Multichannel LED Driver With CLL Resonant Converter

This paper presents a two-stage multichannel light-emitting diode (LED) driver, which contains a buck converter as the first stage and a multichannel constant current CLL resonant converter as the second stage. The forward current of one LED string is sensed for feedback control to regulate the bus voltage, which is the input voltage of the CLL resonant converter. Meanwhile, the currents of other strings are cross regulated. In contrast, the CLL converter is always working at the series resonant frequency to maintain high efficiency. Therefore, this two-stage LED driver could achieve high efficiency over a wide load range. Furthermore, lower dimming is easy to obtain with the regulation of the first stage. Good current balance is achieved with large magnetizing inductance, even if the loads are severely unbalanced. In addition, since the transformer's turns ratio is 1:3, the charging and discharging process of the junction capacitors of secondary-side rectifiers significantly influences the similar process of the output capacitors of primary-side switches during dead time. Meanwhile, the displacement current flowing through the transformer's interwinding capacitor impacts on the current balance among multiple LED strings. With these considerations, a two-stage LED driver prototype is built to verify its good current balance and high efficiency.