Comprehensive analysis of LED chip life test (two)

According to the requirements of the life test conditions, LED can be connected in parallel and in series with two types of connection drive. Parallel connection: more than half of the LED positive and negative, and the negative pole and the parallel connection, which is characterized by the operating voltage of each LED, the total current.

Effect of packaging technology on life test

Packaging technology has great influence on the life test, although the use of transparent resin package, available microscopy to directly observe the internal solid crystal, bonding, for failure analysis, but not all of the packaging defect can be observed, such as: bonding and weld quality conditions is closely related to temperature and pressure, and temperature too much, too much pressure will make the chip deformation stress, causing dislocation, even dark crack, affect the luminous efficiency and lifetime. The stress change of wire bonding and resin package attractive, such as heat dissipation, expansion coefficient are the important factors influencing the life test, the life test results than bare crystal life test, but for the current low power chip, increasing the scope of quality assessment, life test results closer to the actual situation, there are certain the reference value for the production control.

Design of life test stand

The life test bed is composed of the life test unit board, the platform and the special power supply equipment, and can carry on the 550 groups (4400) LED life test at the same time. According to the requirements of the life test conditions, LED can be connected in parallel and in series with two types of connection drive. Parallel connection form: the multiple LED anode and cathode, anode and cathode connected in parallel, which is characterized by the working voltage of each LED, the total current is Ifn, in order to achieve consistent working current of the If of each LED, each LED forward voltage requirements should be consistent. However, there are some differences between the device parameters, the forward voltage Vf and LED decreases with the increased temperature, probably because different LED radiation condition difference, caused the current of If difference, poor heat dissipation of the LED, Wen Sheng is larger, the forward voltage Vf decline is larger, due to current If rise.

Comprehensive analysis of LED chip life test (two)

Although the resistance can be reduced by adding series resistance, but there are many disadvantages, such as complex circuit, large difference of working current If, LED can not be applied to different VF, etc. Therefore, it is not appropriate to use parallel connection driver. Series connection type: cathode is more than LED on the cathode connected in series, the utility model has the advantages of the working current of each LED, should be on the current limiting resistor R, as shown in Figure 2 single series circuit, when a LED is open, it will result in a series of 8 LED out, from the principle of LED chip open the possibility of. We believe that the life test of LED, with constant current drive and series connection work is preferred.

The common 78 series power circuit IC LED constant current drive circuit, which is characterized by low cost, simple structure and high reliability; by adjusting the resistance value of the potentiometer, can facilitate the adjustment of the constant current; for the power supply voltage range, the drive current is accurate and stable, the power supply voltage change has little effect. We take the two circuit as the basic route, and construct the life test unit board in parallel, each unit can carry on the (88) LED life test at the same time. The platform is a general standard combined rack, which can be easily loaded and unloaded by reasonable wiring.

The special power supply equipment, the output is 5 36V DC voltage safety load capacity is 5A, which has 2 microcomputer timing control function, can automatically open or close, 5 road input and output respectively indicating, figure three is a diagram fatigue test system.

The advantages of the design of the life test stand:

[1] life test current accurate, adjustable, constant;

[2] with microcomputer timing control function, can automatically open or close;

[3] can be applied to different VF LED at the same time, without having to adjust;

[4] uses the unit combination structure, may increase the life test unit at any time, realizes the on-line operation;

[5] uses low-voltage power supply to ensure safety performance.