March 03, 2021

Research on High Power LED Packaging Technology and Driving Method for Lighting

-23-Aging of the phosphor may also cause the solder of the flip-chip solder to melt and the chip to fail. Moreover, when the temperature rises, the LED chromaticity deteriorates. This is because as the temperature rises, the luminescence peak of the blue light shifts toward the long wavelength direction, which is the same as the change in the forbidden band width of the InGaN semiconductor with temperature, and the YAG phosphor is most sensitive to the 460 nm light, as the blue wavelength shifts red, The absorption rate of YAG phosphor decreases, the total luminescence intensity decreases, and the white chromaticity deteriorates.

Therefore, how to improve the package structure to meet the heat dissipation problem caused by the increasing chip area has become the primary consideration for high-power LED packaging; there are two main methods for solving the thermal problem: improving the quality of GaN wafers and improving internal quantum efficiency. Improve the luminous efficiency of the chip and fundamentally reduce the generation of heat.

Improve the structure of the LED to make it easier to dissipate heat.

In terms of packaging, we mainly use the following methods to help dissipate heat: a flip-chip structure (see), using Sisub-mount to dissipate heat, because Si is a good conductor of heat, the effect is much better than the film with positive light Sapphire to cool.

Front side light and flip-chip solder b. The GaN chip is adhered to a 15.5X 15.5mX2mm square A1 heat sink with a very thin thermal paste. It is more beneficial to heat than the *5 LED is only cooled by a bowl die. Transmission. c Use the packaged LED on the appropriate heat sink. (Lumileds and GE products also require a heat sink or fan, see.

3 chromaticity uniformity problem LEDs used for illumination are often used in clusters, and the uniformity of chromaticity between devices must be controlled, the "luminous flux-drive current curve". It can be seen from the figure that the luminous flux satisfies the linear relationship with the change of the driving current, indicating that the current density is not saturated at the operating current of 350 mA. There is no significant decrease in light extraction efficiency. Therefore, we believe that in the case of good heat dissipation, increase the drive current to 400mA, our packaged high-power LEDs can work well.

4 LED drive efficiency problem LED is a current-driven device, its brightness is proportional to the forward current before the brightness reaches saturation. But the dramatic increase in power has caused large-size white LEDs to encounter some new problems in drive solutions. First of all, high-power white LEDs and traditional white LEDs are current-driven devices that are sensitive to voltage fluctuations. The LED is very steep near its operating point. The voltage changes by 0.1V and the current varies by more than 50mA, so high-power white LEDs require constant current drive. Secondly, the driver circuit must be able to output a large amount of power, so the boosted charge-voltage converter and the step-up DC-DC converter adopted by the conventional small-sized chip are no longer applicable.

At the same time, the reliability of the LED driver must be considered. The series-parallel array drive must consider the balance of current and voltage between devices. The reason is that the current high-power white LED die manufacturing technology is still immature, and the uniformity and reliability of the device have yet to be improved, even if the electrical parameters are very consistent. The factory products, the inconsistency of the attenuation characteristics, will cause the difference of the I-V curve, which causes the current imbalance between the simple series-parallel devices to damage the device. In addition, voltage surges must be considered to damage the device.

In response to this situation, we have designed a low-cost solution to form a constant current drive module with a switching regulator 1C (such as the LM2575) as the core. It has high conversion efficiency and has additional functions such as input and output voltage protection and PWM dimming. The output current can be set to 350mA or 700mA, and the output voltage can be adjusted between 3 and 15V.

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