PWM Dimming Method for Capacitor-Clamped Current-Sharing Circuit in LED Backlight System

When multichannel light-emitting diode (LED) strings are applied in display backlight application, it's necessary to keep the brightness identical and adjustable. This paper deals with the capability of pulsewidth modulation dimming control method for capacitor-clamped current sharing circuit. By detecting the rising-edge of dimming pulse and turn on the switches of current sharing circuit in rotation, this method can achieve current sharing at any dimming frequency and dimming ratio. What is more, to improve the dynamic response of LED current and achieve a high dimming ratio, a control method based on the output voltage feedforward is proposed, which can decrease the rising time of the LED forward current significantly. The operational principle and parameter design are analyzed in detail. Based on this analysis, a three-string current sharing circuit driven by Buck converter is fabricated and tasted in the laboratory. Experimental waveforms demonstrate the validity.