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Power supply circuitry is always limited by cost, PCB area, height and by the desire to reduce complexity. The majority of systems today are mixed-signal, and in all but the simplest cases, the various analog and digital circuits need several different supply voltages to operate. One way to add additional outputs to a power supply without adding additional control ICs is to replace the standard inductor of a buck regulator with a multi-winding inductor.
One winding is energized by the buck regulator, and there are many ways to create a second output by rectifying and filtering the voltage induced in the second winding. Buck regulators are found in nearly every power supply architecture, and the multi-winding inductor can be custom-wound to provide various turns ratios and even multiple outputs, but in the interest of controlling both complexity and cost, this application note will focus on off-the-shelf, 1:1 coupled inductors. Würth Elektronik eiSos offers several families of 1:1 coupled inductors with varying power levels and pinouts, many of which are suitable for adding a secondary output to a buck regulator.
Using a coupled inductor can help derive a second output voltage without the cost of another complete switching power supply, but the tolerance of the secondary output voltage and the DCM threshold in the primary are difficult to predict with pure mathematical expressions. Successful 1:1 coupled buck designs should be lab tested thoroughly, over line, load and temperature. In most cases the secondary output voltage tolerance is too wide, line regulation too high and load regulation too high to be used directly. For all of these reasons a linear regulator is the recommended way to provide a well-regulated secondary output.
Download the white paper here, or take a look at the range of inductors from Würth Elektronik.