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Super bright LED driver

Published on Mar 15 2011 // Lighting Display circuits

Abstract: After briefly introducing the characteristics of ultra bright LED features based on the details of the LED current limiting resistor, linear regulator and switching regulator such as drive, introduced on this basis, driven by super bright LED MLX10801 chip features and applications. Given the current major super bright LED driver chips main features.
Keywords: LED: Super bright: constant current drive
Introduction
LED (Light Emitting Diode, LED) invented in the 20th century, 60’s, it is the use of semiconductor materials in the combination of electrons and holes and release energy, making energy band (Energy Gat,) rank changed to light-emitting display its the release of energy. LED with a small size, long life, low driving voltage, low power consumption, fast reaction rate, good shock resistance, etc., are widely used in signal indicator, digital display and other fields. With the continuous advancement of technology, super bright LED’s has been successfully developed, particularly the successful development of white LED, making it more and more used in the decorative lights, and even lighting.
l Super bright LED features
Compared with traditional lighting, super bright LED has the following advantages:
1) Long life, reliable, durable, extremely low maintenance LED can be used continuously 105h, longer than ordinary incandescent bulbs, 100 times;
2) high efficiency can now be reached 201m / w. 2005 is expected to reach 501m / W [1], LED) of the spectrum almost entirely focused on the visible light spectrum, the luminous efficiency of up to 80% ~ 90%, LED energy than the energy-saving lamps even 1 / 4;
3) color, light color with 12-inch pure red traffic lights, for example, low-efficiency 140W incandescent light as the light source, white light produced by 2000lm The red filter, 90% light loss, only red light under the 2001m, while in Lumileds Lighting Company with 18 lights red LED light source design, including circuit losses, including, consumes only 14W, the light can produce the same effect;
4) fast car lights lit LED light source is an important area of ​​application, the LED response speed (ns level), high in the car to install LED brake lights, can reduce rear-end accidents.
Highlighted in recent years has been in the car for nearly LED light has been applied, such as the German Audi Audi A86.O, Italy Fioravanti Geneva Motor Show in 2003 introduced the concept car on the Yak, the United States not long ago at the Detroit Auto Show Ford launch of the Model U have begun to highlight the LED for the headlight design.
Despite the super bright LED has many advantages, but there are still the following disadvantages:
1) low power single LED available in the market in general 5W less power, but no higher power LED, which is currently the preferred LED lighting is difficult to become the biggest bottleneck;
2) the need to strictly control the temperature of LED is a semiconductor material, with as a PN junction diode, due to highlight the power of the diode is relatively large, so the same with the power semiconductor devices, need to consider thermal issues will directly affect the junction temperature is too high LED life, and will increase the LED’s light fades, in serious cases will be LED burned;
3) In addition to low power high prices, the price is the LED lighting is difficult to become a major factor, although the LED current has been most understanding, most people have been good, but its high price can hardly be acceptable to consumers, the current single yellow LED about O. 6 yuan / month, green and blue single LED at 1.8 yuan / month or so, the price of white LED has reached 2.2 to 5.5 yuan / month or so; if the combination of dozens of single LED, which will greatly increase the cost of such as putting an LED installed in the lawn, the lawn of their unit price is equivalent to several times normal, LED lighting to become the next mainstream source, it must be moving in the direction of a large lumen, the cost may be reduced only for markets There may be a breakthrough.

 Super bright LED driver circuit 1

ultra bright LED features
HPWA-xH00 Lumileds Lighting Company is a super bright LED, this paper is an example of ultra bright LED features. Figure 1 shows the forward voltage drop (VF) and forward current (IF) curve, the curve shows, when the forward voltage exceeds a certain threshold (about 2V), known as after the turn-on voltage, can be approximated that, IF and VF is proportional. Table 1 is the current major super bright LED’s electrical characteristics. Table 1 shows, super bright LED current up to the maximum IF 1A, while the VF is usually 3 ~ 4V.

 Super bright LED driver circuit 2

Characteristics of the LED light is usually described as a function of current, rather than a function of voltage, luminous flux (φV) and IF the curve shown in Figure 2, therefore, a constant current source driver can better control the brightness. In addition, from Table 1 shows the range LED’s forward voltage drop is relatively large (up to 1V above), but by the Figure 1 shows curves of VF-IF, VF will cause small changes in the larger, IF changes, which cause large changes in brightness. Therefore, the use of constant voltage source drive LED brightness can not guarantee consistency and reliability of LED life and lumen. Therefore, the ultra bright LED is usually driven by constant current source.

 Super bright LED driver circuit 3

Figure 3 is HPWA-xH00 LED temperature and flux (φV) curve, Figure 3 shows the flux and inversely proportional to temperature, 85 ℃ 25 ℃ when the flux is half the time, and a 40 ℃ 25 ℃ time when output is 1.8 times. Temperature on the wavelength of LFD has some impact, therefore, good heat dissipation is guaranteed LED brightness constant.

3 super bright LED drive circuit
LED power levels due to the restrictions, often to simultaneously drive multiple LED brightness to meet the demand, therefore, require specialized driver circuits to light LED. The following brief LED drive the main circuit.
3.1 The current limit circuit resistance

Super bright LED driver circuit 4

Shown in Figure 4, current limiting resistor drive circuit is the most simple drive circuit, current limiting resistor according to equation (1) basis.

Super bright LED driver circuit 5

Where: Vin input voltage for the circuit:
VF is the forward current IED;
VF is the forward current of LED, IF when the pressure drop;
To prevent the anti-VD diode drop (optional);
y for each of the number of LED strings;
x is the number of parallel LED string.
Figure 1 can be linearized LED mathematical model

 Super bright LED driver circuit 6

Where: Vo is the opening of a single LED voltage drop;
Rs for a single linear LED equivalent series resistance.
Then (1) the calculation of the current limiting resistor can be written as

Super bright LED driver circuit 7

Super bright LED driver circuit 8

When the resistance is chosen, the resistor current limiting circuit for the IF and the relationship between VF

Super bright LED driver circuit 9

From (4) shows a simple resistive current-limiting circuit, but the fluctuations in the input voltage, the current through the LED will follow the change, so adjust the poor performance. In addition, because the access resistance R loss of power xRIF, so efficiency is low.

Super bright LED driver circuit 10