Power Tip: SEPIC converter makes an efficient bias supply

Figure 1 shows a SEPIC converter which, like a flyback, has a minimal parts count. In fact, this circuit would be a flyback, if C1 were removed. This capacitor provides voltage clamping of the semiconductors to which it connects. When the MOSFET is turned on, the reverse voltage on D1 is clamped by the capacitor through the MOSFET. When the power switch is turned off, the drain voltage rises until D1 conducts. During the off time, the MOSFET drain voltage is clamped by C1 through D1 and C2. A SEPIC converter with multiple outputs puts a constraint on the winding ratios. One of the secondary windings needs to have a 1:1 turns ratio to the primary, and C1 must connect to it. In the example circuit shown in Figure 1, the 12-V winding has a 1:1 ratio, but it could have used the 5-V winding instead.

The circuit in Figure 1 has been built and tested. It was operated both as a SEPIC with C1 in place, and as a flyback with C1 removed. Figure 2 shows the MOSFET voltage stresses in both operating modes. In the flyback mode, the MOSFET drain went to nearly 40 V, while in the SEPIC mode the drain voltage was only 25 V. So the flyback design would have to use a 40- or 60-V MOSFET, while the SEPIC design could use a MOSFET rated for only 30 V. In addition, the high-frequency (greater than 5 MHz) ringing would be problematic for EMI filtering.