary mode between CCM and DCM) to achieve soft switching, using the L6562A controller. High power factor is achieved by reshaping the peak current near the zero crossing of the input AC line.