Research On Dimming And Current Sharing For LED Backlight Driver

Light Emitting Diodes(LEDs)are widely used as backlight in panel display because of their advantages such as high luminous efficiency,good color performance,long life and environmental protection.Multiple LED strings in parallel is the common type in high power backlight system due to the power limitation of single LED.Directly connecting LED strings in parallel does not guarantee current balancing with the negative temperature coefficient and the manufacturing tolerance in I-V characteristics,and on the other hand,dimming function is required to improve the picture quality,so for LED backlight,it's necessary to keep the brightness identical and adjustable.The main research objective of this thesis is an LED driver which can achieve high-precision current sharing and wide-range dimming.In this paper,current sharing is achieved by a capacitor crossed circuit based on the capacitor charge conservation principle.The current sharing circuit is of simple structure,low voltage stress and no magnetic component,so it can be easily integrated.To achieve wide-range dimming,PWM shunt dimming is applied,and the unbalanced situations are analysed.Then,three dimming control methods are proposed to realize current sharing at any dimming ratio: the frequency-matching method,the frequency-multiplication method and the sequentially-turn-on method,and the working principles are explained in details.Finally,considering of current sharing effect,current ripple and complexity,the sequentially-turn-on method is adopted.Based on the above analysis,this paper completes a 90 W LED backlight drivers with 4 strings.The design of system and parameters is given in details,and at the same time,an improved control method based on the output voltage feed-forward(OVFF)is proposed to optimize the dimming current.Experimental results demonstrate the feasibility of the proposed schemes.