LED drive power failure analysis

- Sep 21, 2018-

LED drive power failure analysis

Compared with the LED light source, the structure of the LED driving power supply is more complicated, and there are more trade-offs, so that the LED driving power source often fails before the LED light source. 

According to statistics, more than 80% of the failures of the whole lamp are caused by a power failure. After long-term empirical analysis in the laboratory, there are many reasons for LED driver power failure, which can be summarized into the following seven categories.

1. Aging of electronic components

ALIGHT will use physical and chemical methods for various failures such as open circuit, short circuit, burnout, leakage, functional failure, unqualified electrical parameters, and unstable failure of resistors, capacitors, diodes, transistors, LEDs, connectors, ICs, etc. Analytical means, from the macro and micro analysis of the cause of failure, and propose improvements for customers.

2.PCB quality issues

ALIGHT is responsible for various failure problems such as PCB, PCBA wetting failure, blasting, delamination, CAF, open circuit, short circuit, etc., using physical and chemical analysis methods to analyze the cause of failure from macro and micro, and propose improvements for customers. direction.

3. LED power supply is poorly cooled

The drive circuit consists of electronic components, and a few components are very sensitive to temperature. For example, electrolytic capacitors, the current electrolytic capacitor life estimation formula is "every 10 degrees decrease in temperature, the life is doubled", poor heat dissipation is likely to lead to a much shortened life, early failure, resulting in LED voltage failure, lamp failure.

 Especially for the built-in power supply (the power supply placed in the whole lamp), the heat source with large heat will increase the heat conduction and heat dissipation pressure of the whole lamp, the temperature of the LED will increase, and the light efficiency and life will be greatly reduced. Therefore, when designing LED power supply, you should pay attention to its own heat dissipation problem. 

Therefore, the initial evaluation of the design of the luminaire, the design of the power supply simultaneously, can solve the above problems. In the design, it is necessary to comprehensively consider the heat dissipation of the LED and the heat dissipation of the power supply, and the overall temperature of the lamp is controlled, so that a better lamp can be designed.

4. Problems in power supply design

(1) Power design. Although the LED light efficiency is high, there is still 80% to 85% of the thermal energy 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 is 45 to 55 ° C, and the power supply is in a high temperature environment for a long time. Under the work, to ensure the longevity, it is necessary to increase the power margin, generally retaining 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 about 20°C, and the temperature of the component accessory should reach 65~75°C. Some components drift at high temperatures and even shorten their lifetime. Therefore, the device should be selected for long-term use at higher temperatures. Pay special attention to electrolytic capacitors and wires.

(3) Electrical performance design. The switching power supply is designed for the parameters of the LED, mainly the constant current parameter. The magnitude of the current determines the brightness of the LED. If the batch current error is large, the brightness of the whole batch of lamps is not uniform. Moreover, changes in temperature can also cause the output current of the power supply to shift. 

Generally, the batch error is controlled within ±5% to ensure the brightness of the lamp is consistent, and the forward voltage drop of the LED is deviated. The constant current voltage range of the power supply design should include the voltage range of the LED. When multiple LEDs are used in series, the minimum voltage drop multiplied by the number of series is the lower limit voltage, and the maximum voltage drop multiplied by the number of 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 left 1~ 2V margin.

(4) PCB layout design. LED luminaires leave the power supply small in size (unless the power supply is external), so the PCB design requirements are higher and there are more factors to consider. The safety distance should be sufficient. The input and output isolation power supply is required. The primary side circuit and the secondary side circuit require a withstand voltage of 1500 to 2500 VAC, and at least 3 mm distance must be left on the PCB. In the case of a metal-enclosed luminaire, the layout of the entire power supply must also take into account the safe distance between the high-voltage part and the outer casing. If there is no space to ensure the safety distance, other measures must be taken to ensure the insulation, such as punching holes on the PCB, adding insulation paper, potting insulating glue, etc. In addition, the layout of the board should also consider the heat balance, the heating elements should be evenly distributed, and can not be placed in a concentrated manner to avoid local temperature rise. Electrolytic capacitors are kept away from heat sources, slowing down aging and prolonging service life.

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5. Lightning strike damage

Lightning strikes are a common natural phenomenon, especially during the rainy season. The damage and losses it brings are measured by hundreds of billions of dollars each year. Lightning strikes are divided into direct lightning strikes and indirect lightning strikes. Indirect lightning mainly includes conducted lightning and induced lightning. Because the energy impact caused by direct lightning is very large, the destructive power is very strong, and the general power supply is unbearable. Therefore, the indirect lightning type is mainly discussed here.

The surge impact formed by lightning strikes is a transient wave, which is a transient disturbance, which can be a surge voltage or a surge current. Transfer to the power line along a power line or other path (conducting lightning) or by an electromagnetic field (inductive lightning). Its waveform characteristics are first rising rapidly and then slowly decreasing. This phenomenon can have a fatal effect on the power supply, and the instantaneous surge impact generated by it far exceeds the electrical stress of the general electronic device, resulting in direct damage to the electronic components.

6. Grid voltage exceeds power load

When the wiring of the same transformer grid branch is too long and there are large power equipment in the branch, when the large equipment starts and stops, the grid voltage will fluctuate drastically, and even the grid will be unstable.

 When the instantaneous voltage of the power grid exceeds 310 VAC, the drive may be damaged (even if there is a lightning protection device, because the lightning protection device responds to pulse spikes of several tens of microseconds, and the power grid fluctuation may reach several tens of milliseconds or even hundreds of milliseconds) . Therefore, special attention should be paid to the large-scale electric machinery on the street lighting branch network. It is best to monitor the fluctuation range of the power grid or to supply power from a separate grid transformer.

7. Solder joint failure

The power package mainly involves the connection process between the PCB board and the components, in which the solder joint plays an important role. The main function of the solder joint is to realize the mechanical connection and electrical connection between the electronic component and the substrate (the PCB power supply is for the PCB board), and the solder joint quality seriously affects the reliability of the device. Solder joint failure comes from welding failures in production assembly, such as solder bridging, solder joints, voids, and Manhattan. 

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, thermal stress is generated in the solder joints, and periodic changes in stress may cause fatigue damage of the solder joints, eventually leading to fatigue Invalid.