The reason of broken for power supply

As an indispensable part of LED lighting, LED power supply has increasingly stringent requirements for its electronic packaging technology. It not only needs to have excellent weather resistance, mechanical properties, electrical insulation properties and thermal conductivity, but also needs to take into account potting. Adhesion of materials and components. In the use of LED power supply, what are the reasons for the failure of LED driver? Let's take a look at it !

Causes of broken for LED power supply

1. Aging of electronic components

Including resistors, capacitors, diodes, triodes, LEDs, connectors, ICs and other devices open circuit, short circuit, burnout, leakage, functional failure, unqualified electrical parameters, unstable failure and other failure problems.

2. PCB quality problems

Including PCB, PCBA poor wetting, board explosion, delamination, CAF, open circuit, short circuit and other failure problems.

3. Poor heat dissipation of LED power supply

The drive circuit consists of electronic components, a few of which are very sensitive to temperature. For example, electrolytic capacitors, the general life estimation formula of electrolytic capacitors is "every time the temperature drops by 10 degrees, the life doubles". Poor heat dissipation may lead to a greatly shortened life and premature failure, resulting in LED voltage failure and lamp failure. Especially for the built-in power supply (the power supply placed in the whole lamp), the power supply with high heat generation will increase the heat conduction and heat dissipation pressure of the whole lamp, the temperature of the LED will rise, and its luminous efficiency and life will be greatly reduced. Therefore, when designing LED power supplies, attention should be paid to its own heat dissipation.

4. Problems in power supply design

(1) Power design. Although the LED light efficiency is high, there is still 80% to 85% of heat loss, resulting in a temperature rise of 20 to 30K inside the lamp. If the room temperature is 25°C, the inside of the lamp will be 45 to 55°C, and the power supply is in a high temperature environment for a long time In order to ensure the service life, it is necessary to increase the power margin, and generally keep 1.5 to 2 times the margin.

(2) Component selection. When the internal temperature of the lamp is 45-55°C, the internal temperature rise of the power supply is still about 20°C, and the temperature of the component accessories should reach 65-75°C. The parameters of some components will drift at high temperature, and even the life will be shortened, so the device should be used for a long time at a higher temperature, and special attention should be paid to electrolytic capacitors and wires.

(3) Electrical performance design. The switching power supply is designed for the parameters of LEDs, mainly the constant current parameters. The magnitude of the current determines the brightness of the LEDs. If the batch current error is large, the brightness of the whole batch of lights will be uneven. Moreover, temperature changes can also cause the output current of the power supply to shift. Generally, the batch error is controlled within ±5% to ensure that the brightness of the lamp is consistent. The forward voltage drop of the LED has deviations. The constant current voltage range of the power supply design must include the voltage range of the LED. When multiple LEDs are used in series, the minimum voltage drop multiplied by the number in series is the lower limit voltage, and the maximum voltage drop multiplied by the number in series is the upper limit voltage. The constant current voltage range of the power supply is slightly wider than this range. Generally, the upper and lower limits are set at 1~ 2V headroom.

(4) PCB layout design. The size of LED lamps left for the power supply is small (unless the power supply is external), so the requirements for PCB design are relatively high, and there are many factors to be considered. The safety distance should be enough, and the input and output isolation power supply is required. The primary side circuit and the secondary side circuit require a withstand voltage of 1500-2500VAC, and a distance of at least 3mm should be left on the PCB. If it is a lamp with a metal casing, the layout of the entire power supply should also consider the safe distance between the high-voltage part and the casing. If there is no space to ensure a safe distance, other measures must be used to ensure insulation, such as drilling holes on the PCB, adding insulating paper, and potting insulating glue. In addition, the layout of the board should also consider the heat balance, and the heating elements should be evenly distributed, and should not be placed in a centralized manner to avoid local temperature rise. Electrolytic capacitors are kept away from heat sources to slow down aging and prolong service life.

5. Lightning damage

Lightning strikes are a common natural phenomenon, especially during the rainy season. Lightning strikes are divided into direct lightning strikes and indirect lightning strikes. Indirect lightning mainly includes conductive lightning and induction lightning. Because the energy impact brought by direct lightning is very large and the destructive power is extremely strong, the general power supply cannot bear it, so the main discussion here is indirect lightning.

The surge impact formed by lightning strike is a kind of transient wave, which belongs to transient interference, which can be surge voltage or surge current. It is transmitted to the power line along the power line or other paths (conducted lightning) or through an electromagnetic field (inductive lightning). Its waveform is characterized by a rapid rise and then a slow decline. This phenomenon will have a fatal impact on the power supply. The instantaneous surge impact it generates far exceeds the electrical stress of general electronic devices, and the direct result is damage to electronic components.

6. Grid voltage exceeds power supply load

When the branch wiring of the same transformer grid is too long and there are large power equipment in the branch, the grid voltage will fluctuate violently when the large equipment starts and stops, and even cause the grid to be unstable. When the instantaneous voltage of the power grid exceeds 310 VAC, the driver may be damaged (even if there is a lightning protection device, it will not work, because the lightning protection device is to deal with pulse spikes at the level of tens of microseconds, and the fluctuation of the power grid may reach tens of milliseconds, or even hundreds of milliseconds) . Therefore, special attention should be paid when there are large electric machinery on the power grid of the street lamp lighting branch. It is best to monitor the fluctuation range of the power grid, or supply power by a separate power grid transformer.

7. Solder joint failure

Power packaging mainly involves the connection process between PCB board and components, in which solder joints play an important role. The main function of solder joints is to realize the mechanical connection and electrical connection between electronic components and substrates (PCB boards are used in LED power supplies), and the quality of solder joints seriously affects the reliability of devices. On the one hand, the failure of solder joints comes from welding faults in production assembly, such as solder bridging, virtual soldering, voids, and Manhattan phenomenon. On the other hand, during the service process, when the ambient temperature changes, due to the difference in thermal expansion coefficient between the components and the PCB board, thermal stress is generated in the solder joints, and the periodic changes in stress will cause fatigue damage to the solder joints, eventually leading to fatigue invalidated.

The above reasons for the failure of LED power supply are shared here. As the global LED lighting manufacturing industry continues to transfer to China, it will further increase the market demand of the domestic LED drive power supply manufacturing industry. The proportion is expected to increase further.