According to the normal process, before a lamp bead is shipped as a qualified product, it should undergo a pressure test. However, it is said that the manufacturer actively ignores this link in the production process, because there is a great chance that the lamp will die in the process of the withstand voltage test. In fact, even if the lamp with a voltage of 3.75KV cannot pass the 3KV withstand voltage test, why is this result?
There are two main reasons for LED damage in the pressurization process. One is that the voltage is exceeded, and the other is that the current is exceeded. The leakage current of the withstand voltage test is set at about 10 mA, and generally does not exceed the allowable current value of the LED. The biggest cause of LED damage is the voltage excess. This article will analyze the situation of voltage excess, and interested friends can take a look.
The luminaire is composed of three parts: a driving power LED and a heat sink. The withstand voltage test is generally to test the withstand voltage between the input end of the drive power source and the outer casing of the lamp that the human body can access. The withstand voltage is achieved by insulation. The insulation from the input to the outer casing consists of two parts, one is the insulation of the primary and secondary of the power supply (except for the non-isolated power supply), and the other is the insulation between the lamp bead and the heat sink (generally integrated with the outer casing). AC-powered lamps with AC high voltage for withstand voltage test.
Assume that the applied AC high voltage is VC, the voltage divided by the power source Y capacitor is VCY, and the distributed capacitance of the aluminum substrate is divided into voltage VCY1/2, then VC=VCY+VCY1/2. We know that the capacitance tolerance Xc=1/(2πfC), the larger the capacitance, the smaller the capacitive reactance, and the smaller the voltage divided in the series circuit, the opposite is also true. In this example, the Y capacitance of the power supply is fixed, between 1000p and 2200P. Assuming that the distributed capacitance value of the aluminum substrate is equal to the Y capacitance value of the power source, the voltage divided by the aluminum substrate is equal to the power source to which the power source Y capacitor is divided, and both are 1/2 VC. If CY1/2 is less than CY, VCY1/2 will be greater than VCY, that is, the voltage applied to the aluminum substrate is higher than half of the withstand voltage test high voltage value, and the smaller the distributed capacitance, the higher the voltage. At this time one.
Second, the copper foil of the aluminum substrate has positive and negative poles, and the two copper foils are not connected together. The LEDs connected in series are connected between the two poles, and the distributed capacitance of the two copper foils is discrete. The difference in the shape of the copper foil makes the distribution capacitance different in size, and the divided high voltage is also different. If the voltage of the negative electrode is higher than the positive electrode, the secondary winding of the power supply transformer and the rectifier are turned on, and the voltage is forcibly leveled to the positive voltage, which does not cause the LED to overshoot, and if the positive voltage is higher than the negative voltage, At this time, there is a current (this current is different from the leakage current) flowing through the LED, which is why the LED will flash during the withstand voltage test. The higher the voltage, the brighter the LED. When it is too large to exceed the LED withstand voltage, it will damage the PN junction of the LED. Another case where the positive voltage is higher than the negative voltage is that the negative electrode is broken down and is at zero potential. At this time, the positive electrode is only enough to be divided into a very low voltage to cause LED damage. Therefore, the main cause of LED damage is that the voltage difference between the positive and negative electrodes is too large. This pressure difference is determined by the shape and position of the positive and negative copper foil on the aluminum substrate.
Through the above introduction, it can be seen that the excessive pressure difference between the positive and negative electrodes is the main cause of LED lamp bead damage. If the voltage is too large, the PN junction of the LED will be damaged, and the result of the dead light will be obtained. I hope that everyone can understand the increase test of the LED lamp bead after reading this article, and have a better understanding of the dead light caused by the excessive voltage.