Single-output switching regulator circuits can often be adapted to multiple output configurations with a minimum of changes, but these transformations usually call for custom wound inductors. A series of standard inductors, featuring quadrifilar windings, allows power supply designers to take advantage of these modified circuits but without the risks of a custom magnetics development program.
The circuit shown in Figure 1 fulfills a recent customer requirement for a 9 V to 12 V input, 5 V/800 mA and –5 V/100 mA output converter. It employs a 1:1 overwinding on what is ostensibly a buck converter to provide a –5 V output. The optimum solution would be a bifilar wound coil with heavy gauge wire for the main 5 V output and smaller wire for the overwinding. To avoid a custom coil design, an off-the-shelf JUMBO-PACTM quadrifilar wound coil is used. This family of coils is wound with 1:1:1:1 sections. In the application of Figure 1, three sections are paralleled for the main 5 V winding and the remaining section is used for the –5 V output. This concentrates the copper where it is needed most – on the high current output.
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| Figure 1. | 5 V Buck converter with –5 V overwinding. |
Efficiency with the outputs loaded at 500 mA and –50 mA is over 80%. Minimum recommended load on the –5 V output is 1 mA to 2 mA, and the – 5 V load current must always be less than the 5 V load current.
