Multiphase Converter ICs

The trend toward higher current lower voltage microprocessors has created a need to supply up to 100A at voltages in the neighborhood of 1V. The multiphase converter answers this need. Multiphase converters employ two or more identical, interleaved converters connected so that their output is a summation of the outputs of the cells, as shown in Fig. 1.

To understand the advantages of the multiphase converter we must first look at the shortcomings of single-phase converters relative to supplying high current and low voltage. With a conventional single-phase converter, the output ripple and dynamic response improve with increased operating frequency. In addition, the physical size and value of the output inductor and capacitor also reduce at higher frequencies. Unfortunately, after the frequency reaches a certain limit, the converter switching losses increase and lower the converter's efficiency, forcing a design tradeoff between operating frequency and efficiency.

To overcome these single-phase frequency limitations, the multiphase cells operate at a common frequency, but are phase shifted so that conversion switching occurs at regular intervals controlled by a common control chip. The control chip staggers the switching time of each converter so that the phase angle between each converter switching is 360°/n, where n is the number of converter elements. The outputs of the converters are paralleled so that the effective output ripple frequency is n × f, where f is the operating frequency of each converter. This provides better dynamic performance and significantly less decoupling capacitance than a single phase system.

Current sharing among the cells is necessary so that one does not hog too much current. Ideally, each multiphase cell should consume the same amount of current. To achieve equal current sharing the output current for each cell must be monitored and controlled.

The multiphase approach also offers packaging advantages. Each converter delivers 1/n of the total output power, reducing the physical size and value of the magnetics employed in each phase. Also, the power semiconductors in each phase only need to handle 1/n of the total power. This spreads the internal power dissipation over multiple power devices, eliminating the concentrated heat sources and possibly the need for a heat sink. Even though this uses more components, its cost tradeoffs can be favorable.

Multiphase converters have important advantages:

  • Reduced rms current in the input filter capacitor, allows use of a smaller and less expensive types
  • Distributed heat dissipation, reduces the hot-spot temperature and increasing reliability
  • Higher total power capability
  • Increased equivalent frequency without increased switching losses, which allows use of smaller equivalent inductances, that shorten load transient time
  • Reduced ripple current in the output capacitor reduces the output ripple voltage and allows the use of smaller and less expensive output capacitors

    • Multiphase converters also have some disadvantages that should be considered when choosing the number of phases. Those disadvantages include:

  • The need for more switches and output inductors than in a single-phase design, which leads to a higher system cost than a single-phase solution, at least below a certain power level
  • More complex control
  • The possibility of uneven current sharing among the phases
  • Added circuit layout complexity

As current requirements increase, so does the need for increasing the number of phases in the converter. ICs providing just two phases are not adequate because of their limited output current range. An optimum design requires tradeoffs between the number of phases, current per<