1 Introduction

Isolated bias rails are common in many systems or subsystems, such as telecommunication equipment, medical equipment, and industrial factory automation. It is mandatory by safety standards to protect users from the hazardous voltage of a power supply, or the isolation is installed to break the ground loop interference for noise-sensitive applications.

In many cases, simple, low part count, isolated power supplies working from an input voltage are needed. This design solution is for when regulation may not be as important, but cost and board area are. A popular solution to these requirements is an isolated buck power supply, which is a synchronous buck converter, with the inductor replaced by a coupled inductor or flyback-type transformer. There is no need for an optocoupler or auxiliary winding because the secondary output closely tracks the primary voltage, resulting in smaller solution size and cost. The flybuck can support a simple, small, and cost effective power solution making it suitable as a flyback alternative.

This application report presents the basic operating principles of a flybuck converter, shows some typical operating current and voltage waveforms, and the key design equations are derived. The design example shows a step-by-step procedure for designing one nonisolated and two isolated outputs with the synchronous buck regulator TPS62933F.

The TPS62933F device is a 3.8-V to 30-V input voltage range, 3-A switching regulator, that has two integrated switching FETs and internal loop compensation. By integrating the MOSFETs and employing the SOT-5x3 package, the TPS63933F achieves high-power density and offers a small footprint on the PCB.