Fundamentals of Power Electronics by Robert W. Erickson

Author:Robert W. Erickson
Language: eng
Format: epub, pdf
Publisher: Springer-Verlag Wien 2012
Published: 2013-11-12T16:00:00+00:00

converters are quantitatively quite different from their respective CCM transfer functions. The basic DCM buck, boost, and buck-boost converters essentially exhibit simple single-pole transfer functions, in which the second pole and the RHP zero are insignificant. So the basic converters operating in DCM are easy to control; for this reason, boost and buck-boost converters are sometimes purposely operated in DCM for all loads. The transfer functions of higher order converters such as the DCM Cuk or SEPIC are considerably more complicated; but again, the small inductor value causes one pole to shift to high frequency, where it has negligible practical effect. Since most converters operate in discontinuous conduction mode at some operating points, small-signal ac DCM models are needed, to prove that the control systems of such converters are correctly designed. More detailed analyses predict that the DCM buck, boost, and buck-boost converters indeed exhibit a second pole and a RHP zero, at frequencies near or exceeding the switching frequency [2,4]. A small-signal model of the DCM switch network is derived by linearization of the loss-free resistor model.

It is also possible to adapt the CCM averaged models developed in Chapter 7, to model converters containing switch networks that operate in the discontinuous conduction mode. In the generalized switch averaging technique, an effective switch conversion ratio µ (t) is defined, which is a generalization of the duty cycle d (t) of CCM switch networks. With this approach, equations and models that are valid for the continuous conduction mode are adapted to the discontinuous conduction mode by simply replacing d (t) with µ (t). The generalized switch averaging procedure is described in Section 10.3, and is used in the analysis of an example DCM buck converter.

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