Modeling And Simulation Of The LED Constant Current Driver

LED lights have been widely used due to their advantages of energy saving,high color rendering,long life,good dimming ability,wide color gamut and environmental friendiliness.Because LED needs constant current drive,many LED constant current control methods had been proposed.In order to study different constant current control methods,based on the non-isolated DC-DC Buck LED driver,these control methods such as peak current control(PCC)with constant frenquency(CF)and slope compensation,PCC with constant off-time(COT),hysteresis current control(HCC)with adaptive offtime(AOT),etc.are modeled.Meanwhile,theoretical analyses of these control methods are carried out in depth.On the basis of theoretical analyses,a new constant current control method which is named as average current-detection and hysteresis turn-off(ACHT)with enhanced adaptive off-time(EAOT)is proposed.Firstly,the state space averaging method is used to model the non-isolated DC-DC Buck LED-driver,then its static working point,AC small signal model and various transfer functions are obtained from the proposed model.And these characteristics are compared with those of the ideal non-isolated DC-DC Buck circuit.Secondly,the average output current and the stability of different current control modes are discussed.Analysis shows that neither of the current control modes considered can output constant current and stabilize the system at the same time.Thirdly,based on the non-isolated DC-DC Buck LED driver,an ACHT with EAOT control method is proposed,the proposed control method is composed of the Hysteresis turn-off loop and the EAOT loop.The Hysteresis turn-off loop is used to control on-time while the EAOT loop is used to control off-time.Theoretical analysis proves that the proposed control method can output constant current independent of the duty ratio and peripheral circuit parameters,and the system based on this control method can work stably.Furthermore,based on the model of Buck circuit with LED which has been put forward by this paper,the AC small signal models of different control methods are established,then the transfer function of the current control loop and the output-input transfer function of different control methods are obtained.Above transfer functions are simulated based on Matlab to obtain the bode diagrams and the pole-zero plots of different systems.The characteristics of different systems are analyzed.Using Hspice,the proposed control method is simulated based on 1μm 700 V BCD process.Simulation results show that the error between the average current and the pre-defined average current approximately equals to 0.5%.