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Employ SEPIC-Ćuk converter to generate dual supply rails

Posted: 13 Dec 2011     Print Version  Bookmark and Share


This topology can be built with three single-winding inductors, two coupled inductors, a custom 1:1:1 transformer, or a six-winding Hexapath device from Coilcraft (or equivalent). Coupling the inductors reduces current ripple in the inductors by a factor of two (again, Reference 1), significantly reduces the complexity of the small-signal model, and enables higher bandwidth by eliminating the SEPIC and Ćuk resonances.

One of the main advantages of this topology is that a single, standard current-mode boost controller (such as the ADP1621 or ADP1613) can implement both negative and positive outputs, with feedback taken from the SEPIC (positive) output. If the components are chosen according to the following rules, the small-signal model for this complicated converter will look nearly identical to a single-output current-mode SEPIC:

1. Use the same capacitance on each output.

2. The capacitance of C2 should be slightly larger than that of C1. These capacitors are generally ceramic, so the difference in dc bias must be taken into account.

3. Coupled inductors should be used, such that L1 is coupled with L4 and L2 is coupled with L3. Identical inductors should be chosen for the both the SEPIC and Ćuk outputs.

By nature, the Ćuk (negative) output of the SEPIC-Ćuk is unregulated; thus, some amount of load variation occurs with output current, particularly with load mismatch, as compared to the SEPIC (positive) output. Note that the tracking is much better than a similarly configured flyback converter, especially in the case of transient or load mismatch, because the coupling between channels is a direct connection rather than through the transformer with its inherent leakage inductance.

With an identical load on both supplies, at steady state, the most significant error terms are the mismatch in DC resistance of the inductors and the forward voltage of the diodes, both of which can be made quite small relative to the output voltage.

With substantial load mismatch, the error grows (figure 4). Therefore, in some applications it may be necessary to put a small dummy load on one or both the channels to keep both supplies in their regulation window. Note that analogue components such as op amps are generally insensitive to dc changes in their power supplies, as long as sufficient headroom is available.

Figure 4: Relative voltage regulation between rails with differential loading.

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