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In recent years, LEDs have been increasingly used for back-light illumination for high-performance digital television applications. This is usually a wide-screen TV application in which array-connected LED strings and boost converter configuration are used, and dimming capability is a necessity. In such a control scheme, the LED current is precisely controlled by high-switching-frequency boost converter, but the dimming control is achieved through the use of low-frequency LED pulse-width-modulation (PWM) cycle. This control scheme presents a special problem that must be addressed in the design to avoid LED color temperature deviation and shortening of LED life, besides feedback stability considerations. In this paper, a small-signal control model is first established including the effects of LEDs as the load. From the model, control loop gains are used to investigate the feedback stability issue. Then the issue of the LED transient current overshoot caused by the PWM dimming scheme is addressed by using the converter closed-loop output impedance concept. It is observed from the output impedance model that under normal conditions, the mathematical expression can be simplified from 6th order to 2nd order that makes the model practically useful for addressing the issue. The model was verified by experimental and simulation results. |
