The applications of describing functions to estimate the nonlinear inductance for large power filter design

Conventionally, passive filter design is very important to many power electronic circuits and power systems. For the design of these power electronic circuits and power systems, how to obtain accurate corner frequencies becomes a very important issue. It is known that when the load is large, the inductance for an inductor always changes. This is due to the fact that when the current load is high, the inductance always suffers from saturation or hysteresis. Because the inductance is nonlinear, it is very difficult to estimate the actual value for the nonlinear inductance during large current situations. In this paper, the authors explore the saturation phenomenon for a nonlinear inductance. With the describing function method, one can linearize the nonlinear inductor and then estimate the inductance during large current situations. Therefore, the bandwidth for low-pass, high-pass and notch filters can be also calculated very accurately. It can be found by the describing function method that when the current is very large, the corner frequencies or bandwidth frequencies will drift to a larger value in the low-pass filter. The drift value of bandwidth frequencies can be easily calculated by the describing function. This phenomenon can be verified by simulation and experimental results.