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Utilise two phases to cut current and improve EMI

21 Sep 2015  | Goran Perica

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Figure 3 shows a two-phase converter, which is similar to the single-phase converter in figure 1 except for the addition of an identical second-phase power stage. The second phase halves the peak inductor, MOSFET, and output-diode currents. The 50% lower peak output-diode currents produce 50% lower output ripple (figure 4). Also, the output-ripple-current frequency doubles, thus making it easier to filter out with an additional LC filter if necessary.

Figure 4: The output ripple current is 50% lower for a two-phase SEPIC. The output ripple voltage is 50% lower than that of a single-phase design with the same output capacitors.

The benefits of using a dual-phase converter become clear when you consider output-capacitor ripple current (figure 5). The two-phase converter's output-capacitor ripple current is always lower than that of an equivalent single-phase converter. Depending on the duty cycle, the two-phase converter's output-capacitor ripple current can approach 0A at a 50% duty cycle. Inductor ripple current is still present, and you can reduce it by using larger inductors.

Figure 5: The normalized output-capacitor ripple current for single- and dual-phase SEPICs shows lower output ripple with a two-phase design.

Using a two-phase converter means that you can use smaller inductors, MOSFETs, output diodes, and output capacitors than you can use in an equivalent single-phase converter. Because high-power designs may need to use more than one MOSFET anyway, a dual-phase design may need only one additional smaller inductor and one smaller diode. Output LC filters can also be smaller because of the doubling of the output ripple frequency. Finally, the EMI performance of a dual-phase SEPIC should be better than that of a single-phase converter because of lower current slew rates and smaller current loops.

About the author
Goran Perica contributed this article.

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