Or, when is a Fly-Buck better?
Figure 1. Isolated Converters (Topology vs. Power)
There are a wide variety of applications that require bias rails, which must be isolated from the main power source. Traditionally, engineers have used a host of schemes to generate these rails, including flyback converters, low-side or push-pull drivers with transformers, isolated modules, or proprietary integrated solutions. The Fly-Buck™ converter (or isolated buck converter) is becoming very popular as a low power isolated bias solution because of its simplicity, ease-of-use, low component count, and the availability of wide-Vin integrated reulators like TI's LM5017 family of parts. It is not uncommon for both customers and field engineers to ask why they should select a Fly-Buck solution rather than just sticking with a flyback solution. To answer this, I need to rephrase the question: “when is a Fly-Buck solution better?” This blog identifies a number of application attributes that if present, will make Fly-Buck more attractive.
Low Power (10W, 5W, 2.5W, 1W) Isolated Rails
Fly-Buck converters can work at different power levels depending on the input voltage, RDSON of the FETs, resistance of the windings, the leakage of the coupled inductor and so on. But it naturally results in a simpler solution than, say a Flyback at lower power levels, because of the integrated FETs and absence of isolated feedback loop. The most compelling solutions have been in < 5W where the complexity of a secondary feedback flyback isn’t justified. See Figure 1 above.
Multiple Isolated and/or Non-isolated Outputs
Fly-Buck converters provide good cross regulation without an optocoupler feedback circuit. Since an opto-based feedback only works for one of the isolated outputs, the advantages of a dedicated feedback loop becomes less important when there are multiple outputs.
The Fly-Buck converter offers the primary non-isolated buck output at no additional cost. Therefore, it results in a simpler design for applications where both isolated and non-isolated outputs are required.
Regulation ± 5%
Because Fly-Bucks are primary side regulated, it is most suitable for applications that do not require very accurate isolated output regulation. MOSFET or IGBT gate drive circuits fall in this category.
But better regulation is possible with post regulators. This topology is also suitable for applications where the isolated output is allowed to have 5-10% variation and there are multiple post regulators that bring the voltage down to specific load voltages.
Wide Vin rail
Typically discrete designs involving push-pull or driver-transformer combinations require a regulated rail on the primary to generate a stable secondary rail. The LM5017 family of Fly-Buck regulators can work from widely varying rails up to 100V.
So when answering the question "when is a Fly-Buck solution better?" Consider the power level, number of output rails, regulation target, and input voltage range when making your decision.
References
1. Texas Instruments “Designing an isolated buck (Fly-Buck) converter”
2. LM5017: 100V, 600mA Constant On-Time Synchronous Buck Regulator
3. Texas Instruments Engineer It video, "How to use Fly-Buck DC/DC converter topology"
4. Wide Vin DC/DC Power Solutions from Texas Instruments