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Utilise two phases to cut current and improve EMI

21 Sep 2015  | Goran Perica

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In dc/dc converter applications in which the input voltage may be lower or higher than the output voltage, you can use either a flyback converter or a single-ended-primary-inductor converter (SEPIC). SEPICs offer lower input-current ripple and higher efficiency than do flyback designs. Both converters suffer from relatively high output-current ripple, especially at high load currents and low input voltages. As output-current ripple increases, so does the circuit's output-filter-capacitance requirement, which increases size and cost. You can reduce output-current and -voltage ripple without increasing the application size and cost by using a multi-phase SEPIC or flyback converter. Using a multi-phase flyback circuit also greatly reduces the input-current ripple.

Figure 1: A single-phase SEPIC has low input ripple currents and is suitable for power levels of 5 to 50W.

To evaluate the benefits of a dual-phase versus a single-phase SEPIC, this Design Idea compares two designs running at 300kHz switching frequency. For consistency, both examples use the same power components, resulting in twice the output power in the two-phase design.

Figure 2: The single-phase circuit in Figure 1 has a peak capacitor output current of about 14A (bottom trace).

The single-phase SEPIC circuit can generate 3A of output current (figure 1). SEPICs are typically 1 to 2% more efficient than flyback converters. Figure 2 shows the output diode's current (bottom trace) at minimum input voltage and maximum load and the output-voltage ripple (top trace). The circuit's output capacitors must handle the peak output-diode current of 14A. Even though the circuit uses four low ESR (equivalent-series-resistance) output capacitors, output-voltage ripple is still 110 mV p-p. The aluminium output capacitor, COUT2, doesn't help much in reducing the output ripple due to its much higher ESR. COUT2 mainly helps reduce load transients by adding bulk capacitance to the output rail.

Figure 3: By adding second power stage and shifting the phase by 1808, you can reduce the output ripple currents by more than 50%.

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