What causes LED driver failure?
Usually, the main function of the LED drive is to convert the input AC voltage source into an output voltage. As a key component in LED lighting, the quality of LED driver directly affects the reliability and stability of the overall lamp. This article will start from LED driver and other related technologies and customer application experience to analyze many failure situations in lamp design and application.
1. Don’t understand the working characteristics of LEDs
A customer once required the input power of the lamp to be a fixed value with a fixed 5% error. The output current could only be adjusted for each lamp to reach the specified power. Due to different working environment temperatures and different lighting times, the power of each lamp will still vary greatly.
Although customers make such requests, they have marketing and business factors to consider. However, the volt-ampere characteristics of LED determine that the LED driver is a constant current source and its output voltage changes with the change of the LED load series voltage Vo. When the overall efficiency of the driver remains unchanged, its input power changes with Vo.
At the same time, the overall efficiency of the LED driver will increase after thermal balance. Under the same output power condition, the input power will decrease compared with the startup time.
Therefore, when formulating requirements, users of LED drivers should first understand the working characteristics of LEDs and avoid proposing indicators that are inconsistent with the principles of working characteristics. At the same time, avoid indicators that far exceed actual needs, and avoid excess quality and cost waste.
2. Failure during testing
A customer once purchased LED drivers from many brands, but all samples failed during the testing process. Later, after on-site analysis, we found that the customer used an auto-coupling voltage regulator to directly power the LED driver for testing. After power-on, the voltage regulator was gradually increased from 0Vac to the rated working voltage of the LED driver.
Such a test operation can easily cause the LED driver to start and work with load at a very small input voltage. This situation will cause the input current to be much larger than the rated value. Internal input-related devices, such as fuses and rectifier bridges, thermistor etc. fail due to excessive current or overheating, causing the driver to fail.
Therefore, the correct test method is to adjust the voltage regulator to the rated operating voltage range of the LED driver, then connect the driver and power on for testing.
3. Different loads, different test results
When the LED driver is tested with an LED lamp, the results are normal, but when tested with an electronic load, the results may be abnormal. The LED driver is designed to comply with the working characteristics of LED lamps. The test method closest to reality and real application should be to use LED lamp beads as loads and connect ammeters and voltmeters in series for testing.
4. The fluctuation range of the power grid exceeds the reasonable range
When the branch wiring of the same transformer grid is too long and there is large power equipment in the branch, when the large equipment starts and stops, the grid voltage will fluctuate violently, even causing grid instability. When the instantaneous voltage of the power grid exceeds 310Vac, the driver may be damaged. Therefore, special attention should be paid when there is large-scale electric machinery on the street lighting branch power grid. It is best to monitor the fluctuation range of the power grid, or use a separate power grid transformer to supply power.
5. Lines trip frequently
Too many lights on the same branch lead to overload on a certain phase and uneven power distribution between phases, resulting in frequent line tripping.
6. Heat dissipation of LED driver
When the driver is installed in a non-ventilated environment, the driver housing should be in contact with the lamp housing as much as possible. If conditions permit, apply thermal conductive glue or a thermal pad on the contact surface between the housing and the lamp housing to improve the heat dissipation performance of the driver. Ensure the life and reliability of the driver.
7. The Vf variation range of LED lamp beads is not considered, resulting in low lamp efficiency and even unstable operation.
The load end of LED lamps is generally composed of a number of LEDs connected in series and parallel, and its operating voltage Vo=Vf*Ns, where Ns represents the number of LEDs connected in series. The Vf of LED changes with temperature. Generally, under constant current, Vf becomes lower at high temperature and Vf becomes higher at low temperature. Therefore, the load operating voltage of LED lamps at high temperatures corresponds to VoL, and the load operating voltage of LED lamps at low temperatures corresponds to VoH. When selecting an LED Driver, it must be considered that the output voltage range of the driver is greater than VoL~VoH.
If the maximum output voltage of the selected LED driver is lower than VoH, the maximum power of the lamp may not reach the actual required power at low temperatures. If the minimum voltage of the selected LED driver is higher than VoL, the output of the driver may exceed the operating limit at high temperatures. range, the operation is unstable, and the lamps may flicker.
However, considering the comprehensive cost and efficiency, we cannot blindly pursue the ultra-wide output voltage range of the LED driver, because the driver efficiency is the highest when the driver voltage is only within a certain range. Once the range is exceeded, the efficiency and power factor (PF) will become worse. At the same time, if the output voltage range of the driver is designed to be too wide, the cost will increase and the efficiency cannot be optimized.
In summary, many details need to be paid attention to in the actual application of LED drivers, and many problems need to be analyzed and adjusted in advance to avoid unnecessary failures and losses!
