Study Of Current Observer And Advanced Digital Control Strategies For DC-DC Converter

As a kind of power conversion circuit, Switching Mode Power Supply (SMPS) converter is widely used in power supply systems. Compared to conventional power converters, SMPS converter is more efficient, stable and smaller in size. Recently, digital control for SMPS converter is becoming popular, since lower power consumption, higher system compactness and flexibility are required by applications. Therefore, a lot of studies focus on improving converter performance under digital control. This paper focuses on improving converter dynamic and steady state performances, and studies multiple models and advanced control strategies for Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) converters.With respect to converter modeling, a new total differential method is proposed to simplify the modeling process and acquire an accurate model that takes converter parasitics into consideration. For CCM converters, both basic and accurate small signal models are acquired. Through comparison and simulation, the accurate model is proved to be more accurate at low and medium frequencies. For DCM converters, basic small signal model is deduced, which is accurate enough to describe system frequency characteristic. Furthermore, an accurate current model is derived with consideration of parasitics, where the damping effect on inductor current is considered. Compared to the conventional current model, the new model is more accurate in describing inductor current and output current average value. These studies provide basis for designing current observer and advanced digital current model control strategies.Furthermore, advanced digital current mode control strategies are studied for CCM and DCM converters. In order to improve current response of CCM converters, digital dead-beat current mode and Predictive Current Mode (PCM) controllers are studied. For the dead-beat controller, a linear extrapolation method is used to estimate inductor current of the next switching cycle. Furthermore, duty cycle of the next beat is calculated, which regulates inductor current to the reference current in two switching cycles. Based on circuit parameters, digital PCM controller calculates inductor current slopes for current prediction and control, where the inductor current can be regulated to the reference current in one switching cycle. In order to improve dynamic performance of DCM converters, digital charge balance control strategy is studied, which controls the charge current of output capacitor to achieve fast voltage regulation. A current observer is used to estimate the output current, while a voltage controller outputs a reference current based on the estimated current and charge balance principle. Finally, based on the reference current, the current controller modifies the output current to re-balance the output capacitor charge.For Sensorless Predictive Current Mode (SPCM) controller, the influence of parasitics on current observation and output voltage steady state error is studied. Based on accurate modeling for the converter, it is proved that parasitics will cause inaccurate and non-convergent current observation, and lead to output voltage steady state error. Furthermore, comprehensive compensation and self-correction compensation strategies are proposed to improve the accuracy. The comprehensive compensation strategy includes compensations for both current observer and voltage sampling, which are used to eliminate influences of parasitics and voltage ripple on current observation. The self-correction compensation uses an integrative feedback to correct the observed current. The correction factor is designed to improve the observed current convergence and dynamic response speed of output voltage. Finally, effectiveness of the compensations are verified through simulations and experiments.With respect to digital charge balance control, the observed current error and reference current error are proved through the accurate current model for DCM converters. It is found that output voltage steady state error can be caused by single and unequal compensations for the errors. Furthermore, a dual current errors compensation strategy is proposed, where a current error observer is used that equally compensate the errors. Through the compensation, the current errors are eliminated, and converter dynamic and steady state performances are improved.