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Multiphase converters: Beyond high current apps

Posted: 04 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:power supplies? multi-phase converters? power amplifier? inductor?

Traditionally, power supplies with multiple phases have been employed in applications such as servers, desktop personal computers and laptop computers. These systems require very high current, low-voltage supplies to power core processors. The currents in these applications can be in the hundreds of amperes. In recent years, designers of high-end audio systems have started to take advantage of the multi-phase approach as well.

The main benefits of multi-phase converters are ripple current cancellation and lower per-phase currents. These conditions can lead to a number secondary improvements such as lower output voltage ripple, smaller size, higher efficiency, lower thermal dissipation and better transient performance. Usually multi-phase converters are not considered for lower power systems because of cost and complexity. However, there are a number of devices that are pushing the multi-phase converter power levels to lower and lower levels.

A power amplifier in an automotive application takes the car battery and boosts it to a higher voltage. Some audio systems have peak powers in the thousands of watts. These systems benefit from a multi-phase approach as it helps to reduce stress on the components. In both applications multiple phases are used to share the current, so that reasonably-sized components can be used. There are many benefits of multi-phase converters than also can be applied to lower current and lower power systems.

The ripple current cancellation of multi-phase designs can be useful to reduce the stress on input/output capacitors. Figure 1 shows how the output (a) and input (b) ripple currents in a buck converter are reduced, based on the number of phases and the duty cycle.

Figure: Capacitor ripple current cancellation in a buck converter.

The reduction in per-phase current can lead to reductions in component size and power stress. Conduction losses are calculated using equation 1:

The losses increase as the square of the current. If a two-phase approach is used, losses can be reduced by a factor of four. These losses are applicable in FETs, inductors and other resistive elements in the circuit. The buck converter relies on the inductor as the energy storage element. The energy stored in the inductor is shown in equation 2:

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