Fly-Buck: Frequently Asked Questions (FAQs)

Other Parts Discussed in Post: LM5017, LM5160A

What is Fly-Buck™ ?

Fly-Buck is a Synchronous Buck Converter with the inductor replaced by a transformer or a coupled inductor or . The secondary winding is diode rectified to generate an isolated output voltage (VOUT2) which is related to the primary output voltage (VOUT1) through the turns ratio of the transformer.


For more information on Fly-Buck operation, please refer to the application note "Designing an Isolated Buck (Fly-Buck) Converter." For more details on Fly-Buck applications and design resources, please visit



Is Fly-Buck the same as Flyback?

No. Fly-Buck and Flyback are totally different topologies. They can both be used to generate multiple isolated outputs. Fly-Buck is Trademark of Texas Instruments.


Do I need an opto-coupler with Fly-Buck devices?

No. The Fly-Buck does not need either an opto-coupler or an extra transformer winding for isolated feedback. It is essentially a primary side regulated topology (PSR). The isolated output (VOUT2) in Fly-Buck is regulated because of the magnetic coupling to the primary output voltage (VOUT1). The secondary output capacitor is magnetically connected to the primary output voltage through the leakage of the transformer during the off-time.


In some cases an opto-coupler can be used to improve the isolated output regulation. This is not very common though and takes away from the simplicity of the solution. For those interested please refer to the two-part blog series "Improving Fly-Buck Regulation Using Opto."


For more information on Fly-Buck operation, please refer to the application note "Designing an Isolated Buck (Fly-Buck) Converter."


When should I use Fly-Buck for my isolated rail, and why?

Fly-Buck can be used anywhere there is a need for a low power isolated output rails. The input is usually a wide input rail (up to 100 V). It usually results in simpler solutions at lower power levels (<12 W or so). Fly-Bucks are suitable also for generating multiple-output rails, inverting rails, and bipolar rails.


Is Fly-Buck only used for a single isolated rails?

Fly-Buck can be used for single or multiple output isolated or non-isolated rails. In fact the more isolated or non-isolated outputs there are, the better is the return on the cost of the regulator.


Does Fly-Buck need a load on the primary output to work properly?

Fly-Buck converters (LM5017 family, LM5160A) do not need any load on the primary to work properly. The secondary or isolated outputs can be loaded even when there is zero load on the primary output. The synchronous rectifiers in Fly-Buck converters allow the primary inductor current to turn negative. Therefore the primary output capacitor charge balance is maintained even with secondary output loaded and no load on the primary output.


How is the cross regulation on outputs in Fly-Buck?

The cross regulation is dependent on the coupling (or leakage), the winding resistances, and the switch resistances. To the extent a general statement can be made, the cross regulation between multiple isolated outputs of a Fly-Buck converter is no different from other isolated topologies, e.g., multi-output Flyback or Forward.


Is there a pre-load or minimum load required on the isolated output in a Fly-Buck?

Just like other isolated topologies with diode rectification (as different from synchronous rectification), there is a small amount of leakage energy that gets transferred to the secondary in every switching cycle. A small amount of preload might be needed for isolated outputs. The amount of preload depends on the leakage, frequency, and the current flowing in the windings to some extent. Usually the preload resistor is in the order or 1k-10k. It is also possible to use a zener based clamp instead of a preload resistor. This avoids power loss in the preload circuit under loaded conditions.

It is worth pointing out that the preload requirement in Fly-Buck is usually smaller than in a Flyback converter.