Design Of A Triac Controlled AC-DC LED Driver Chip With Wide Tuning Range And Two-Stage Dimming

With the increasingly worsening of environment problems as energy crisis and haze, light-emitting diode (LED) would be widely used in the lighting area all over the world due to its high luminous efficiency, long-life, energy conservation and environmental protection. As the core component of the LED lighting circuit, the LED driver chip should be innovated and developed constantly to keep pace with the enlarging LED market. In the dimming and lighting filed, TRIAC dimming products occupies the main market as its advantage in energy conservation, adjustable brightness, low-cost, low volume and so on. As the TRIAC dimmer is widely used in resistance load, the LED convertor, which is the nonlinear load, should be introduced the power factor correction (PFC) to reduce the harmonic pollution and meet the TRIAC dimmer. On the contrary, as the conduction angle of the TRIAC dimmer is limited, the dimming range of the LED luminance may be limited as well. So, there is an important issuse to design a TRIAC dimming LED driver chip with wide dimming range for resaerchers.Aiming at the restricted dimming range problem caused by the limited conduction angle of TRIAC dimmer, the proposed LED driver chip improved and optimized the dimmer control circuit on the basic of constant current and power factor correction. By introducing the conduction angle compensation technique, when the TRIAC dimmer turns to the maximum conduction angle, the output current can up to the maximum current, namely the rated current, and when the TRIAC dimmer turns to the minimum conduction angle, the output current can low to zero. So, the output current can be regulated in a widely dimming range. On the other hand, aiming at the fact that our eyes are sensitive to the LED brightness variation in large current lighting, but not sensitive in low current lighting, the dimming circuit is operated with further optimization and two-stage dimming curve is designed. The output current can be regulated rapidly in large current lighting, but slowly in low current lighting, so the LED luminance can be regulated uniformly in the whole dimming range. By analyzing the formulas of the rapidly dimming curve l1 and slowly dimming curve l2, the design parameters of the curves can be correspond with the parameters in conduction angle compensation circuit, pull current control circuit and dimming curves switching circuit, and then the system model is designed in SIMPLIS. The proposed design philosophy is verified by the simulation of the system model, and the whole system circuit is designed in cadence software based on the system model.The proposed control chip is fabricated in TSMC 0.35?m 5V/650V CMOS/LDMOS process, and verified in a 21V/500mA circuit prototype. The test results show that, in 90V/60Hz-132V/60Hz ac input range, the voltage linear regulation is 2.6%, power factor is 99.5%, and efficiency is 76%. Moreover, in the dimming mode, the maximum dimming current is 516mA, corresponded with the rated current 510mA, the minimum dimming current is almost zero, and the rate of the minimum and maximum current is less than 1%. Therefore, the proposed control chip has a promising application in TRIAC-dimmable LED driver.