Study Of Key Technologies Of Digitally Controlled DC-DC Converters In Boundary Conduction Mode

DC-DC converters operating in Boundary Conduction Mode(BCM)have the potential to achieve small size,high efficiency and high bandwidth,and are widely used in lowmedium power fields such as automotive electronics,communications,and renewable energy.Since modern switching power supply equipment has higher requirements on integration,power efficiency and application stability,related model improvement and performance optimization have been hot areas of research.With respect to current model,converter model and device model,this thesis studies the mode realization,general control strategy and efficiency improvement of digital controlled BCM converter.Aiming at the cost and robustness of current mode implementation for BCM converter,the research on the current inner loop of the converter is carried out,where a ripple current mode controller without a current sensor is achieved.Variations of inductor current are quantified considering influence of resistive parasitic parameters such as the forward voltage drop of the diode and the on-resistance of inductor and switching device,where an inductor current damping model is proposed.Based on the current damping model,it is proved that valley value of the inductor current at the end of switching cycle under SRCM control shows a dropping trend,where the self-convergence to BCM operation is realized.At the same time,considering the mismatch between actual inductance and controller inductance,the relative error between actual current and ideal current under the steady-state damping model is studied,which proves that the BCM under SRCM control is not affected by the inductance mismatch.Furthermore,considering inductor current and output voltage during transients,a controller limitation method to prevent the converter current mode deviation is studied.The converter not only eliminate the current sensor,but also achieves inductor-deviationtolerance BCM operation.Finally,in a boost converter prototype,steady-state and transient experiments under inductance mismatches are designed to verify the current mode stability and transient robustness of the converter under the SRCM controller.Aiming at the restriction that multiple transient responses of BCM converters cannot be optimized simultaneously under linear control,the research on the linear controller of the voltage outer loop controller is carried out,where a general control strategy result based on the principle of linear superposition compensation is obtained.Based on the closed-loop transfer function for input voltage,reference voltage and load,and considering the influence of sampling and calculation delay effects on system modeling in digital control,an accurate closed-loop small-signal model of the converter is obtained.The amplitude-frequency response of the model is highly matched with the circuit model,which verifies the effectiveness of the closed-loop small-signal model as a design reference.According to the principle of linear superposition,results of each compensator is summed as the reference current,and then the on/off time of the main switch is modulated by the SRCM controller.In order to eliminate the divergence problem that may be caused by the integral term in the compensator,the compensation loop is modified to suit digital implementation.Finally,this strategy not only optimizes the transient process simultaneously,but also provides a method to adjust the compensator directly based on the characteristics of the closed-loop transfer function.Finally,aiming at the accuracy of the current zero-crossing point when achieving soft switching of BCM converter,the research on the resonance process involving nonlinear capacitors have been developed,where a piecewise equivalent capacitor model that can calculate the accurate soft switching time is obtained.In the process of soft switching,the capacitance participating in the resonance process exhibits a strong nonlinear characteristic with voltage change.The influence of capacitance with the voltage change is studied,the capacitance is piecewise linearized based on the variation degree,where a piecewise equivalent capacitance model is proposed.Based on this model,the segmental resonance equations in the soft switching process of the BCM converter are studied.According to the resonance equations and the actual characteristics of the converter,the negative to positive zero-crossing point of the inductor current is analyzed.The current zero-crossing point corresponds to the soft switching time of the BCM converter,where accurate soft switching time is obtained.Compared with the traditional constant value equivalent model,the soft switching time based on the piecewise equivalent capacitance model can better solve the soft switching deviation problem.Finally,in a Boost converter principal prototype based on Ga N high electron mobility transistors and Si C diodes,the accuracy of the theoretical soft switching time based on the piecewise equivalent capacitance model is verified,which improves the overall efficiency of the converter and achieves 99.15% peak efficiency at output power level of 200 W.