Further Converters

SEPIC and “PESCI” Converters
These two are combinations of Boost and Buck Converters, lying halfway between the Boostbuck (Cuk) and the Buckboost (Flyback.) They both feature non-inverting outputs when not isolated, but share noise problems with the Flyback.

Note that of the four topologies mentioned here, three suffer from pulsating currents, while the fourth does not. This is a good example of the 3/4 | 1/4 rule mentioned earlier.

It is interesting to note that, when isolated, all four may be made to share a common isolation transformer! and all feature a D/D’ DC gain. But the Boostbuck (Cuk) alone impresses no DC current in the transformer’s core: the 3/4 | 1/4 rule again!


The Two Landsman Derived Converters
These two converters arise from supplying the Landsman Converter with an input or output inductor. His original circuit, also called the Single Inductor Cuk Converter, is not really a proper Topology at all. The central inductor’s current path is not well defined, and is very load dependent, as pointed out by Sabi Saberwal at Caltech, soon after the circuit’s conception.

Interestingly, the addition of a small inductance at either the input or output of this conversion stage yields a true switched-mode Topology. These correspond, in turn, to the Coupled Inductor Cuk Converter with zero input or output ripple–except that, being discreet elements, they are incapable of exhibiting negative induction*. This prevents them from nulling out esl and stray inductance in the source or load, preventing achievement of true zero ripple.

The Landsman circuit by itself is noteworthy for being its own dual, but no use has been found for this circumstance. In fact, this circuit is not used in industry, and remains a laboratory curiosity.

*I just mean a negative inductor value! The equivalnet inductors in the standard T model of the coupled inductor (same model as is used for the two winding transformer,) may be negative. When added to the stray inductance of the input, for example, they reduce that path’s impedance to 0 Henrys. This forces the switching ripple to flow there, rather than in the output path. This is how the Coupled Inductor Cuk Converter works!


Full Bridge and Half Bridge Converters
These two buck derived topogies are also old-timers. Both remain in use, despite chronic problems with shoot-through faults. Like the balancing problems associated with push-pull Bucks and Boosts, many complicated measures are taken to avoid shoot-through.

But these add complexity to the switch drive, and are seldom 100% effective. As parts count grows, reliability suffers, and to boot, device matching is still usually mandatory, adding time and expense to the manufacturing process.