1 Introduction
The development of ultra-high-brightness light-emitting diodes ( LEDs ) is more rapid than previously anticipated. More than a decade ago, LEDs with luminous intensities up to or exceeding 100 cd were defined as ultra-high-brightness LEDs, and the current level has already exceeded 1300 cd. A few years ago, LEDs operated at currents of only 10 mA to 30 mA, luminous efficiencies of only 101 m to 201 m/W, and powers of only a few milliwatts or tens of milliwatts (mW). Lu-mileds Lighting's ultra-high-brightness LEDs (known as LuxeonLED, Luxeon transmitters and Luxeon Star) are already on the market, with operating currents from 200 mA to 700 mA, powers up to 0.7 W to 5 W, and luminous efficiencies above 25 lm /W, green, red and dark blue 350mA LEDs have a luminous flux of 25 lm to 30 lm. LEDs with operating currents of up to 1 A or even greater than 1 A will be placed on the market soon. The luminous efficiency of white LEDs has exceeded 40 lm/W, far superior to incandescent lamps, second only to fluorescent lamps. Light-emitting efficiencies are currently being developed that are not only locked at 100 lm/W to 120 lm/W, but higher LEDs. The US Department of Energy funded a semiconductor lighting project undertaken by the University of California at Santa Barbara and the Twilight Institute Lighting Center. The luminous efficiency is locked at 200 lm/W, which is much higher than fluorescent lamps, high-pressure sodium lamps and metal halide lamps. The goal of this research and development project is to replace all traditional electric light sources. The industry generally believes that the development of LED will bring a revolution in the field of lighting. Ultra-high brightness LEDs are the most competitive solid state light sources in the 21st century. In addition to large-screen displays, cars and traffic lights, ultra-high-brightness LEDs will replace incandescent, tungsten-halogen and fluorescent lamps in more than a decade.
2 offline driving scheme for high current LED
Ultra-high-brightness, high-current LEDs can be powered by automotive DC power supplies when used in automotive applications. If high-current LEDs are used for landscape lighting, tree and lawn lighting, car parking lots, traffic safety island signal lights, large-screen display backlights, etc., 220V (or 110 V) power frequency mains supply should be used.
The simplest solution for driving ultra-high brightness Luxeon LEDs is to use the bridge rectifier capacitor filter circuit shown in Figure 1. The output of the power supply step-down transformer is 14Vac (Vin), and the output voltage of the bridge rectifier filter circuit is VDC=1.2Vin. The forward operating current of the LXHL-MWlD Luxeon LED is ILED=350 mA, forward voltage drop (@350 mA) VLED=3.5 V.
Figure 1: Luxeon LED driven by a bridge rectifier filter circuit
The resistance of the current limiting resistor R1 is
R1=VDC-3*VLED=1.2*14-3*3.5V=18Ω (1)
R1=20 Ω can be selected. When the AC line fluctuation range is ±10%, the current change rate through the LED is greater than ±25%. Although the LED power supply circuit shown in FIG. 1 is simple, the current regulation performance is poor, and the LED is easily damaged when the voltage fluctuates under a high AC line. Figure 2 shows the Luxeon driver circuit with a linear regulator. Two LXHL-MWlD LEDs are used in series for scene lighting.
MC7809 three-terminal regulator output voltage Vout=9V, the resistance of resistor Rl
R1=(Vout-2*VLED)/ILED
=(9V-2*3.5V)/0.350A (2)
=5.7Ω
The power consumption PR1 on R1 is PR1=I*I*R1*R1=0.350*0.350*5.7=0.698W (3)
Linear Regulator Power Consumption PIC1
PIC1=(1.2Vin-Vout)*ILED=(1.2*14V-9V)*0.350A
=2.73W (4)
The LED power supply circuit shown in Figure 2 has excellent current regulation performance (usually ±5%), but the power loss is large and the system efficiency is low.
Figure 2: Luxeon LED Driver Circuit with Linear Regulator
Another driver scheme is based on a flyback switching power supply topology configuration of a control circuit (such as the NCPl200). The switching regulator circuit is more complicated, but it can obtain excellent constant current characteristics with an efficiency of up to 80%. Although this scheme also uses a transformer, it is stepped down at a high frequency because of its size and weight. The transformer is much smaller and lighter.
3 Luxeon Star LED Driver Circuit
3.1 Key Features of NCPl01x ON Semiconductor recently introduced the NCPIOlx series of low standby power off-line switching power supply self-powered monolithic transfer switches that integrate a fixed frequency current mode controller and a 700 V power MOSFET. This switch can be used not only for auxiliary power supplies such as UBS, TV, etc. for charger low-power AC/DC adapters, but also for driving Luxeon Star LEDs. The main features of the NCP101x series of transfer switches are as follows:
◠Internal 700 V MOSFET on-resistance RDS(on) is l1/22 Ω;
â— Current mode fixed frequency operation (65 kHz/100 kHz/130 kHz);
â— The skipping period at low peak current eliminates audible noise;
â— Dynamic self-powered (DSS), no bias winding required;
â— Internal soft start of 1 ms;
â— Frequency jitter improves EMI performance;
â— If auxiliary winding is used, stop DDS operation and the standby power is lower than 100 mW;
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