| The electrolytic capacitor is the key component that limits the lifetime of LED driver.Therefore,to prolong the overall long lifetime of LED lighting products,it is necessary to reduce the storage capacitance and use other kind of capacitor instead of electrolytic capacitor.Although such a two-stage configuration can help LEDs achieve good operating performance,too many components,large size,low efficiency,and relatively high cost are its main drawbacks.In addition,PWM dimming has become the essential function to regulate lighting leveles for human needs as well as to achieve energy saving.In order to design a high efficiency,long lifetime LED driver,this paper is dedicated to study electrolytic capacitor-less LED drivers with PWM dimming function.Recently,a quasi-single-stage LED driver is presented.This LED drivers consist of a front-end AC/DC converter with two outputs containing a large low frequency ripple and a back-end current regulator.The first output is cascaded by current regulator.Then its output voltage is connected in series with the secondary output.In order to achieve constant current output,the key point is to control the ripple of current regulator output voltage so that is has the same magnitude,but out of phase,to the low frequency ripple of the secondary output.It can cancel electrolytic capacitor.However,the LED drivers only focuse on driving single-string LED without PWM dimming function which cannot be applied to multi-strings LED output directly.Thus,this paper studies the topology that a front-end flyback AC/DC converter cascades a back-stage non-isolated DC/DC converter and the design of the system parameters under the PWM dimming function.In the quasi-single-stage LED drivers,the conventional CRM flyback converter is usually employed as the front-end AC/DC converter.The high PF,zero-current-switching for primary-side MOSFET are its advantages.However,the wide switching frequency range over the entire load and the half line cycle,resulting in high switching power loss,is the main disadvantage.In order to solve this problem,the quasi-resonant technology is imposed to flyback AC/DC converter with power factor correction function.By introducing a minimum turn-off time control,the valley switching for the main switch can be obtained.Furthermore,the switching frequency range over half line cycle can greatly reduced.Hence,the efficiency of LED drivers can be improved.The principle of the quasi-resonant flyback AC/DC converter,power factor and control strategy is dervied in detail.In order to demonstrate the feasibility of the proposed solution,two 40W experimental prototypes with two LED strings,have been designed,fabricated,and tested in the lab.The design consideration,including selecting the bus voltage,parameters design for front-end quasi-resonant flyback AC/DC converter and back-end floating buck converter are described in detail.The experiment results verfiyed well the features of the proposed scheme. |
