Combined power electronic circuit and control loop simulation: why? And how?

Power electronic systems provide a considerable challenge to CAD packages due to their strong nonlinearities and widely varying time constants. This is particularly so if an attempt is made to simulate the entire system including the power circuit, load characteristics, control loops and thermal effects. Due to the lack of available tools and desktop computing power it has been normal practice to study each of these aspects independently. This has limited the effectiveness of CAD for power electronic systems since important interactions are inevitably either overlooked or simplified to the point that they are unrealistic. In addition, considerable ingenuity has been required on the part of the user to partition the problem in such a way that meaningful results are obtained in a reasonable computation time. The situation is however changing and tools are now becoming available which enable us to get closer to a “complete system” simulation. In this paper we concentrate on combined simulations of power electronic circuits (including representations for the load and supply) and their control systems principally for the purpose of control system design and validation prior to prototyping. The need for a combined approach is illustrated with reference to the design of an active filter system. A “wish list” of desirable features for a CAD package is outlined. Some commonly used programs are compared and our experiences in applying them to this problem are discussed