by Vijay Choudhary and Sourav Sen, Texas Instruments
Over the last few years, Fly-Buck™ topology has received quite a bit of interest from designers of various industrial applications. The Fly-Buck isolated topology provides a cheaper alternative compared to more conventional isolated topologies. In part one of a two-part blog series, we will briefly discuss operation of the Fly-Buck topology and provide a simple design approach to improve isolated output regulation.
The Fly-Buck converter is derived from a synchronous buck converter by replacing the output filter inductor with a coupled inductor or a flyback type transformer. The operation of the Fly-Buck topology is explained in great detail in . Although the Fly-Buck topology has been known for some time, the availability of integrated high voltage synchronous COT regulators like the LM5017, without any need of external compensation, has made its adoption simpler. We can now find this topology being used extensively in PoE (33VIN-57VIN), telecom (48 VIN) and in other isolated bias application spaces.
The basic Fly-Buck converter as given in Figure 1 regulates the primary output while the secondary isolated output ‘follows’ the regulated primary output. The nominal secondary output voltage is given by:
Where N1/N2 is the turns-ratio of the transformer and VF is the forward bias voltage drop of the secondary rectifier diode.
Figure 1. A Fly-Buck converter with a primary and an isolated output
The secondary voltage regulation is affected by many factors which include the input-output voltage duty ratio, the leakage inductance of the transformer, the resistive drop in the current circulation path during the power transfer (at off-time, TOFF), and the diode forward voltage drop variation with temperature and forward current, IF. All these factors degrade the secondary output voltage regulation when compared to the actively controlled primary output voltage. In some applications, it is often desired to have a much tighter regulation on the isolated output over the line voltage and the load current range than what can be achieved with the circuit as shown in Figure 1.
A complete LM5017 based Fly-Buck converter circuit can be reviewed in Figure 2. In this schematic, the typical value of the RFB2 needed to maintain the nominal regulated primary voltage of 12V measures 8.25k. The typical values RFB2 and RFB1 can be easily arrived at based on the typical value of VFB pin voltage at 1.225V and the corresponding voltage divider circuit (RFB2//RFB1). The application diagram given in  discusses this calculation in more detail.
Figure 2. The LM5017 based Fly-Buck converter circuit without opto-coupler based regulation circuit
With this configuration, the primary output voltage is very well regulated, as expected, but the secondary output voltage can be observed losing regulation with secondary load current. See Figure 3. With an increase in the line voltage, the secondary output voltage deviates further away from the nominal secondary output voltage at 5V as seen in Figure 3 as well.
Figure 3. The LM5017 isolated output voltage regulation
In order to improve this isolated output voltage regulation issue, a simple yet repeatable solution can be used here as well. This design includes a dedicated feedback compensation circuit, as has been used in numerous other isolated topologies. Based on just an opto-coupler and the shunt regulator, LM431A, one can design a simple isolated compensation circuit, which can regulate the secondary side. We will discuss the compensation circuit and the results in part two of this blog series.
Texas Instruments “Designing an isolated buck (Fly-Buck) converter”
LM5017: 100V, 600mA Constant On-Time Synchronous Buck Regulator
“Designing with TL431” by Ray Ridley
Visit ti.com/fly-buck for more details and design resources
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