Planar Magnetics Design for Low Voltage High Power DC-DC Converters
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Planar magnetic technology is widely used in a variety of applications from telecommunications to power electronics. In particular, the use of planar inductors and transformers in power electronics have burgeoned as a result of increasing switching frequencies. However, the design and construction of these magnetic structures becomes more difficult as switching frequencies increase. At high frequencies, the skin and proximity effects contribute significantly to a component's total loss. Thus, methods for analyzing the various losses inherent to planar magnetic components were discussed in detail. The air gap necessary for planar inductor design was characterized and discussed in depth. The core geometry, core material, number of turns and gap width affect the inductance of a planar inductor. All of these factors must be carefully considered when designing and building an inductor. A multitude of planar inductors were designed for use in a 12-1 V synchronous buck converter configuration. A simple testing method was proposed that allowed quick and accurate comparison of the various inductors. The inductors were fully characterized through the efficiency of the overall buck converter. Limitations for the developed inductors were found to be core saturation and discontinuous mode operation. Finite element analysis (FEA) was performed to investigate the current distribution throughout the windings. It was quickly concluded that 2D analysis was not sufficiently accurate; consequently, all FEA modeling was performed with the 3D solver. From this analysis, the gap location with respect to the windings was determined to be critical to the level of current imbalance. As such, a conclusion was reached that the windings should be located as far from the gap as possible. A more ideal solution, if available, is the utilization of cores having the air gap located at the top of the middle leg. The most suitable planar inductor determined through preliminary testing was integrated in the most efficient buck converter design. The planar inductor buck converter board performed comparably with a similar board utilizing COTS inductors. A peak efficiency of 94.7% was achieved at the a load current of 6 A. The optimal load current rating of each buck converter phase was determined to be 12 A. For this load current level, the planar inductor performed best at 400 kHz with an efficiency of 92%. The DC resistance of the planar inductor was about 1.5 mOhm, which caused the efficiency of the component to fall off more quickly than the ~1 mOhm COTS inductors.