Current source ballast for high power lighting emitting diodes without electrolytic capacitor

An LED driver requiring no electrolytic capacitor in the whole power conversion process is presented. It consists of two power conversion stages. The first stage is a buck converter operating in discontinuous capacitor voltage mode, so that the input current is continuous. It is used to deliver a regulated current for the second stage. The second stage is a current-fed inverter, in which the semiconductor switches are operated at constant switching frequency and constant duty cycle of 0.5. The power supplying to the LED string is regulated by controlling the duty cycle of the main switch in the front-stage buck converter. The two stages are interconnected by an LC filter, which is designed to attenuate harmonics at double of the line frequency. Instead of using an electrolytic capacitor for the filter, a polyester capacitor of better lifetime expectancy is used. An 18 W experimental prototype has been built and tested.