Research On Digital Control Strategy Of Low Ripple LED Driver

The LED light source is rapidly expanding to the field of high-power lighting in an all-around way,and the matching digital LED driver with long life,low ripple,and high efficiency has become one of the research hotspots.AC-DC two-stage topology can take into account the optimization design of various performance indexes and is the mainstream form of the high-power LED driver.In this paper,a two-stage LED constant-current driver composed of a Boost PFC and an Asymmetrical Half-Bridge(AHB)converter with high conversion efficiency is chosen as the research object.The key technologies,such as the discrete-time small-signal modeling method,adaptive digital predictive current mode control algorithm,and digital adaptive low-frequency ripple compensation control algorithm of AHB LED constant-current driver,are studied.The contents and innovations of this dissertation are as follows:1.Research on discrete-time small-signal modeling method of AHB LED constant-current driver.The discrete-time small-signal modeling method has been successfully applied to Buck,Boost,and Flyback converters,but there is little research on the fourth-order AHB converter.Aiming at model errors caused by digital control loop delay and high-frequency aliasing,a discrete-time mall-signal modeling method of AHB LED constant-current driver controlled by digital current mode is proposed in this dissertation.The discrete-time model of the AHB LED constant-current driver is established based on fully considering the effects of parasitic parameters,loop delay,sampling effect,and load effect with the first-order Taylor expansion used to approximate the complex exponential matrix.Simulation and experiments show that the established discrete-time small-signal model can more accurately describe the resonance peak value,high-frequency dynamic characteristics,and digital loop delay characteristics of AHB LED constant-current driver,which is very suitable for the design of the high-frequency digital controller.2.Research on adaptive Digital Predictive Current mode Control(DPCC)algorithm of AHB LED constant-current driver.The DPCC algorithm can effectively reduce the influence of cycle delay on the digital control performance of the converter,but the theoretical calculation formula of the loss duty cycle is too complicated when applied to the AHB converter.To address the above problem,a simple method for estimating the Zero Voltage Switching(ZVS)loss duty cycle is firstly derived based on the ZVS resonant inductor current in this dissertation.And then,this estimation method is used to form a Digital Predictive Peak Current mode Control(DPPCC)algorithm based on leading-edge modulation,a Digital Predictive quasi-Valley Current mode Control(DPq VCC)algorithm based on trailing-edge modulation,and a Digital Prediction quasi-Average Current mode Control(DPq ACC)algorithm based on double-edge modulation,and the adaptive ability of the algorithms are realized by engineering calibration.Simulations and experiments show that the proposed control algorithm can effectively overcome the effect of digital loop delay on the control performance.The dynamic and static performance metrics of the AHB LED constant-current driver are significantly improved over the entire dimming range.3.Research on digital adaptive low-frequency ripple compensation control algorithm of AHB LED constant-current driver.The non-electrolytic design can effectively prolong the lifetime of the Boost-AHB LED constant-current driver and improve its reliability,but it also has the problem that the output low-frequency secondary ripple current is too large.To solve the above problem,an adaptive low-frequency ripple compensation control algorithm of AHB LED constant-current driver based on DPCC is proposed in this dissertation.Firstly,the optimal duty ratio compensation curve of low-frequency secondary ripple is given based on an in-depth analysis of low-frequency ripple transmission characteristics of the AHB LED constant-current driver.And then,aiming at constructing the optimal duty ratio compensation curve,the feedforward low-frequency ripple compensation control algorithms of the single closed-loop controlled AHB LED constant-current driver are investigated,which are analog classical feedforward compensation control algorithm,analog double feedforward compensation control algorithm,and the digital adaptive optimal feedforward compensation control algorithm.Furthermore,based on the comprehensive analysis of the current mode’s low-frequency ripple compensation control characteristics,the proposed adaptive low-frequency ripple compensation control algorithm based on DPCC is studied.Simulation and experiments show that the compensation control algorithm can adaptively reduce the low-frequency ripple current flowing through LED in the whole dimming range according to the changes of average input voltage,input ripple voltage,and load,and the scintillation percentage of the light source are all prior to the requirements of IEEE Std.1789-2015 NOEL standard.