For many years, light emitting diode (LED) is widely used status display with dot matrix display panel. Now, not only can choose just recently developed by the blue and white products (commonly used in portable devices), but also to the existing green, red and yellow product selection. For example, the white LED is considered to be the ideal backlight color display. However, it must pay attention to these inherent characteristics of the new LED products, you need to design an appropriate power supply. This paper describes the old and new types of LED's features, and performance requirements of drive power.
Standard red, green and yellow LED
LED work to make the most simple way is to use a voltage source through a resistor and LED connected in series. As long as the operating voltage (VB) remains constant, LED to constant-intensity light can be issued (although the ambient temperature as the light intensity will decrease.) By changing the resistor value can be optical section to the required emphasis on strength.
5mm in diameter for a standard LED, Figure 1 shows the forward voltage (VF) and forward current (IF) of the function curve. Note that LED's forward voltage drop with the increase of forward current. Assuming that a 10mA forward current of the green LED should have a constant 5V operating voltage, then the RV equivalent series resistance (5V-VF, 10mA) / 10mA = 300. Such as data tables in the typical operating conditions given in the graph (Figure 2) shows, the forward voltage is 2V.
Figure 1. The standard red, green and yellow LED with a 1.4V to 2.6V of forward voltage range. When the forward current of less than 10mA, the forward voltage varies only a few hundred millivolts.
Figure 2. Series resistance and voltage source LED driver provides a simple way.
Such commercial diodes GaAsP (GaAsP) made. Easy to control, and is well known to most engineers, they have the following advantages:
The resulting color (emission wavelength) in the forward current, voltage and temperature variations to maintain considerable stability. Standard green LED emission wavelength of about 565nm, tolerance only 25nm. Since the color difference is very small, at the same time several of these parallel LED driver does not present a problem (see Figure 3). Forward voltage of the normal changes in light intensity will have slight differences, but this is secondary. Usually you can ignore the same manufacturer, the same batch of LED differences.
High forward current to about 10mA, the forward voltage is very small. Red LED changes of approximately 200mV, the other colors around 400mV (Figure 1).
In contrast, for less than 10mA forward current, blue and white LED forward voltage is smaller. Cheap lithium batteries can be used directly, or three NiMH batteries.
Figure 3. The figure shows the number of parallel drive while red, yellow or green LED of the structure, with very little color difference or luminance difference.
Therefore, standard LED drive current consumption is very low. If the LED drive voltage higher than the maximum forward voltage, the boost converter does not require expensive or complex current source.
LED or even directly by lithium batteries or 3 NiMH to drive, as long as a result of battery discharge due to reduced brightness to meet the requirements of the application.
A long period of time are unable to provide blue light emitting LED. Design engineers can only use existing colors: red, green and yellow. The early "blue" devices are not really blue LED, but the scattering material surrounding the incandescent blue.
A few years ago, the use of pure silicon carbide (SiC) materials, developed the first "true blue" LED, but their luminous efficiency is very low. The next generation of devices using the GaN-based material, the luminous efficiency can reach several times the original product. The current manufacturing crystal blue LED epitaxial material is indium gallium nitride (InGaN). Emission wavelength range of 450nm to 470nm, InGaN LED can produce five times the light intensity in the GaN LED.
True white LED emission does not exist. This device is very difficult to manufacture because of the characteristics of LED emission only one wavelength. White does not appear in the color spectrum; an alternative approach is the use of synthetic white light of different wavelengths.
White LED design uses a small trick. In the blue light emitting InGaN-based material is covered conversion materials, such materials will be issued by the Blu-ray excitation yellow. So get a mixture of blue and yellow, it seems that the white in the eye (Figure 4).
Figure 4. White light LED emission wavelength (solid line), including blue and yellow regions of the peak, but it appears to the naked eye is white. The relative light sensitivity of the naked eye (dotted line) as shown.
White LED color defined by the color coordinates. X and Y coordinate values according to the International Commission on Illumination (CIE) of 15.2 calculated from the requirements specification. White LED's detailed description of data usually increases as the forward current due to changes in color coordinates (see Figure 5).
Figure 5. Forward current is changing the white LED (OSRAM Opto Semiconductors of the LE Q983) color coordinates, and hence a change in the quality white light.
Unfortunately, the use of InGaN LED technology is not like the standard green, red and yellow as easy to control. InGaN LED display wavelength (color) will change with forward current (Figure 6). For example, the white LED color changes presented by converting the material produced in different concentrations, and blue light emitting InGaN material changes in forward voltage as a result the wavelength change. Can be seen from Figure 5, color changes, X and Y coordinates of the mobile means that the color change (as mentioned above, there is no clear white LED wavelength.)
Figure 6. The increase in forward current by changing the emission wavelength of a blue LED to change color.
When the forward current up to 10mA, the forward voltage of a great deal. The range of variation is about 800mV (some changes in the diode model will be even greater.) Battery discharge voltage changes caused by changes in color therefore, because the voltage is changing the forward current. At 10mA forward current, forward voltage of about 3.4V (the value will vary with the supplier is different, ranging from 3.1V to 4.0V). Similarly, among the different LED current - voltage characteristics are also quite different. Direct use of battery-powered LED is very difficult, because the vast majority of the battery will discharge the voltage is lower than with the minimum required LED forward voltage.