Research On Control Strategy Of Buck-Boost Coverter With Constant Power Load

With the cascade system composed of DC/DC converter is becoming more and more popular in the fields of microgrid,new energy automobile,aviation,ship,building power system,navy submarine power system and so on,more and more attention has been paid to the instability caused by the constant power load(CPL).In the cascade system,the back-end closed-loop feedback DC/DC converter is equivalent to a constant power load when it is stable,and the negative impedance characteristic of the converter will seriously affect the stability of the system.Therefore,a combined control strategy with PID control and virtual damping method is designed in this paper,and the control strategy is applied on the cascade circuit of DC/DC converter,so as to solve the instable problem which is caused by the CPL.The studied object of this paper is composed of two stage cascaded circuit:A front-end digital Buck-Boost circuit load by a closed-loop Buck converter which works as a CPL.The controller of Buck-Boost converter is designed based on the combined control strategy with PID control and virtual damping method.Firstly,the characteristics of CPL is analyzed,and the Buck-Boost circuit is modeled and analyzed by state space averaging method.The reason that a CPL causes the instability of system is analyzed by the small signal model of the converter.Secondly,the integrated control strategy of virtual damping method and PID regulation is introduced.The reason that the virtual damping method can eliminate the influence of CPL is systematically analyzed,and the system not only can operate stably but also has good dynamic performance when the voltage closed-loop PID control is combined with.Then a simulation model of Buck-Boost converter with CPL is built by the Simulink tool of MATLAB,and the simulation results verify that the proposed control strategy can well solve the problem of instability that caused by CPL.Finally,based on DSP28335,a 240W prototype of digital Buck-Boost converter using the combined control strategy with PID control and virtual damping method is developed,and a closed-loop Buck converter is developed as a CPL to cascade with the Buck-Boost converter.The experimental results verify the feasibility and rationality of the proposed control strategy.