Should You Use Buck or Inverting Topology for Fly-Buck

Fly-Buck converters are a common topology of choice for generating low power isolated outputs, bipolar (+/-) outputs and multi-output rails. Fly-Buck topology use is common in telecom, PoE, custom PoE, industrial and automotive applications. The most commonly used Fly-Buck topology uses buck configuration for the non-isolated output. This configuration is shown in Figure 1.

Figure 1. Fly-Buck converter utilizing buck (step-down) configuration for the non-isolated output

This topology is best suited for higher VIN, low VOUT applications where the input voltage is greater than two to three times the target isolated VOUT. In these applications, the buck non-isolated stage provides a simple circuit and low inductor current ripple. The buck primary stage is also useful for applications where the non-isolated output is also needed in the system.

In applications where the input voltage (VIN) and the isolated output voltage (VOUT2) are close together, a minor variation of the Fly-Buck topology is advantageous, in which the non-isolated or the primary side of the converter is configured in the inverting buck-boost configuration. The inverting buck-boost configuration works at much smaller duty cycles for the same VIN, VOUT combination. Furthermore, the inverting buck-boost configuration can generate non-isolated outputs equal to or even greater than the input voltage. It results in better regulation for the isolated output while utilizing simpler transformer designs.

 Figure 2. Fly-Buck converter utilizing inverting buck-boost configuration for the isolated output 

Figure 3. Inverting configuration has smaller duty cycles for the same input and output voltages.

The choice of a buck or inverting configuration is dependent on the application. In summary, use buck-configured design if:

  • VIN is two to three times or higher than the intended VOUT
  • The non-isolated (primary) output is needed in the application and is positive.

On the other hand, use inverting buck-boost configuration if:

  • VIN is close to the VOUT
  • The non-isolated (primary) output is not needed in the application or if needed, is negative. A common need for negative voltage is in bipolar (+/-) supply generation.



  • Hi Denis,

    Yes, it is possible to use the buck derived configuration of Fly-Buck to create +/- 15V or any other +/- combination. There are reference designs on LM5017 webpage that do this.

  • Let's say that I'd like to generate negative isolated Vout using the first configuration (there Vin > 50V). Is it possible/permissible to connect isolated ground with "system ground" and provide in that way split rail bias power supply for op amps (i.e. +/-15V)?

  • Excellent blog Vijay.  The buck-boost configuration provides a lot of advantages over the typical flybuck configuration.  With a buck-boost, you can use a 1:1 coupled inductor in low input voltage applications, whereas a flybuck might not allow it.  Most standard coupled-inductor products only come in a 1:1 turns ratio.  Also, as you pointed out the regulation is better with the buck-boost.  On a lighter note, perhaps we should refer to this variation of the topology as a flybuck-boost? :)

  • Great comparison Vijay! In addition to this, for some applications a potential advantage of the inverting toplogy Fly-Buck is it does not always require a synchronous buck converter operating in CCM. A buck toplogy Fly-Buck almost always does and often the buck converter needs to support a higher low-side MOSFET sinking current to allow more power to be transferred to the isolated output. When using the inverting topology Fly-Buck the buck converter does not need to sink current in the low-side switch. The voltage and current waveforms look just like a flyback. The buck converter then can operate in DCM at light loads allowing better efficiency at light load.