The single-ended primary-inductor converter—the SEPIC converter—is capable of operating from an input voltage that is greater or less than the regulated output voltage. Aside from being able to function as both a buck and boost, the SEPIC design also has minimal active components, a simple controller, and clamped switching waveforms which provide low noise operation.
It is often identified by its use of two magnetic windings. These windings can be wound on a common core, in the case of a single dual-winding coupled inductor, or they can be two independent inductors. Designers are often unsure of which approach is best and whether there is any real difference between the two. This article looks at each approach and discusses the impact each has on a practical SEPIC converter design.
Figure 1 shows the basic SEPIC converter with a coupled inductor. When the FET (Q1) turns on, the input voltage is applied across the primary winding. Since the winding ratio is one-to-one, the secondary winding is also imposed with a voltage equal to the input voltage.
Figure 1: The basic coupled inductor SEPIC converter.
But because of the polarity of the windings, the anode of the rectifier (D1) is pulled negative and reversed bias. With the rectifier biased off, the output capacitor is now required to support the load during this on-time period. Additionally, this forces the AC capacitor (C_ac) to be charged to the input voltage.
While Q1 is on, current flow in both windings is through Q1 to ground, with the secondary current flowing through the AC capacitor. The total FET current during the on time is the sum of the input current and the output secondary current.
When the FET turns off, the voltage on the windings reverses polarity to maintain current flow. The secondary winding voltage is now clamped to the output voltage when the rectifier conducts to supply