The separation of variables approach to formulate the averaged models of DC-DC switch-mode power converters is presented in the paper. The proposed method is applied to basic converters such as BUCK, BOOST and BUCK-BOOST. The ideal converters or converters with parasitic resistances, working in CCM and in DCM mode are considered. The models are presented in the form of equation systems for large signal, steady-state and small-signal case. It is shown, that the models obtained by separation of variables approach differ in some situations from standard models based on switch averaging method.
The averaged models of switch-mode DC-DC power converters are discussed. Two methods of averaged model derivation are considered - the first, based on statespace averaging and the second, on the switch averaging approach. The simplest converters: BUCK, BOOST and BUCK-BOOST working in CCM (continuous conduction mode) or DCM are taken as examples in detailed considerations. Apart from the ideal converters, the more realistic case of converters with parasitic resistances is analyzed. The switch averaging approach is used more frequently than the other and is believed to be more convenient in practical applications. It is shown however, that in the deriving the averaged models based on the switch-averaging approach, some informalities have been made, which may be the source of errors in the case of converters with parasitic resistances, or working in DCM mode.