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Power Tip #7: Efficiently Driving LED's Offline
Consider isolation, PFC, and topology options
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By
Robert Kollman, Texas Instruments
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Page 1 of 2
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Power Management DesignLine
(01/07/2009 11:34 AM EST)
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While we may be years away from a viable screw-in LED replacement for an incandescent bulb, there is a growing movement to use LEDs in architectural lighting due to their improved reliability and potential energy savings. As is the case with most electronics, a power supply will be needed to convert input power to a form usable by the LEDs. In a street light application, a possible configuration is a series of 80 LEDs creating a 300V/0.35 amp load. In choosing a power supply topology, requirements regarding isolation as well as power factor correction (PFC) need to be established. Isolation requires a significant safety trade-study involving the need for providing shock protection versus complicating the power supply design. In this application, where there is high voltage at the LED, isolation is deemed not necessary. PFC is necessary as this product is for European markets where PFC is required for lighting over 25 watts.
Three candidate power supply topologies can be considered for this application: buck, transition mode flyback and transition mode (TM) single ended primary inductor converter (SEPIC). The buck can be used very effectively to meet harmonic current requirements when the LED's voltage is on the order of 80 volts. In this case, the higher load voltage deems the buck not feasible. The trade then comes down to the flyback and SEPIC. The SEPIC has the advantage that it clamps the switching waveforms on the power semiconductor allowing the use of lower voltage and, hence, more efficient parts. In this application, this would provide an estimated two percent efficiency improvement. Additionally, there is less ringing in the SEPIC, making EMI filtering easier. Figure 1 provides a schematic of the power supply.
Figure 1: Transition Mode SEPIC Functions as a Simple LED Driver.
(Click this image to view a larger, more detailed version)
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