| In recent years, as a green light source, due to the advantages of the energy conservation, pollution-free, high efficiency, long operating time and no mercury content, LED lighting has developed rapidly. The demand of LED drive chip also grows sharply. Based on this background, this paper designed a LED drive chip applied in a small and medium power dissipation situation.The chip drives the Flyback converter in the quasi-resonant mode to realize the boundary conduction mode(BCM). The power mosfet is turned on at the bottom of VDS(Drain-source voltage) to reduce the switch loss. In order to minimize the interference to the power grid, the chip has the single stage power factor correction(PFC) function without sampling the input AC voltage, and it reduces the number of chip pins too. The output information is sensed by the auxiliary winding, which is called the primary-side feedback. Without the opto-coupler, the outter circuit can be simplified and the reliability of the system can be improved. The auxiliary winding also provides the power for the chip, so the dissipation on the startup resistor decreases. The output current is estimated by the primary side peak current and the conduction time of secondary winding. Based on the estimation of output current, the system can adjust the on time of the power mos to realize the constant output current.The chip mainly includes the estimation of the output current circuit, the single stage PFC circuit, the peak current sampling and compensation circuit, the conduction time of secondary winding detection circuit, the error amplifier and various protection circuits.The entire circuit has been verified by simulation with the csmc0.25 um 50 VBCD process. The results are as follows: its start time is about 460 ms. The power factor is greater than 0.9 when the AC input voltage varies from 100 V to 280 V. The system efficiency is larger than 0.85. The linear adjustment rate is 0.02 mA/V. The load regulation time is smaller than 0.1 s. |
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