Research On Switching Converter Based On Variable Frequency Control

DC/DC converters are most used converter in DC micro-grid. These converters require the controller with excellent performance in order to achieve a stable output voltage. Therefore, studying DC-DC converter control strategy deeply has important practical value. DC-DC converter has a wide range of control strategy. Variable frequency control is one of a kind. Constant on-time(COT) control which is attached to variable frequency control is widely used, because of its simple structure, easy to implement, excellent transient performance.Firstly, the principle of COT control is introduced in this paper and its steady state is analyzed. The results show that steady-state error exists between the output voltage of the traditional COT control Buck converter with the reference voltage. The small signal model of the COT control Buck converter is established by utilizing describing function method. As a theoretical comparison study, the small signal modeling of the COT control Buck converter based on the three terminal switch equivalent model is introduced in detail. The results show that the condition of keeping the system stable is that the output filter capacitor equivalent series resistance(ESR) should be greater than the critical ESR.Secondly, for the output voltage steady-state error problem of the COT control, the integral compensation measure is used. The mathematical model of the V~2 COT control with integral compensation is established and the analyses in time domain and frequency domain are done. The results show that the steady-state error problem of the COT control can be eliminated by integrating compensation, but the sub-harmonic oscillation problem in the COT control is unable to resolve. For solving such a problem, the inductor current ramp compensation is proposed. Modeling and analyses show that increasing the inductor current slope compensation is equivalent to increasing the output voltage ESR, thereby enhancing the stability of the system. However, if the inductor current slope compensation is not selected appropriately, the output voltage may drop when the load steps. In order to improve the adverse effects caused by the inductor current compensation, the capacitor current slope compensation is proposed and compared with the inductor current slope compensation in the frequency and time domains. The results show that the output impedance of the capacitor current slope compensation is far less than that of the inductor current slope compensation, and the transient performance of the capacitor current slope compensation is better than that of the inductor current slope compensation.Then, the small signal model of the voltage mode control Buck converter is established. The performance of the voltage mode control is compared and analyzed with that of the COT control in the frequency and time domains. The results show that the COT control is superior to the voltage-mode control in the dynamic performance.Finally, a COT Controlled Buck Converter with simple compensation is attempted to propose and its operating principle is described. The critical ESR value of the COT Controlled Buck Converter with simple compensation is derived. The results show that no matter how small ESR is, the pulse bursting phenomenon will not exist in the improved Converter.