Electrons and holes are combined with each other and release energy, so that the energy band (Energy Gat,) rank changes to its light-emitting display
The energy released. LED has a small size, long life, low driving voltage, low power consumption, fast response rate, shock resistance
Jia, etc., are widely used in signal indication, digital display and other fields. With the progress of technology, ultra-bright LED
Development has been successful, especially in the successful development of white LED, making it more and more used in decorative lanterns, and even
Lighting.
Compared with the traditional lights, super bright LED has the following advantages:
1) long life, reliable and durable, very low maintenance costs LED can be used continuously for 105h, longer than regular incandescent bulbs 100 times;
2) high efficiency can be achieved now 201m / w. 2005 is expected to reach 501m / W [1], LED) spectra of several
Almost all concentrated in the visible light spectrum, the luminous efficiency of up to 80% ~ 90%, LED energy-saving lamps also energy than 1/4;
3) colorful, pure light color to 12-inch red traffic lights, for example, it uses low-efficiency 140W incandescent lamp as
As the light source, 2000lm white light generated by the red filter, 90% light loss, only 2001m red,
In LumiledsLighting company used 18 red LED light source design, including circuit losses into account, only consumption
Power 14W, light can produce the same effect;
4) light speed car lights is an important area of application of the LED light source, the LED response speed (ns level),
Install high LED brake lights on the car, you can reduce the incidence of rear-end accidents.
In recent years, Bright LED has been applied in the last light of an automobile, for example, the German Audi Audi A86.O, Italy
Italy Fioravanti company at the 2003 Geneva Motor Show concept car Yak, US Ford Motor Company in Detroit recently
The law of Model U motor show have begun to highlight LED headlamp design.
Although super bright LED has many advantages, but there are still the following disadvantages:
LED lighting is difficult to become the biggest bottleneck of choice;
2) need to strictly control the temperature of the LED is a semiconductor material, and having a general diode as the PN junction, as highlighted two
Diode power is relatively large, so the same power semiconductor devices, you need to consider the heat problem, excessive junction temperature directly
Effect of LED life, and increases the LED light decline in serious condition LED will burn;
3) In addition to the high price of power is low, the price is difficult to become a major factor in LED lighting, LED although most people have now been
Know, and most people are good, but its high price is difficult to be accepted by consumers, the current single yellow LED about O.
6 yuan / month, green and blue LED monomer at 1.8 yuan / month or so, the price of white LED reach 2.2 to 5.5 yuan
/ Month or so; if the dozens of single LED combinations, the cost will be greatly increased, as the LED mounted on a lawn lamp
, The unit price is equivalent to the general lawn several times, LED light source to become the mainstream of the future of lighting, it must move
Large lumen direction, will it be possible to reduce costs, the market may have a breakthrough.
2, Features of Ultra-bright LED
Features. (IF) is a graph showing the relationship between the forward voltage drop (VF) and forward current of the curve shows that, when the forward voltage exceeds a certain
Threshold (about 2V), that is commonly referred to after the turn-on voltage can be approximated that, IF and VF proportional. Current ultra-bright
Up to the highest IF LED 1A, and VF is usually 3 ~ 4V.
Since the optical characteristics of the LED is usually described as a function of current, rather than a function of voltage, luminous flux (φV) and IF Guan
Line curve shown in Figure 2, therefore, the use of constant current source driver can better control the brightness. In addition, from Table 1, LED
Range of forward voltage drop variation is relatively large (up to 1V above), but by Figure 1 VF-IF curve indicated, VF tiny
Changes will cause large, IF change, causing large changes in brightness. Therefore, the constant voltage source LED driver can not be guaranteed
Brightness consistency and impact of LED reliability, lifetime and lumen. Thus, ultra-bright LED constant current source commonly used
drive.
3 Drive Circuit
Due to limitations LED power level, typically to simultaneously drive a plurality of LED brightness to meet the needs, therefore, require specialThe gate drive circuit to light LED. Briefly describe the main circuit LED driver below.
3.1 resistive current limiting circuit
3.2 as a linear regulator
Core linear regulator is to use the power transistor operating in the linear region or MOSFFET as a dynamically adjustable resistor
Control the load. Linear regulators have a parallel and series two.
Since the shunt regulator needs a resistor in series, the efficiency is not high, and relatively wide input voltage range of circumstances
Under difficult to achieve a constant adjustment.
Figure 5 (b) shows the series regulator when the input voltage switching regulator 3.3
The drive technology is not only limited input voltage range, and low efficiency. In general LED for low power driven,
Since the current is only a few mA, so the loss was not obvious, when used as a current several hundred mA or more bright LED
Drive, the power loss of the circuit became more serious problem. Switching power supply is the most efficient energy conversion,
It can reach 90%. Buek, Boost and Buck-Boost converter can be used with equal power LED driver, only
To meet the LED constant current driver, instead of using the detected output current detection output voltage feedback control.
Figure 6 (a) for the introduction of the LED drive circuit Buck Converter with traditional Buek different converters, power switch S moved
Back inductance L, so that S source grounded, thus facilitating the S driver, LED and L series, and the freewheeling diode D and
The anti-parallel series circuit, the driver circuit is not only simple and does not require an output filter capacitor, reducing costs. but,
Buck converter is a buck converter, does not apply to low input voltage or a plurality of LED series of occasions.
4 Introduction of Drive Control Chip
According bright LED high-power constant-current drive characteristics, many companies have launched a special highlight LED Driver IC,
For example: Melexis, lnfinton (Infineon), Lienear Technology (Linear Technology), Supenex Inc, Analog. Devices (ADI)
Wait.
4.1 introduction of MLxl0801 chip
MLXl0801 is Melexi8 of a public TV for automotive applications, LED driver chip, the chip can also be used to relay
Etc. coil driver, and as electronic fuse. Principle MLx10801 chip block diagram shown in Figure 7, which integrates power
Rate MOSFET, the maximum drive current of 350mA. When used as an LED driver that has the following characteristics:
- Shao external application circuit is simple;
- Internal / external temperature detection and protection;
- High efficiency switching power supply drive;
- You can control the brightness by PWM input;
-LED Parameters can be adjusted and can be stored in on-chip NV memory.
MLX10801 although only 8 pins, but powerful, Table 2 MLXl0801 pin function description.
4.2 Comparison of high-power LED driver chip
A comparison of the current drive control chip performance power IED, IED in the application of high-power drive control chip, according to
According to different applications to choose:
1) When the optional power devices require higher power is not integrated in the chip controller, so you can follow the actual work
Rate demand individually selected power devices;
2) When you need a higher conversion efficiency, such as portable devices, choose the drive circuit of switching power supply;
3) When applied to the high reliability of the device, you can select with a temperature protection, fault alarm and other comprehensive control functions