A Study Of Control Methods In Switching DC-DC Power Converters

One-cycle control, current mode control, and feedforward control are three powerful control methods in dc-dc switching power converters. The one-cycle controller has excellent line ripple cancellation capability. However, its transient response is not satisfactory and in steady state it has output offset error. So, in the thesis three new control methods are presented to improve the one-cycle control features for dc-dc switching power converters. The feedforward control method is an alternative for line ripple cancellation. Because the amplitude of sawtooth signal in conventional controller IC's is fixed, the conventional feedforward control method is not applicable. With reference to this fact, two new feedforward methods are presented in this thesis. Finally, a new analysis of chaos, in dc-dc switching power converters under control by current mode control is presented. The first method which is the subject of chapter 3 combines the one-cycle controller with a hysteresis-like non-linear controller. The role of the non-linear controller is to select the reference value for the one-cycle controller, due to the output error value, in order to make the response follow the reference faster. By this way, the combined controller has better dynamics than that of one-cycle controller. The results of this method have been approved by circuit level simulations. The second method which is the subject of chapter 4 combines the one-cycle controller and a PID controller. Because the one-cycle controller is essentially a nonlinear controller, for the design of PID parameters a linear model of the one-cycle controller is needed. Firstly, a new linear model of the one-cycle controller has been derived. By use of this model it is possible to modify the conventional method of PID controller design when it is used with conventional PWM to be adequate for our case in which a PID controller is used with a one-cycle controller. In this method, the derived model of the one-cycle controller is substituted in place of the pulse width modulator model. Furthermore, the conventional procedure of PID-PWM controller design is changed to the PID-one-cycle controller design procedure. The results of this method have been approved by simulations and an experimental prototype. The third method which is the subject of chapter 5, offers a combination of the current mode controller with the one-cycle controller. In this method, by eliminating inductor current at transient, the overshoot of the inductor current and consequent output voltage is reduced. The results of this method have been verified by simulations and an experimental prototype. Two new feedforward controllers will be proposed in chapter 6. The idea of first feedforward control method is to divide reference voltage to the input voltage. By mathematical analysis and simulations, it is shown that in this method, the line ripple can be cancelled out completely and also opposite to conventional feedforward method, because the reference signal instead of sawtooth signal has been used, the method is practical. The second feedforward method is more practical. This method is based on subtraction of a fraction of the ac input signal from the reference signal. In this method the line ripple cancellation ratio is less than that of first method but remains satisfactory. A simulation of the practical implementation of the circuit shows the capability of the method. Finally, chapter 7 is devoted to a new analysis of chaos and bifurcation in dc-dc power converters under control by current mode control. Despite many works which have been done in this field, some of the basic features of bifurcation diagram are still not clear. In this chapter, the equations which are responsible for the pattern and critical values of the bifurcation diagram have been derived. In addition, a new type of three-dimensional bifurcation diagram has been proposed.