Cross regulation of the two circuits was measured and the SEPIC was substantially better. In both, the 5 V held at 5.05 V, the loads were varied from zero to full load, and input voltage was set to 12 or 24 V. The 12 V on the SEPIC remained within a 10% regulation band, whereas the 12 V on the flyback went to 30 V (high line input, no load on the 12 V, full load on the 5 V). Efficiency between the two configurations was the same but could have favored the SEPIC, if the power parts had been selected consistent with its lower voltage stresses.
2. A SEPIC dramatically reduces EMI and voltage stress. The top image has C1 removed, while the bottom image has C1 installed.
To summarize, the SEPIC is a valuable topology for non-isolated power supplies. It clamps the MOSFET voltage stress to a value equal to the input plus the output voltage and eliminates EMI seen in a flyback. The reduced voltage stress may allow the use of lower voltage parts, resulting in a more efficient and less costly supply. The reduced EMI will simplify compliance testing of the final product. Finally, if configured as a multiple output supply, it will improve cross regulation when compared to a flyback.
Previously published Power Tips articles by Robert Kollman are available.
For more information about this and other power solutions, visit ti.com/power-ca.
About the author
Robert Kollman is a senior applications manager and a Distinguished Member of the Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-Megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University. You can reach Robert at email@example.com.
"No need to fear: SEPIC outperforms the flyback," John Betten and Robert Kollman, Power Management DesignLine, January 25, 2006.