How to Use a Multiphase Boost Converter

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Figure 1

   

Abstract

When a system requires higher voltages than are available, a boost converter is a good way to meet this requirement. However, the classic standard boost topology is not the only option. A better available solution might be a phase-shifted multiphase boost converter. These offer higher efficiencies at high loads and reduce the requirements for output and input capacitors.

Introduction

Switch-mode power supplies based on the boost principle can convert a low voltage into a higher voltage. For this purpose, the standard boost topology is used as shown in Figure 1. This topology ensures that the output side receives pulsed currents from the inductor. However, since a voltage converter requires a fixed output voltage, output capacitor C2 has an important task. The capacitor must average the pulsed currents into a fixed output voltage. To perform this task successfully, output capacitors in boost regulators typically must be quite large by having, for example, high capacitance values. They must also have the lowest possible equivalent series resistance (ESR), parasitic resistance, and low equivalent series inductance (ESL), parasitic inductance.

To reduce this high demand on the output capacitor, it is advisable to design a multiphase boost converter. In this case, two boost regulators work in parallel and are connected to the same output capacitor. The two channels are controlled with a time lag of 180°. The circuit diagram is shown in Figure 2. Here, output capacitor C2 receives energy twice in one cycle, once from L1 and once from L2. To obtain a voltage ripple similar to that of the circuit in Figure 1, only about half the C2 capacitor size is required.

Multiphase boost regulators have advantages not only in terms of output capacitors, but also in relation to input capacitors. On the input side, a boost regulator does not have pulsed currents, as the inductance limits the current increase and decrease. However, two phase-shifted coils, as shown in Figure 2, can also limit the input current fluctuation. This allows the input capacitor, C1, to be reduced in size as well.

A multiphase boost converter also increases the conversion efficiency. By dividing the power over several paths, the peak currents per component decrease and thus efficiency increases.

Figure 3 shows a practical implementation with an integrated circuit, an LT8349. This is a two-phase synchronous boost converter. Its voltage range is designed to increase or stabilize battery voltage. If a higher current is drawn from batteries for a short time, the battery voltage drops temporarily. A two-phase boost converter is ideal for such operation. Due to the phase-shifted behavior, a current with a higher continuity is taken from the battery.

Another special feature of the solution with an LT8349 is its ability to achieve very high efficiency even at low load currents. To be particularly efficient in this mode of operation, one of the two phases can be switched off at low loads. At low load currents, the battery is not particularly stressed anyway, and the circuit works with one phase. If higher load currents of several amps are required, the second phase switches on automatically and offers all the advantages of two-phase operation. This shutdown of a phase in low load operation is referred to as phase shedding.

The example circuit in Figure 3 converts 2.5 V supply voltage to 6 V output voltage. At 3 A load current, an efficiency of 92% is achieved. With a load current of only 2 mA, an efficiency of 90% can be measured.

Conclusion

There are special ways to operate a boost converter. Two-phase operation offers advantages in terms of efficiency. This applies to both high and low load currents. A specially adapted integrated circuit makes this unique operation very easy.

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