How to Design a Fly-Buck DC/DC Converter

When we look at DC/DC converters that use a transformer, there are two topologies that most people think of:

• Flyback converters, which can convert DC/DC or rectified AC/DC
• H-bridge converters, which can offer very high output and even bidirectional output

There is another type of DC/DC converter that is used less often, but it allows for low-voltage operation at multiple rails with smaller transformers. This is the fly-buck converter; it allows for switched-mode operation at multiple output voltages in a small footprint. When multiple rails are needed with large step-down values, the fly-buck converter offers significant advantages compared to other DC/DC converter topologies.

The Fly-Buck Converter Topology

The fly-buck converter topology mixes the best aspects of flyback converters and buck converters into a single circuit topology. This type of converter is a simple extension of the buck converter topology to multiple rails with a multiple-winding transformer, similar to what would be done with a multi-rail flyback converter or some H-bridge converters.

Some of the important characteristics of fly-buck converters include:

• High efficiency power conversion
• Use of a single switching node, which carries less noise than using multiple buck converters
• Galvanic isolation of secondary rails
• Ability to
• Ability to use standard driver or controller

The topology of a typical fly-buck converter is shown below. This converter uses a transformer (TX1) to provide coupling of the switching waveform from MOSFET M1 onto a secondary rail. As the primary rail switches, its switching waveform inductively couples onto the secondary rail via the transformer. The inductances of the transformer play a dual role Finally, D1 is selected such that it rectifies the voltage on the output side.

Example fly-buck converter circuit with two output rails.

(Alt Text: fly-buck converter)

Currently, this converter design shows a secondary isolated rail with the same ground net as the primary rail. However, the secondary rail can be galvanically isolated from the primary rail by using different ground nets in the PCB.

Resistors in the Converter Design

In the above diagram, the resistors R1 and R2 would be used in a simulation to mimic a load. Since this converter is typically desired to run in continuous conduction mode (CCM), we would have R1 and R2 placed as very large resistors; this would mimic inputs into an integrated circuit.

You will also notice several resistors placed around the converter (R3-R6). These resistors are generally very small (on the order of 1 Ohm) and they provide damping of ringing waveforms that can originate from the switching node, as well as at the output of the converter from the discharging capacitors (C1 and C2).

These resistors help reduce the ringing and overshoot typically observed during startup of the converter, making them important for passing EMC testing. However, they also slightly reduce the efficiency of the converter, so they should be sized carefully. Make sure to simulate the output transient response, as well as any filters on the input/output, to verify that the converter will not exhibit excessive overshoot and ringing.

The resistors above in the above fly-buck converter circuit can reduce ringing and overshoot on the primary and secondary rails.

How to Implement Multiple Secondary Isolated Rails

The diagram above shows a single secondary that is isolated from the primary rail. However, a tertiary rail (or even more rails) could be created with a multi-winding transformer. The turns ratios (equivalent to the inductance ratios) across the transformer will set the coupled voltage and current on the secondary rails in the circuit.

The secondary voltages in a fly-buck converter can be set by adjusting the turns ratios in a multi-winding transformer.

Because the ripple on the output and the coupled rail voltage depends on the winding inductance in the secondary current loop, it could be challenging to set the secondary rail to hit both a voltage target and a maximum ripple target simultaneously. The right approach here is to size the secondary winding inductance to reach the required rail voltage. If the ripple on the secondary side is too large, then a discrete inductor can be added in series with the transformer winding to increase the total inductance in the current loop.

Is There a Fly-Boost Converter?

While it is less common, there is a fly-boost variant of the fly-buck converter. This type of converter would have the typical boost topology, but with the primary-side inductor replaced with a transformer. The duty cycle sets the voltage/current for the primary rail, while the transformer inductances set the step-up/step-down capability for the isolated rail(s). A simple change in placement to match a standard boost topology will provide fly-boost functionality.

When you’re ready to design and simulate your fly-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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