Fly-Buck™ converter is a multi-output converter topology implemented with a synchronous buck converter on the primary side and additional isolated outputs can be produced like in a conventional flyback converter on the secondary side of a transformer, It has been widely used in various applications due to many inherent advantages.
This article show cases a simple and cost-effective Fly-Buck™ solution using the LMR38020 device from Texas Instruments. The operating principle and step-by-step design procedures are presented, along with experimental results and some design tips for optimal design.
Fly-Buck™ is a trademark of Texas Instruments.
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Conventionally, the flyback converter topology has been a very popular solution for applications that need multi isolated output voltages. However, Flyback converter design has to employ either an opto-coupler or an auxiliary winding as the feedback circuit for output regulation. The loop compensation becomes difficult and sometimes tricky. And the use of optocoupler not only increases the solution cost but also reduces the circuit reliability. To overcome these drawbacks, the Fly-Buck™ converter topology, also called isolated buck, are introduced.
A Fly-Buck™ converter is one of the most suitable options for low power applications in industrial automation, communication power supplies, intelligent electric meters, and so on. The Fly-Buck™ has the merits of low component count, simple design, high efficiency, and good transient response when compared with the conventional flyback converters.
The LMR38020 is a 4.2 V to 80 V, 2-A synchronous buck converter in the HSOIC-8 package. It's internal compensation saves external component and simplifies the IC pin out, making the LMR38020 ideal for Fly-Buck™ converter applications.
This article presents the basic operating principles of a Fly-Buck™ converter by going over key waveforms and design equations. The step-by-step design procedure is given through an example of one non-isolated and two isolated outputs.
The Fly-Buck™ converter is based on standard buck converter topology in which the regular inductor is replaced by a coupled inductor or transformer such that one or multiple isolated secondary outputs can be produced. Figure 2-1 shows a Fly-Buck converter with one non-isolated output and one isolated output. Additional isolated output can be easily obtained by more secondary windings coupled to the transformer core.
Basically the closed loop operation is still a buck converter and it regulates the primary output voltage. The secondary output voltage is also regulated via cross regulation by winding coupling.
Therefore the Fly-Buck converter is able to produce a tightly regulated primary output voltage, along with one or more isolated outputs without the need of an optocoupler. This means that designing a Fly-Buck™ converter is relatively straightforward and similarly to designing a typical buck converter with minor adjustments.