1996, Nichia Nakamura's found a blue LED flashlight
, white LED is regarded as the most development potential of the lighting source components, so the white LED performance improvement and commercialization, immediately became the focus of national research. At present, white LED flashlight has been used in public places trail lights, automotive lighting, traffic signals, portable electronic products, LCD monitors and other fields. Also has rich RGB color temperature and high luminous efficiency white LED characteristics, generally considered ideal for liquid crystal display backlight light source, the manufacturers launched white CREE flashlight driver circuit and related components. View of this, and future developments briefly discusses features of the flashlight LED drive circuit.
Constant current drive reasons
1.1 luminosity of white LED forward current specification
White CREE flashlight forward voltage is usually regulated to 20mA minimum of 3.0V, up to 4.0V is simply apply some Shun voltage forward current for a wide range of changes.
Figure 1 is a randomly selected three white LED samples A, B two LED flashlight company's products forward voltage and forward current characteristics of the test results. According to test results show that when using 3.4V forward voltage to drive the six white LED forward current of 10 to 44mA range change significantly. Table 1 shows the electrical and optical properties of the white LED.
CREE flashlight luminosity and chromaticity of white is fixed current measurement, so as to obtain the desired luminance and chrominance, is typically driven with a constant current.
Table 2 for the optical coordinate rank (RANK) (IF = 25mA, Ta = 250C).
1.2 Avoid forward current beyond the allowable current value
To ensure the reliability of the white LED is basically need to try to avoid white LED forward current exceeds the absolute maximum design value (freeze value).
Figure 2, white CREE flashlight freeze Shun current of 30mA, to allow forward current continued to decay with the rise of the ambient temperature, if the ambient temperature of 50 ° C, usually a forward current can not exceed 20mA. In addition, the use of a constant voltage drive mode is difficult to control the inflow of the current value of the LED flashlight, and therefore will not be able to maintain the reliability of the LED.
2 white LED driver
Figure 3 is a the drive white CREE flashlight commonly used four power circuit; Figure 4 is a regulation accuracy characteristics of the six random sampling of the stability of the white LED.
Figure 4, test results show that, REGULATOR load characteristics appear in white CREE flashlight VF of the corner, i.e. the cross point is a stable operation point of each white LED FIG.
2.1 voltage regulator drive
The circuit of Figure 3 (a), respectively, using the voltage regulator can control the LED current Ballast resistor, the advantages of this circuit is a voltage regulator variety, the degree of freedom of the designer can choose, but with the voltage regulator, LED flashlight contacts only; drawback is the loss of electricity resulting BaLLast the efficiency decreases. In addition, LED's forward current for precision control.
As can be seen in Figure 4 (a), random sampling of six white CREE flashlight forward current from 14.2mA to 18.4mA distribution range is very wide, so manufacturers A LED (average) forward current of up to 2.0mA. In contrast, Figure 4 (b) used in the circuit ReguLator although there is a small, low-cost advantages, disadvantages may not be able to meet the performance and reliability requirements, which means that the usefulness of this circuit is relatively weak.