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High voltage LED structure and technical analysis - (AddTime : 2013-01-25)

In recent years due to the progress of technology and efficiency, LED application more and more widely, Along with the led application upgrade, the market for led demand and, in a more power and higher brightness, namely appellative high power led development direction.

For the design of high power LED, at present due to the large size more than single low voltage DC LED is given priority to, practice has two, one for the traditional level structure, the other is a vertical conductive structure. The first kind of practice is concerned, its process and general small size grain are almost the same, in other words, the section structure is the same, but different from the small size grain, high power LED often need to operate in the large Current, a little under unbalanced P, N electrode design, will lead to serious Current cluster effect (Current crowding), the result in addition to make LED chip can not reach the brightness of the design in the outside, also can damage the chip's Reliability (Reliability).

Of course, to the chip maker/chip factory is concerned, the legal process Compatibility (Compatibility) high, no need to buy new or special machine, on the other hand, for the downstream system in factory, peripheral collocation, such as power aspects of design, and so on, the difference is not big. But as stated above, in the big size LED to the current uniform diffusion and is not an easy thing, the more difficult the more size; At the same time, because the relationship between the geometric effect, large size 200W led Flood light extraction efficiency are often the small size of the low.

Graph: low voltage diodes, communication diodes and high pressure diode drive mode difference (click on image to enlarge).

The second approach is the first kind of complicated many, because at present the commercialization of blue LED are almost grew up in sapphire substrates on, want to vertical conductive structure, must first and conductivity base plate do joint, then will not conductive sapphire substrates to be removed, then finish the following process; Current distribution is concerned, the vertical structure, less need to consider horizontal transmission, so the current uniformity than traditional level structure is preferred; In addition, the basic physical principle, we in good conductivity material with high thermal conductivity characteristics, through the replacement base plate, we also improve the heat dissipation, reduced the junction temperature, so as to improve the luminous efficiency and indirect. But it the biggest drawback is that, due to the process complexity increase, leading to yield a traditional level structure, low production cost much higher than. High light emitting diodes (HV LED) basic structure and key technology Crystal yuan photoelectric global first put forward the high light emitting diodes (HV LED) as a high power LED solutions; The basic structure and AC LED the same, but the chip area is divided into multiple cell after a series. Its characteristic is that the chip can vary according to the input voltage demand determines the cell number and size, etc., is equal to do customized service. Because for every single cell can be optimized, so can get better current distribution, and improve the luminous efficiency.

High light emitting diodes and general low voltage diodes in the most technically the main difference of the three, the first for groove (Trench). The purpose of the groove in the plural of single cell independent, so its groove below needs to achieve the insulating substrate, its depth in different epitaxial structures and different, generally about 4 ~ 8 um, groove width has no certain restriction, but too wide groove represents effective luminous area reduction, will affect the HV LED luminous efficiency performance, so need to develop GaoShenKuan ratio process technology, reduce process line width to increase the luminous efficiency.

The first two insulating layer (Isolation), if the insulation layer does not have the good insulation properties, will make the whole design failure, the difficult point is to must be in GaoShenKuan than the groove on the overlying coated with good sex, membranous close and good insulation film, this also is single crystal AC LED process on the key.

The third is the chip of the interconnection between conductor (Interconnect). Generally speaking, to do a good link, wire in the jumper need a relatively flat surface, a deep step structure will allow conductor structure, weak in high voltage, high current drive are prone to damage, resulting in the failure of the chip, so planarization process development becomes important. The ideal state is lagging, can together will deep groove be flattened, make interconnection wire to smooth connection.

In addition, the high light emitting diodes in application and general low voltage diodes the main differences for, it not only can be applied to Constant DC (Constant DC), as long as the external bridge rectifier, it can also used in communication environment, very elastic. In the high light emitting diodes, external rectifier part with AC LED the homogeneous gallium nitride practice and change the silicon rectifier, not only makes the low energy consumption, but also prevent excessive reverse bias to the impact of the chip, Finally, because high light emitting diodes is AC LED less internal bridge the luminous area, make relatively high luminous efficiency, durability is better.

As a large size, high power LED solutions

High light emitting diodes efficiency is better than the conventional low light emitting diodes, mainly attributable to small current, the design of the cell can evenly spread will be current, and enhance the light extraction efficiency. In some applications of, in addition to need to consider the chip itself efficiency, and finally the selling price of the product is also an important indicator; For example, in the current lighting field, 200W led Flood light source is still not be regarded as ZhuLiuXing product, the key point lies in its price is still on the high side. 200W led Flood light source for the high cost of the tableland, in addition to the price of the chip itself, but still need to consider the whole BOM (Bill of material, BOM), such as light-emitting diodes in essence is a polar components, must supply a consequent bias to light, so the general 200W led floodlighting light source inside must be additional communication turn direct current (AC/DC) power conversion system, this is must pay cost.

And because LED itself small volume, easy to concentration, heat caused by the so-called Hot spots (Hot spot) phenomenon, make luminous element itself a shorter life. In order to solve the problem of hot, 200W led Flood light source on the thermal design is also essential, the thermal design by metal heat sink, but the most common metal fin in addition to increase the weight of the light source, but also increase the cost of the light source. Due to the high light emitting diodes itself high efficiency, reduce waste heat and cooling demand, and then cut costs; From the power transfer point of view, high voltage small wattage power converter such as flyback extension servant type circuit, in addition to small volume outside, because the element less, the cost is low. Therefore, high pressure light-emitting diodes is not only the advantages of the chip itself, it can directly or indirectly to further enhance the efficiency of the whole module.

All in all, in the application and design, single chip of high light emitting diodes has the following benefits:

1, save transformer energy conversion loss and reduce the cost.
2, in addition to high voltage dc applications outside, use external bridge rectifier circuit can also design for ac operated.
3, small volume not occupied space, to packaging and optical design has excellent use elastic.
4, in addition to the red fluorescent powder outside, also can use blue, red HV LED tie-in appropriate yellow, green fluorescent powder into higher efficiency high CRI warm white LED.

At present in the crystal yuan photoelectric, will first according to the customer's various parameters needs, do design criteria of the basic inspection; Further according to relevant optical, electric and thermal model simulation, decision unit cell size, number and the final product, then present form to practice verification; And according to the practice of the collected data, we design a test, or modified to achieve the result of optimization. At present crystal yuan photoelectric r&d center has embarked on high pressure light emitting diodes related analog optical, electric and thermal modeling