With the recent advancement of fast power switching devices, MOSFET, SIT and IGBT, a fast and simple method called instantaneous control has been often used for power conversion circuits. According to the method, detected information for load voltages and currents is processed with control circuits which consist mainly of analog operational and logic devices, and switching states in main circuits are determined with appropriate gate-signals fed back instantaneously in each operational case. There are interactions between main and control circuits and these two have to be solved simultaneously for circuit operational analysis. According to the conventional analysis method, there was a problem in equation formulation processes. But it is overcome by formulating main circuits with modified nodal equations (MNA) and control circuits with signal flow equations. Mixed circuits of two system types of elements can be analyzed. Further, equation formulation stamps, which are an advantage of the MNA, are deduced for main control elements. As an application example, an inverter system with the modified hysteresis control is analyzed for validation.