The choice of the number of phases in a multiphase synchronous buck converter is probably the first decision you will have to make when designing a notebook DC-DC converter. Here are some of the considerations we had to take into account to make a decision: 1. Current per phase. This single parameter will influence the power train components like MOSFETs and inductors and will also influence the performance of the converter as expressed by the efficiency. A three phase would offer smaller number of inductors and MOSFETs and is likely to take less PCB real estate but this will increase the conduction losses for a given MOSFET by a factor of (30/22.5)2 or a factor of 1.78. This means we will have to use much larger switching devices to achieve good conduction losses but larger MOSFETs have larger Qg and hence slower rise and fall times leading to higher dynamic losses. 2. A larger number of phases will distribute the losses and the stresses amongst larger number of components leading to higher efficiency, lower average operating temperature and better reliability 3. Other considerations are availability of a heatsink or airflow, limitations on PCB board space, the PCB number of layers, copper thickness used on the PCB to name a few 4. A compromise was found at four phase leading to a DC-DC converter that delivers excellent efficiency and low operating temperature When the four phases are implemented side by side as can be seen in Fig. 2, the inner two phases are usually running at a higher temperature than the outside two. This can be remedied by fine adjustments to the current sharing pins of the PWM controller allowing for slightly higher phase current for the outer laying phases while slightly reducing the phase current for the inside two until approximate temperature equality is achieved. This leads to higher reliability and better performance.
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