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Power tip: Selecting turns ratio for DC-DC converter

Posted: 09 May 2014 ?? ?Print Version ?Bookmark and Share

Keywords:fly-buck? power supply? buck regulator? power switches? coupled inductor?

There are times when you have a requirement for a simple, low-power isolated output voltage from a wide-ranging input source. Regulation may not be important, but cost and board area may be. A good solution to these requirements is a fly-buck power supply, which is simply a buck regulator with a coupled-winding.

Regulation is achieved by rectifying the secondary winding of the coupled inductor, when the low-side primary switch is on. This reflects a voltage to the secondary that is set by the output voltage of the buck times the turns-ratio of the coupled inductor.

Figure 1 shows how simple a fly-buck can be. In this design, the sync-buck power switches are contained within the control IC and it only takes a handful of discrete parts plus a transformer to complete the design. The real trick for a successful design is the specification or selection of the coupled inductor. In particular, requirements for turns, leakage inductance, and magnetizing inductance need to be established.

Figure 1: The Fly-Buck is a simple way to provide a regulated, isolated output. (Click on image to enlarge.)

In the circuit shown in figure 1, the turns-ratio of the transformer is established by the primary and secondary output voltages. It will simply be the ratio of the primary voltage to the secondary voltage plus allowances for the diode (D1) voltage and any winding resistance drops. In this case, the relationship between the primary output voltage and the minimum input voltage needs to be understood. Clearly, the buck cannot provide an output higher than the input. If the two are too close together, the circuit may not function properly. You may be limited by the maximum duty cycle of the control since the output voltage is approximately the duty factor times the input.

The second challenge is in the circuit operation at extremely high duty factors, where the currents can be become quite high. These high currents can result from both charge conservation and the basic circuit operation. From charge conservation, the output capacitor is only charged when the switch node is low. During the remainder of the period, it sources the load current. On an average basis, to conserve charge:

This result is plotted in figure 2 where Icharge/Iout is plotted versus D. At duty factors above 75%, the ratio is above three and climbs quite rapidly with increasing duty factor. The high current impacts regulation of the secondary output. During diode conduction, the coupled inductor places a reflected primary output voltage across the series combination of the coupled inductor leakage inductance, series parasitic resistances and the output filter capacitor.

Figure 2: Capacitor C7 Charge Current Is High for High Duty Factor or Vo Near Vin.

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