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Led Driver Failure Analysis

May. 05, 2020

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Compared with the LED light source, the structure of China LED driver for sale is more complex, and there are more trade-offs, so the LED driver often fails before the LED light source.According to statistics, more than 80% of the total lamp failure is due to power failure.There are many reasons for LED driver failure, which can be classified into the following categories.

1. Aging of electronic components

Including resistance, capacitance, diode, triode, LED, connector, IC and other devices open circuit, short circuit, burn, leakage, functional failure, electrical parameters unqualified, unstable failure and other failure problems.

2. PCB quality problem

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

3. Poor heat dissipation of LED power supply

The 12w mini LED driver is made up of electronic components, a few of which are very sensitive to temperature.For example, the commonly used life estimation formula of electrolytic capacitor is "for every 10 degree decrease in temperature, the life will be doubled". Poor heat dissipation is likely to lead to greatly shortened life and premature failure, leading to 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 will increase the heat conduction and heat dissipation pressure of the whole lamp, and the LED temperature will increase, and its light efficiency and life will be greatly reduced.Therefore, when designing LED power supply, we should pay attention to its own cooling problem.Therefore, at the beginning of the lamp design evaluation, the power supply design synchronization, can solve the above problems.In the design, the heat dissipation of LED and power supply should be considered comprehensively, and the temperature rise of the lamp should be controlled overall, so as to design a better lamp.

12w Mini Led Driver

 12w Mini Led Driver

4. Problems in power supply design

(1) power design.Although LED light efficiency is high, but there is still 80% ~ 85% heat loss, resulting in 20 ~ 30K temperature rise inside the lamp, if the room temperature is 25℃, there is 45 ~ 55℃ inside the lamp, the power supply for a long time in the high temperature environment work, to ensure the life must increase the power margin, generally retained 1.5 ~ 2 times of the margin.

(2) element selection.When the internal temperature of the lamp is 45 ~ 55℃, the internal temperature rise of the power supply is still about 20℃, then the temperature of the component accessories should reach 65 ~ 75℃.The parameters of some components will drift at high temperature, and even their life will be shortened. Therefore, it is necessary to choose devices that can be used for a long time at higher temperature. Special attention should be paid to electrolytic capacitors and wires.

(3) electrical performance design.The switching power supply is designed for LED parameters, which are mainly constant current parameters. The magnitude of current determines the brightness of LED. If the error of batch current is large, the brightness of the whole batch of lamps will be uneven.And temperature changes can also cause the output current of the power supply to shift.In general, 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 low voltage drop multiplied by the number of series is the lower limit voltage, and the 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, 1 ~ 2V margin is left for each upper and lower limits.

(4) PCB board design.The size of LED lamps left for the power supply is small (unless the power supply is external), so the PCB design requires higher requirements and there are many factors to be considered.The safe distance should be kept enough, the input and output isolated power supply is required, the primary side circuit and the secondary side circuit are required to withstand voltage 1500 ~ 2500VAC, and a distance of at least 3mm should be left on the PCB.If it is a light fixture with a metal shell, then the cloth plate of the whole power supply should also consider the safety distance between the high-pressure part and the shell.If there is no space to ensure a safe distance, other measures should be taken to ensure the insulation, such as punching holes in the PCB, adding insulating paper and sealing insulating glue.In addition, the cloth should consider the heat balance, heating elements should be evenly distributed, not centrally placed.

5. Lightning damage

Lightning strikes are a common natural phenomenon, especially during the rainy season.The damage and losses are estimated at hundreds of billions of dollars a year worldwide.Lightning strikes can be divided into direct lightning strikes and indirect lightning strikes.Due to the energy impact brought by direct lightning is very large, the destructive force is extremely strong, the general power supply is unable to bear, so the main discussion here is indirect lightning.

The surge impact caused by lightning is a transient wave, which is a transient disturbance. It can be the surge voltage or the surge current.Transmission to a power line along a power line or other path (conducting lightning) or through an electromagnetic field (inductive lightning).The waveform is characterized by a rapid rise and then a slow fall.This phenomenon can have a fatal effect on the power supply, which produces a transient surge impact far beyond the electrical stress of ordinary electronic devices, resulting in a direct result of electronic component damage.

6. The grid voltage exceeds the power supply load

When the branch distribution line of the same transformer is too long and there are large power equipment in the branch, when the large equipment starts and stops, the network voltage will fluctuate violently, even causing the network instability.When the transient voltage of the grid exceeds 310 VAC, the driver may be damaged (even if lightning protection device is available, it is not effective, because lightning protection device is in response to a pulse spike of tens of microseconds, while the grid fluctuation may be tens of milliseconds, or even hundreds of milliseconds).Therefore, special attention should be paid when there are large power machines on the power grid of street lighting branches, and the fluctuation of the power grid should be monitored, or the power is supplied by a separate power grid transformer.

7. Solder joint failure

Power package mainly involves the connection process between PCB board and components, in which solder joint plays an important role.The main function of solder joint is to realize the mechanical connection and electrical connection between electronic components and the substrate (LED power supply is for PCB). The quality of solder joint seriously affects the reliability of the device.On the one hand, solder joint failure comes from welding faults in production and assembly, such as solder bridging, virtual welding, void, Manhattan phenomenon.

On the other hand, in the process of service, when the ambient temperature changes, due to the difference in thermal expansion coefficient between components and PCB board, thermal stress is generated in the solder joint. The periodic change of stress will cause fatigue damage to the solder joint and eventually lead to fatigue failure.

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