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Power tip: Choosing turns ratio for converter

Posted: 07 Mar 2014     Print Version  Bookmark and Share

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

The current waveform in the secondary shown in the bottom traces of figure 3 are strongly influenced by the leakage inductance which will impact regulation. The leakage inductance determines how quickly the current in the secondary winding can ramp. With small amounts of leakage inductance, the currents ramp quickly to a high value which charges the output capacitor quickly. As the inductance is increased, the current rise is slowed which can result is less charge being supplied to the output capacitance and less output voltage.

 

Figure 3: Recharge current wave shape is strongly impacted by leakage inductance (Green = 10 nH, Red = 100 nH, Blue = 1 uH).

Figure 4 shows the simulated impact of the leakage inductance on the secondary output regulation. This chart plots primary output voltage and secondary output voltage as a function of duty factor and leakage inductance. This was based on a 1:1 transformer with a 2.5 uH primary inductance and varying amounts of leakage. The input voltage was 5 V. The primary was loaded with 1 A of current and the secondary was loaded with 0.2 A.

Figure 4: Leakage inductance is a killer on regulation.

The first curve is the primary output voltage, which shows a linear relationship between duty factor and output voltage. The remainder shows that there is not a linear relationship for the secondary output voltage.

There are two things that are degrading the secondary regulation. On the left, at lower duty factors, the secondary output voltage is approximately one diode drop less than the primary voltage. This could be improved with synchronous rectification. On the right, at higher duty factors, the shorter conduction time increases the peak currents and the impact of the leakage inductance becomes significant.

With large amounts of leakage, the circuit is probably not usable beyond 50% duty factor or a ratio of 2:1 between input and output. With a nominal amount of leakage, the circuit performed well up to 75% or 1.33:1. Finally, with a heroic leakage inductance, the circuit is good to 83% duty factor or a voltage ratio of 1.2:1. It should be noted as shown in figure 2, the peak and RMS at high duty factors can be quite high. These are stongly influenced by parasitics and the easiest way to understand them is through simulation.

To summarise, the Fly-Buck is a convenient choice for a simple isolated bias voltage, but you need to be careful when considering running at high duty ratios. Peak currents can become quite high. Controlling leakage inductance allows you to push the duty factor, but anything much more than 80% is probably impractical.

About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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