Multiple topologies and designs are available for providing a positive (Pos VOUT) and a negative voltage (Neg VOUT) rail from a positive input voltage (VIN). For example, there are dedicated devices containing a boost converter plus an inverting buck-boost converter (TPS65130, TPS65131, TPS65133), devices with boost or LDO plus negative charge pump (TPS65132, LM27762), but also single inductor multiple output split-rail converters (TPS65135). Aside from these devices, designers can also choose from several topologies and related devices typically used for isolated power conversion.¹ These devices are also used for non-isolated applications as well. Open loop push-pull (SN6501, SN6505, SN6507) or LLC (UCC25800) devices belong to this category as well as primary-side-regulated flyback.² Furthermore, the use of buck converters in fly-buck or fly-buck-boost topology is a possible solution.

The latter two devices have the advantage of choosing devices from a huge buck converter portfolio and of being usable for applications with a wide VIN tolerance range. The use of these device is also supported by a range of off-the-shelf coupled inductors offered by different inductor vendors. The largest offering of those off-the-shelf inductors is available for a turns ratio of 1:1, enabling straight-forward inductor selection but leading also to specific limitations in regards of the desired designed for duty cycle.

Both topologies generate a very well-regulated non-isolated primary output voltage; with a positive output for fly-buck and negative output for fly-buck-boost. The secondary output voltage is usually an isolated output voltage, by following the voltage on the primary side. The energy transfer to the secondary side takes place during the off-time of a switching cycle and leads to a desired duty cycle of less than 50%; leaving more than 50% of the period time available for the energy transfer to the secondary side. Considering the preferable use of a 1:1 coupled inductor, this requirement can be fulfilled for the fly-buck for a VINmin > 2 × VOUT, while the fly-buck-boost is able to fulfill this requirement already for a VINmin > VOUT. Based on these considerations a fly-buck can need a minimum VIN of 30 V to generate the ±15 V, while a fly-buck-boost is able to work with a minimum VIN of 15 V.