Advantages of interleaved boost converters for PFC
The most popular topology for PFC (powerfactorcorrected) preregulators is the boost converter, which has continuous input current that you can manipulate with averagecurrentmodecontrol techniques to force input current to track changes in line voltage. Figure 1 shows a traditional singlestage boost. (To more easily explain the circuit operation, this article refers to dc inputs.) The change in inductor ripple current, ΔIL_{1}, is directly at the converter's input and may require filtering to meet EMI specifications. The diode output current, I_{1}, is discontinuous and requires the output capacitor, C_{OUT}, to filter it. In this topology, the outputcapacitor ripple current, I_{COUT}, is high and is the difference between I_{1} and the dc output current, I_{OUT}.
Interleaving boost converters
Figure 2 shows the functional diagram of a twophase interleaved boost converter, which comprises two boost converters operating 180° out of phase. The input current is the sum of the two inductor currents, IL_{1} and IL_{2}. Because the inductor's ripple currents are out of phase, they cancel each other out and reduce the inputripple current that the boost inductors cause. The best inputinductorripplecurrent cancellation occurs at 50% duty cycle. The outputcapacitor current is the sum of the two diode currents, I_{1}+I_{2}, minus the dcoutput current, which reduces the outputcapacitor ripple, I_{OUT}, as a function of duty cycle. As the duty cycle approaches 0, 50, and 100%, the sum of the two diode currents approaches dc. At this point, the output capacitor has to filter only the inductorripple current.
Inputripplecurrent reduction
The following equations and Figure 3 show how the ratio of inputripple current to inductorripple current, K(D), varies with changes in duty cycle. It is important to remember this variance when selecting inductors for the interleaved boost converter.
Figure 4 shows the normalized output capacitor rms current in a singlestage boost converter, I_{COUT_rms_single}(D), and the normalized rms current in a twostage interleaved boost converter, I_{COUT_rms}(D), as a function of duty cycle. The figure demonstrates that the outputcapacitorripple current in a twophase interleaved boost converter is roughly half that of a traditional singlestage boost converter, reducing the electrical stress on the outputfilter capacitor.
Evaluating inductor size
To evaluate the benefits of interleaving PFC preregulators' reduced boostinductor size, I conducted a mathematical comparison between a singlestage and a twophase boost preregulator (Figure 5). The design requirements were a maximum output power, P_{OUT}, of roughly 350W; a minimum line input, V_{INMIN}, of 85V rms; a maximum line input of 265V rms; and an estimated converter efficiency of 95%. The inductors have a switching frequency, f_{S}, of 100 kHz. The inductors have an inputripplecurrent requirement of 30%, and the inductors of both topologies have the highest inductorripple currents that occur at the minimum input and maximum input current.
I selected the inductors for both designs based on the worstcase ripple current. For a converter for a universal input, this point occurs at the minimum ac input at the peak line voltage, with the converter operating at a minimum duty cycle of 0.67. Figure 6 shows how the duty cycle varies with line voltage V_{IN}(t). Function D_{1}(t) shows how the duty cycle varies with changes in line when the input is at 85V rms. Function D_{2}(t) shows how the duty cycle varies with a maximum input of 265V rms. When the converter is operating at a maximum input of 265V rms, the maximum inductorripple current occurs when the input voltage is at half the output voltage. As the line voltage approaches the output voltage, the duty cycle decreases, reducing the inductorripple current.
The inductorripple current in a singlestage PFC preregulator is evident at the converter's input. A singlestage PFC inductor for a universal input would be roughly 450 µH. I based this calculation on where the inductorripple current was greatest at 85V rms input and 0.67 minimum duty cycle.
The dualinterleaved inductor has the same inputcurrentripple requirements as the traditional preregulator. The change in inductor current in one of the interleaved boost stages is roughly 3.4A. Variable minimum duty cycle at the minimum rms input voltage requires an inductance of roughly 245 µH—about half the inductance a singlestage PFC preregulator at the same power level requires.
Lab results

I evaluated a dualinterleaved boost converter using 200µH inductors for L_{1} and L_{2} and the input current. The worstcase inductorripple current occurs when the converter operates at low input at the peak of the line. The oscilloscope plot in Figure 7 shows the inductor currents of L_{1} and L_{2} with an input of 85V rms. CH_{1} is the rectified line voltage, CH_{2} is L_{1} inductor current, CH_{3} is L_{2} inductor current, and CH_{4} is input current. The currentconversion ratio is roughly 4A/division.
Figure 8a and Figure 8b show the inputline and inductorripple currents at maximum load. The channels of the scope plots are the same as in Figure 7. These waveforms clearly demonstrate a clean inputcurrent waveform for Channel 4. This twophase, interleavedPFC design uses a 220µF output capacitor. At full load for a singlestage, 350W PFC preregulator, the outputcapacitor ripple would be roughly 33.5V. For a twophase, interleaved PFC, the output ripple should be less than half of the single stage. The prototype's outputripple voltage at full load is roughly 13V (Figure 9).
Determining whether the prototype could meet current harmonic specifications EN6100032 requires the prototype's input harmonics' fullload power. The first harmonic is the rms input current at 60 Hz. The proceeding harmonics are well within CH6100032 Class D specifications (Figure 10).
Interleaving PFC preregulators allows powersupply designers to reduce inductor magnetic volume. The inductorripplecurrent cancellation at the input of the power converter allows designers to reduce the inductance by roughly half. Interleaving also reduces the ripple current in the boost capacitor, alleviating electrical overstress on the output capacitor. With no filtering on the prototype circuit, the design achieves EN6100032 Class D currentharmonic specifications. It has a slightly more complicated control scheme with a higher component count, but, in highpower applications, this tradeoff is well worth it.
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