Phase Shifting Optimizes Multistage Buck Converters

Jan 1, 2007 12:00 PM
By Robert Taylor, Applications Engineer, and Wei Liu, Applications Engineer, Texas Instruments, Dall

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In a power-supply topology for a computer system with a 12-V input and a 1-V, 20-A output, currents are high enough that duplicate power stages can be used to reduce stress and spread the thermal loading. Two multiphase converter approaches are possible: one with phase shifting and one without. The supply without phase shifting could be viewed as multiple controllers with a load-share bus. The supply with phase shifting can use the TPS40140 to control the phase of each power stage.^[1]

Typical computer power-supply requirements are presented in Table 1. This supply is subject to substantial surge currents from the system, so transient response is important to maintain small changes in the output voltage. Since this unit is to be used in the computing industry, the two main concerns are the size and cost of the power supply.

The converter with phase shifting significantly reduces both input and output ripple current. Reducing the ripple current will allow for less input and output capacitance, reduce power dissipation and improve efficiency. Both designs use a 6-phase approach to achieve the 120-A design goal. Each power stage is the same for both approaches and optimized to handle 20 A. Fig. 1 shows a comparison of the two approaches. The design on the left shows a configuration that has no phase shift, while the design on the right shows 60 degrees of phase shift between each phase.

Input Ripple Cancellation

The input and output capacitors of a typical notebook or desktop PC contribute a significant amount to the cost of the power supply. Also, the input and output bulk capacitors occupy a large space, which reduces the power density.

In the phase-shifted interleaved supply, the parallel converters are switched at specific phase angles. The angles are evenly distributed so that a maximum ripple current cancellation can be achieved. In the following equations, it is assumed that the input dc current is mainly provided by the input dc source and the ac current is provided by the input capacitors. Also, the parasitic components and the output ripple current are ignored. Eq. 1 shows the normalized input root-mean-square (rms) current, which is defined as a fraction of the output load current:

where k(N_PH, D) = floor(N_PH × D), the floor function returns the greatest integer less than or equal to the input value, N_PH is the number of active phases and D is the duty cycle.^[2]

More on Buck Converters

• Buck-Converter Design Demystified
• Optimizing Voltage Selection in Buck Converters
• Power Conversion Synthesis Part 1: Buck Converter Design
• Improving Efficiency in Synchronous Buck Converters

> Buck Converter Archive 

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