Design Of A LED Driver With Leaner Dimming

LED, as one kind of ordinary light, has the characteristics of long life, energy conservation, environment protection and reliability. As the raising attention paid on energy conservation and environmental protection, LED power supply has attracted public’s attention and been rapidly developed during the last years. Nowadays, the LED industry is driving to maturity stage. And it’s development direction is to be more simple and be more pointed on it’s function. For example, we induce the technology of power factor correction(PFC). It can reduce the pollution on power grid and increase the power efficiency. On the other hand, the induction of the dimming technology makes the dimming with different requirements to be possible.Being one kind of dimming methods, Analog dimming is a simple and cheap dimming solution. It has been used in occasions without stringent specification on color temperature widely. The traditional analog dimming circuit use opto-coupler to realize the feedback of the output signal. In this article, we design a LED driver with primary side control and analog dimming. In this circuit, an adjustable duty cycle oscillation signal produced by external control could change the reference voltage linearly. And the reference voltage will compare with the signal getting by the primary side sampling through the error amplifier, whose output will decided the duty cycle of PWM, and thus adjust the output current of LED power supply linearly. Compare with the traditional dimming structure, the using of the technology of primary side control can make the dimming more accurate and reduce the application costs. In the chip, we use the valley detection technology to turn the primary MOSFET on at the lowest DC voltage. It can reduce the switching loss. Besides, there are some modules like over voltage protection, over temperature protection, over current protection and under voltage lock out to make the chip work normally and safely.In this article, the topology and the structure diagram internal of the chip we design are introduce firstly. Then the calculation of the function of the constant current and PFC is shown. As the design of the whole circuit architecture is finished, the paper design and analyze each part of the dimming loop. Such as the reference production module, the PWM control module, the error amplifier module and the analog dimming module. At last, the paper shows the whole simulation of the circuit using Hspice based on TSMC_ 0.5μm BCD process. The simulation result shows that the constant output current is 175 mA when the input alternating voltage is 220 V. And when the duty cycle of dimming are 80%, 60%, 40% and 20%,the average output current will be 141.1mA, 107.3mA, 74.6mA and 43.5mA. So the circuit we design can control the current of load accurately and achieve linear dimming LED lights.