Basic knowledge and drivers of standard and white LEDs (1)

The simplest way to make an LED work is to connect a voltage source to the LED through a resistor in series with it. As long as the operating voltage (VB) remains constant, the LED can emit a constant intensity of light (although the intensity will decrease as the ambient temperature increases). The light intensity can be adjusted to the required intensity by changing the resistance of the series resistor. For a standard LED with a diameter of 5mm, Figure 1 shows the function curve of its forward voltage (VF) and forward current (IF). Note that the forward voltage drop of the LED increases as the forward current increases. Assuming that a green LED operating at 10mA forward current should have a constant operating voltage of 5V, then the series resistance RV is equal to (5V-VF, 10mA)/10mA=300. As shown in the graph under typical operating conditions given in the datasheet (Figure 2), the forward voltage is 2V. Figure 1: Standard red, green, and yellow LEDs have a forward voltage range of 1.4V to 2.6V. When the forward current is less than 10mA, the forward voltage only changes by a few hundred millivolts. These commercial diodes are made from GaAsP (gallium arsenide phosphorus). Easy to control and well known by most engineers, they have the advantage that the color produced (emission wavelength) remains fairly stable when the forward current, operating voltage, and ambient temperature change. Standard green LEDs emit a wavelength of approximately 565nm, with a tolerance of only 25nm. Since the color differences are very small, there is no problem when driving several such LEDs at the same time (as shown in Figure 3). Normal changes in forward voltage will produce a slight difference in light intensity, but this is minor. Figure 2: A series resistor and voltage regulator provide a simple way to drive an LED. Differences between LEDs from the same manufacturer and the same batch can usually be ignored. For forward currents up to about 10mA, the forward voltage changes very little. The change of red LED is about 200mV, and that of other colors is about 400mV (as shown in Figure 1). In comparison, the forward voltage changes of blue and white LEDs are smaller for forward currents below 10mA. Can be driven directly by cheap lithium batteries or three NiMH batteries. Therefore, the current consumption for driving standard LEDs is very low. If the LED drive voltage is higher than its maximum forward voltage, there is no need for a boost converter or a complex and expensive current source. Figure 3: This figure shows the structure of driving several red, yellow or green LEDs in parallel at the same time, with small color or brightness differences. The LED can even be driven directly from a lithium battery or 3 NiMH batteries, as long as the brightness reduction due to battery discharge is sufficient for the application. Blue LEDs LEDs that emit blue light were not available for a long time. Design engineers can only use existing colors: red, green and yellow. Early "blue light" devices were not true blue LEDs, but incandescent lamps surrounded by blue-scattering material. The first "true blue" LEDs were developed a few years ago using pure silicon carbide (SiC) material, but their luminous efficiency was very low. The next-generation device uses a gallium nitride base material, and its luminous efficiency can reach several times that of the original product. The current crystalline epitaxial material for manufacturing blue LEDs is indium gallium nitride (InGaN). With emission wavelengths ranging from 450nm to 470nm, InGaN LEDs can produce five times the light intensity of GaN LEDs. White LED There is no LED that truly emits white light. Such devices are very difficult to manufacture because LEDs characteristically emit only one wavelength. White does not appear on the spectrum of colors; an alternative is to synthesize white light from different wavelengths. A little trick is used in the design of white LEDs. A blue-emitting InGaN base material is covered with a conversion material that emits yellow light when excited by blue light. As a result, a mixture of blue light and yellow light is obtained, which appears white to the naked eye (as shown in Figure 4). Figure 4: The emission wavelength of a white LED (solid line) includes peaks in the blue and yellow regions, but appears white to the naked eye. The relative light sensitivity of the naked eye (dashed line) is shown in the figure. The color of white LEDs is defined by color coordinates. The values ​​of the X and Y coordinates are calculated according to the requirements of the International Commission on Illumination (CIE) 15.2 specification. Data sheets for white LEDs typically detail the change in color coordinates as forward current increases (as shown in Figure 5). Figure 5: Changes in forward current change the color coordinates of a white LED (LEQ983 from OSRAM Opto Semiconductors) and therefore change the white light quality. Unfortunately, LEDs using InGaN technology are not as easy to control as standard green, red, and yellow light. The display wavelength (color) of InGaNLED changes with the forward current (as shown in Figure 6). For example, the color changes presented by white LEDs arise from different concentrations of conversion materials, and the blue-emitting InGaN material produces wavelength changes as the forward voltage changes. D