Design of high-efficiency LED offline lighting circuit

　　While it may be years before a viable screw-in LED replaces incandescent bulbs, the use of LEDs in architectural lighting is growing, offering greater reliability and energy savings potential. As with most electronics, a power supply is needed to convert the input power into a form usable by the LEDs. In a streetlight application, a possible configuration is 80 LEDs in series to create a 300V/0.35A load. When choosing a power supply topology, requirements for isolation and power factor correction (PFC) need to be determined. Isolation requires extensive safety trade-off studies, including the need to provide shock protection versus complicating the power supply design. In this application, where high voltages are present on the LEDs, isolation is not considered essential, but PFC is essential, as this product is targeted at the European market, where PFC is required for lighting above 25W.

　　There are three power supply topologies to choose from for this application: buck, transfer mode flyback, and transfer mode (TM) single-ended primary inductor converter (SEPIC). When the LED voltage is about 80 volts, the buck topology can be used very effectively to meet the harmonic current requirements. In this case, the higher load voltage will no longer allow the buck topology to be used. Then, the more compromised approach at this time is to use the flyback topology and SEPIC topology. The SEPIC has the advantage that it can clamp the switching waveform of the power semiconductor device, allowing the use of lower voltages, making the device more efficient. In this application, an efficiency improvement of about 2% can be obtained. In addition, there is less ringing in the SEPIC, which makes EMI filtering easier. Figure 1 shows the schematic of this power supply.

　　The circuit uses a boost TM PFC controller to control the input current waveform. The circuit starts by charging C6 offline. Once operating, the controller power is provided by an auxiliary winding on a SEPIC inductor. A relatively large output capacitor limits the LED ripple current to 20% of the DC current. As a side note, the AC flux and current in the TM SEPIC are very high, requiring enameled stranded wire and low-loss inner core to reduce inductor losses.