Design of constant current LED drive system

With the emergence of high-power LEDs, the life of LEDs and power conversion efficiency have become the main considerations when designing LED lighting systems. Based on Fairchild Semiconductor FAN100, a high-efficiency and high-stability LED lighting system is designed. First, the hardware circuit is given, then the performance of the circuit is analyzed, and finally the experimental simulation is performed. From the simulation results, it can be seen that this system is relatively stable within a relatively large temperature fluctuation range.

　　所有发光二极管无论其灯光颜色、尺寸大小或功率有甚不同，只要驱动的电流恒定不变，它们都能充分发挥其性能。发光二极管生产商都会列明产品的规格，例如，数据表上会列出产品在指定正向电流（IF）而非正向电压（VF）驱动下的流明、光束波形及颜色。发光二极管的亮度随电流的大小而不同，且制造出来的发光二极管，其电压与电流曲线稍有差异，因而LED照明的亮度常随电源电压的变动而无法稳定。为维持亮度稳定一致，需要发光二极管恒流驱动器来实现。恒流驱动器可以使得发光二极管工作在固定电流模式，因而亮度稳定性高。恒流驱动器也让发光二极管长期工作在一定电流下，使其维持较长寿命。发光二极管照明优点是节能、安全，但由于恒定电流工作考虑，能耗亦相对增加，因此照明系统设计以低能耗为目标。前面提到恒流驱动器的压降在2 V以内，即是考虑低能耗的设计，若系统的电源端电压与串接发光二极管压降超过2 V以上，则需考虑以电压转换器来达到低能耗目标，但仍维持恒定电流工作模式。低能耗的电压转换器是以开关式方式工作，依据反馈电路控制开关周期，达到稳定输出电压。但为了维持发光二极管恒定电流工作状态，反馈电路是以输出电流来控制转换器开关周期。

　　There are two types of power input systems on the market: one is an AC power input system with a current control module at the back end, and this type of products includes freezer light bars, indoor lamps, street lamps, table lamps, MR16, AR111, etc. The other type is an AC power direct input system that integrates an AC/DC converter and a constant current circuit, and this type of products includes E27 and GU10 bulb-type LED lamps, PAR lamps, T5 and T8 LED tubes.

　　This article uses a constant current source to drive the diode to emit light. When the current of the light-emitting diode decreases, the constant current source circuit collects the changing (decreasing) current value, amplifies it, and transmits it to the control circuit. The control circuit inverts the sampling signal, and the output pulse width increases. The output pulse with increased width drives the power tube of the power conversion stage, so that the secondary output voltage increases. In this way, the voltage across the series LED also increases, so the current flowing through the light-emitting diode also increases, which maintains the current of the light-emitting diode constant. Similarly, if for some reason, the current of the light-emitting diode increases, the control process is opposite. The constant current source drive method can overcome the shortcomings of inconsistent voltage drop of high-power light-emitting diodes and changes in current and luminous efficiency of the tube due to poor temperature characteristics.

　　1 Hardware Circuit

　　1.1 Introduction to FAN100

　　FANl00 is a primary-side regulation PWM controller to meet the key needs of the high brightness (HB) light-emitting diode (LED) market. It uses built-in proprietary TRUECURRENTTM technology and a strict constant voltage (CV) range to achieve the most accurate constant current (CC) control without the need for secondary-side feedback circuits. By accurately maintaining constant current over a wide voltage range, the same circuit can be used for different numbers of strings of LEDs, thereby increasing design flexibility, shortening time to market, and extending the life of HBLEDs. Due to the high integration of these PSR PWM controllers, circuit board space can be saved to keep up with the trend of shrinking bulb form factors.

　　FANl00 has a proprietary power saving mode that provides off-time modulation to linearly reduce the PWM frequency under light load conditions. In addition, they also minimize power consumption (standby power consumption under no load) by reducing secondary-side feedback circuits and components.

　　1.2 Overall circuit

　　FANl00 Description: Pin 1 is analog input, current detection, connected to the current detection resistor for peak current mode control constant voltage mode, for the current detection signal, also provides output current regulation in CC mode. Pins 2 and 6 are ground terminals. Pin 3 is analog output. Pin 4 is analog output, voltage compensation. Pin 5 is analog input, voltage terminal. Pin 7 is voltage reference. Pin 8 is the drive power output.

　　Working process: frequency-hopping PWM working mode, minimized filtering components are used for EMI problem methods. The VDD terminal (pin 7) is equipped with overvoltage protection and pulse current limiting with undervoltage lockout, pulse blocking and CC control to ensure overcurrent protection. The gate output is clamped at 15 V to protect the external MOSFET from overvoltage damage, and the internal overtemperature protection function shuts down and the controller is locked when overheating. The startup current is 10μA, and the low startup current allows the startup power supply of the controller with high resistance and low startup resistance power supply. A 1.5 MΩ, 0.25 W startup resistor, 10μF/25 V is an AC to DC power adapter with a wide input range (100VAC to 240VAC). FANl00 built-in provides better temperature compensation for constant voltage regulation at different ambient temperatures. This internal compensation current is a positive temperature coefficient (PTC) current that compensates for the temperature change of the forward voltage drop diode, which causes the increase of output voltage temperature.

　　The voltage across the current sense resistor is sensed for current mode control and pulses are used to limit the current by pulses. The slope compensation is built in to improve stability and prevent sub-harmonic oscillations due to peak current mode control. The FANl00 has synchronous, active, ramped slopes built in during each switching cycle.

　　The BiCMOS process of the FANl00 output stage is a fast gate driver. Minimize heat dissipation, improve efficiency and enhance reliability. The output driver is clamped by an internal 15 V Zener diode to protect the power MOSFET transistor against unwanted over-voltage gate signals.

　　Overvoltage protection prevents damage caused by overvoltage conditions. When the voltage exceeds 28 V due to abnormal conditions, the PWM pulses drop below the UVLO voltage and are disabled until it drops below 28 V and then restarts. Overvoltage conditions are usually caused by an open feedback loop.

　　1.3 Experimental simulation

　　The relationship between the experimental simulation voltage input and output is shown in Figure 2. The vertical axis is the output and the horizontal axis is the temperature change. It can be seen from Figure 2 that when the input voltage is between 15 V and 17 V, it can be seen that when the temperature rises, the voltage output decreases, so the circuit has a wide temperature fluctuation range.

　　Figure 3 shows the relationship between current input and temperature. When the circuit input current is between 75% and 95%, the output current fluctuation is relatively small. Under such circumstances, the service life of the LED can be extended.

　　2 Conclusion

　　This system can extend the life of the LED and maintain the stability of the output voltage and current within a wide range of temperature fluctuations. In this way, the DC-DC converter from the battery to the LED can both gradually increase the power supply voltage to the standard LED forward voltage and gradually decrease the power supply voltage to the forward voltage while maintaining the LED current constant (for constant brightness). At the same time, when the overall input current is higher, a larger inductor is required, and a current with smaller ripple is also required to limit the peak switch current to below the maximum rated current of the IC.