Buck-derived topologies

Any Buck-derived topology (e.g., the Forward converter, the Half-Bridge, the Push-Pull, the Full-Bridge, etc.) needs an output choke. Otherwise it is akin to running a Buck without its inductor—you can thereby create a dead short cross the input supply rails. 

Off-line converters are derivatives of standard dc-dc converter topologies. For example, the flyback topology, popular for low-power applications (typically <100 W), is really a buck-boost, with its usual single-winding inductor replaced by an inductor with multiple windings. Similarly, the forward converter, popular for medium to high powers, is a buck-derived topology, with the usual inductor ( choke ) supplemented by a transformer. 

Answer Just as the isolated flyback is a derivative of the buck-boost topology, the forward converter is the isolated version (or derivative) of the buck topology. It too uses a transformer (and optocoupler) for providing the required isolation in high-voltage applications. Whereas the flyback is typically suited for output powers of about 75 W or less, the forward converter can go much higher. 

Question 29 How can we tell whether a given topology is buck-derived or not ... 

Coincidentally, the worst-case output capacitor RMS current for all three topologies occurs at the same point at which the general inductor design procedure for each of them is carried out. In other words, this point is Vinmax for the buck, and Vinmin for the boost and buck-boost. So we should have no trouble, directly using the numbers derived from the general inductor design procedure, to find the worst-case RMS current of the output capacitor, using the equations below. 

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