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Signal flow graph nonlinear modelling of N-cell cascade step-down buck converters is presented. General rules useful for drawing the signal flow graphs are evolved. A systematic procedure for developing a unified flow graph model of the cascade buck converters operating in synchronous control is discussed. A simplified procedure is given to deduce large, small-signal and steady-state models from the unified signal flow graph of the converter. A straightforward algorithm is developed to draw the unified graph model of the N-cell cascade buck converter. The unified nature of the proposed method is discussed by taking illustrative examples. Converter performance expressions, small-signal transfer functions and steady state, are derived taking the two-cell buck converter as an example. A large-signal model is developed and has been programmed in a TUTSIM simulator. Large-signal responses against supply and load disturbances are obtained. The validity of the proposed signal flow graph modelling of the cascade buck converters is verified and compared through PSIM simulator results. A few experimental large-signal responses results are provided to verify the proposed method.