In the past, in order to profit from the full white LED beam, the LED industry has developed large-size LED chips in an attempt to achieve the desired goal. However, when the power applied by the white LED continues to exceed 1 W, the beam will decrease and the luminous efficiency will decrease relatively. ~30%, in other words, if the brightness of white LEDs is several times larger than that of traditional LEDs, and the power consumption characteristics are expected to exceed fluorescent lamps, it is necessary to overcome the following four major problems: a. suppress temperature rise; b. ensure service life; c. Improve luminous efficiency; d. Equalize luminescence characteristics.
The specific method for temperature rise is to reduce the thermal impedance of the package; the specific method for maintaining the service life of the LED is to improve the shape of the chip and use a small chip; the specific method for improving the luminous efficiency of the LED is to improve the chip structure and use a small chip; The specific method is to improve the packaging method of LEDs. It is generally believed that white light LEDs are expected to adopt the above countermeasures from 2005 to 2006.
The development of power and latitude to increase the power will cause the thermal impedance of the package to drop to below 10K/W. Therefore, foreign companies have developed high-temperature resistant white LEDs to try to improve the above problems. However, the power of high-power LEDs is actually higher than that of low-power LEDs. More than tens of times, and the temperature rise will also cause the luminous efficiency to drop sharply. Even if the packaging technology allows high heat, the bonding temperature of the LED chip may exceed the allowable value. Finally, the operator finally realized that solving the heat dissipation problem of the package is the fundamental method.
The service life of LEDs, such as the use of silicon sealing materials and ceramic packaging materials, can increase the service life of LEDs by 10%, especially the light-emitting spectrum of white LEDs contains short-wavelength light with wavelengths below 450nm, traditional epoxy sealing materials. It is easily destroyed by short-wavelength light. The large amount of high-power white LEDs accelerates the deterioration of the sealing material. According to the test results of the industry, the continuous lighting is less than 10,000 hours, and the brightness of high-power white LEDs has been reduced by more than half. The basic requirements for long life of the light source.
Regarding the luminous efficiency of LEDs and improving the chip structure and package structure, the same level as low-power white LEDs can be achieved. The main reason is that when the current density is increased by more than 2 times, it is not easy to extract light from a large chip, but the result is luminous efficiency. It is not as good as the low-power white LED. If the electrode structure of the chip is improved, the above-mentioned light extraction problem can be solved theoretically.
Regarding the uniformity of the luminescent characteristics, it is generally considered that as long as the uniformity of the phosphor material concentration of the white LED is improved, the fabrication technique of the phosphor should overcome the above problems.
While increasing the applied power as described above, it is necessary to try to reduce the thermal impedance and improve the heat dissipation problem. The specific contents are as follows:
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