Flyback vs. Buck: Which to Use for DC/DC Conversion?

Flyback converters and buck converters use switching action to provide large step down factors to a high-voltage input, which also allows highly efficient power delivery to a load. The mechanism of power regulation is simple and relies on switching elements in the converter circuitry. These two types of power converters have their own advantages and disadvantages resulting from their topologies. We’ll discuss these advantages and differences in this article.

When To Use Flyback vs. Buck Converters

Flyback converters and buck converters can be used as the main converter topology in switch-mode power supplies, such as bench/brick power supplies. They both perform the same job: step down an input voltage to some desired DC output, and attempt to regulate the output at a constant voltage. It is also possible to regulate at constant current with a sense resistor.

The table below provides some of the main differences between these two types of power converters:

Integrated controllers are available that can aid the design of these power converter circuits. In some cases, the switching element and feedback regulation circuitry will be integrated into the device controller.

The typical cases where flyback and buck converters are used are shown in the diagram below. The AC input for flyback conversion is fed directly to the controller IC, while a buck converter involves stepping down a DC voltage.

Both types of converters offer another usage option: power output at multiple voltages on different rails.

Flyback Over Multiple Rails

Flyback converters and buck converters can be used to deliver power over multiple rails. This involves two types of topologies: multi-coil transformers in flyback converters, or a multi-phase topology for a buck converter.

Flyback converters can receive power on multiple secondary coils from a primary coil, as shown in the example topology below. In this case, the turns ratio for each coil with respect to primary determines the flyback voltage on each output rail. A typical flyback transformer example with multiple-rail output is shown below.

Isolated Buck Converter Over Multiple Rails

It is possible to isolate a buck converter. There are two methods, one requiring an AC input and the other involving coupling from the switching node.

• Use a stepped-down AC input and bridge rectifier to set a pulsating DC voltage at the buck converter input
• Use a fly-buck topology with a DC input

The first route is most common, and it basically puts the transformer before the switching controller in the buck converter. In some ways, this is also safer as the buck converter input will not be exposed to high AC voltages.

The other method to isolate a buck converter while also outputting over multiple rails is to use a fly-buck topology. Fly-buck topologies use a pair of coupled inductors to deliver power to multiple rails. A fly-buck converter topology is shown below.

Example fly-buck converter circuit with two output rails.

In this topology the coupled inductors allow power delivery to a secondary rail, which can be galvanically isolated from the primary rail by giving it a different ground net. This type of multi-rail converter topology will be discussed in another article.

Other Isolated Topologies

Buck converters can be isolated with an AC input or fly-buck topology as outlined above, and flyback converters are natively isolated. However, these are not the only isolated topologies. Other isolated topologies include:

• Cuk converter - This is normally non-isolated, but it can be used in an isolated topology with a transformer, where the inductance of the coils sets the output regulation.
• Forward converter - Similar topology as a flyback, but the forward converter uses an additional choke to store and release energy during switching on the primary side.
• Bridge converter - Half-bridge and full-bridge topologies are used to step down large DC voltages by sending pulses through a transformer. The input could be rectified from AC.
• LLC resonant converter - This type of bridge converter uses a resonant tank on the primary side to set the voltage/current regulation for power delivery.

In summary, many instances where a DC power supply must connect to an AC input will use a flyback converter; the AC input is rectified and then given directly to the flyback controller. In cases where much higher efficiency is needed, a buck converter topology is used. In more specialized cases, where input voltages and output currents are very high, bridge or LLC resonant topologies are preferred.

When you’re ready to design and simulate your flyback converter and buck converter designs, use the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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