| Summary: | 碩士 === 國立中山大學 === 電機工程學系研究所 === 106 === Although conventional LED drivers might be realized with different structures, we select Flyback converter design as the research topic in this thesis, The reason is that not only it provides the stability of the output current, but also meets certain safety requirement.The purpose of the proposed Flyback control chip is to achieve quasi-resonant boundary conduction and provide stable current regulation. Thus, the proposed design
comprises load output current estimation circuit, diode conduction time detection circuit,and pulse width modulation control circuit to generate a switch signal. Besides, the stability is analyzed and a compensation circuit is proposed to filter out unnecessary noise and reduce the ripple coupled in the output.
In order to achieve the quasi-resonant mode control, the accuracy of Valley Detector is very important. When the secondary side inductor current can no longer be supplied to the load, it will cause oscillation between parasitic capacitance of the primary inductor and the power transistor. As soon as the valid valley is detected, an enable signal is delivered to PWM circuit to determine the turn-on time of the power transistor. However, the valley signal is hard to be detected accurately. The worst scenario is that the wrong detection will cause that the power transistor can no longer be turned on. Therefore, a protection circuit is proposed to prevent the case and improve the accuracy such that the efficiency is enhanced. Regarding the chip process variation, a detection circuit is added, which output is used to compensate the phase margin of the operational amplifier.
The output voltage range of the LED driver based on the proposed design is 6 to 60 V, where LEDs are driven by a fixed 0.7 A with a single stage power factor corrector. As a result, it can achieve the maximum efficiency 92.61% given four LED loads.
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