A Design Of Efficiency Improved Single String Linear LED Driving Circuit

With the promotion of energy conservation and emission reduction,LED lighting has been becoming the mainstream products in lighting markets,due to its advantages of low power consumption,long life,pollution free and so on.And,it is well known that the efficiency of LED lighting system is not only related to the luminescent material,but also related to the drive circuit.And,the drive circuit would directly influence the stability and reliability of LED lighting products.And,the single string linear LED lighting system has been widely applied in low power applications because of its simple structure and low cost.Thus,how to achieve the efficiency optimization of single string linear LED lighting system is of practical significance.In this thesis,a single string linear LED drive circuit has been proposed to achieve efficiency optimization through the current shape control.Through the mathematical modeling and theoretical calculation of three different current shapes,it has been concluded that under the concave current,the single string linear LED drive system would achieve higher efficiency relatively.In addition,the influence of the concave current parameter on the efficiency optimization effect is further analyzed.In order to obtain the optimal efficiency,the concave curvature of the concave current is gradually increased.Meanwhile,the maximum value of the concave current gradually increases,and the minimum value of the concave current gradually decreases,thus the current variation during the concave current gradually increases.In order to avoid the flicker caused by the large current variation,the cut-top current control has been proposed.The LED conduction time has been divided into three intervals.The LED current is flat during the intervals around the conduction point and the cut-off point,and the concave current exists during the middle interval.The cut-top current could be regard as the combination of the flat current and the concave current.With the increasing of the flat current interval,the cut-top current approaches the flat current,and the efficiency optimization is gradually weakened.On the other hand,the cut-top current approaches concave current and the efficiency optimization is gradually enhanced.The control chip has been fabricated in TSMC 5V/650 V CMOS/LDMOS 0.35 um process and the performance has been test by a 90V/30 mA prototype.The test results show that when the input voltage ranges from 99 to 121 Vac,the power factor is greater than 0.7,and the efficiency of the system is over 70%.Compared with the flat current under the same conditions,the design of the cut-top current control in this paper can achieve5% system efficiency optimization.Therefore,the control chip designed in this thesis has a good application prospect in realizing the efficiency improvement of single string linear LED drive power supply.