Design and Implementation of Power Drive Controllers for LED Strings

In this thesis, design and implementation of power drives for light-emitting diode (LED) strings is investigated. We particularly focus on design methods for minimizing the size of output filter capacitor in flyback LED drivers. To this end, a novel constant power drive technique was developed to achieve better LED light regulation compared with the constant current technique. We present a filter capacitor minimization algorithm and applying it to an integrated buck-boost/flyback LED driver to achieve a long lasting LED driver. Minimization of the filter capacitor in an ac-dc flyback converter is investigated by utilizing a descent algorithm. The algorithm was proposed using a relationship between input current harmonics and LED electrical and photometric characteristics. The performance of the proposed algorithm in terms of filter capacitor minimization was experimentally verified to achieve input power factor correction along with meeting light flicker requirements. Furthermore, a primary-side constant power drive technique is proposed by utilizing a novel LED power estimation technique and an inner-outer-loop control structure. The proposed technique was implemented on an ac-dc flyback converter to attain simultaneous input power factor correction and LED light regulation. The enhanced performance of LED light regulation for the proposed technique is experimentally verified for different ambient temperatures and compared with the constant current drive method. The above filter capacitor minimization algorithm was utilized and experimentally tested in an ac-dc integrated buck-boost/flyback converter. Utilizing this algorithm, the size of the required filter capacitors can be significantly reduced.