Basically, it can be said that the main function of the LED driver is to convert the input AC voltage source to the current source whose output voltage can vary with the forward voltage drop of the LED Vf.
As the key component of LED lighting, the quality of LED driver directly affects the reliability and stability of the overall lamps. Starting from the related technologies such as LED driver and customer application experience, this paper sorted out and analyzed many failures in the design and application of lamps and lanterns.
1. Failure to consider the range of Vf of LED beads leads to low efficiency and even unstable operation of lamps and lanterns.
The load end of LED lamps is generally composed of a number of LED series and parallel connection. Its working voltage Vo=Vf*Ns, in which NS represents the number of LED series connection. The Vf of LED varies with temperature. Generally, when the current is constant, the Vf becomes lower at high temperature and higher at low temperature. Therefore, at high temperature, the working voltage of the LED lamp load corresponds to VoL, and at low temperature, the working voltage of the LED lamp load corresponds to VoH. When selecting an LED driver, it is necessary to consider that the output voltage range of the driver is larger 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 temperature. If the minimum voltage of the selected LED driver is higher than VoL, the output of the driver may exceed the working range at high temperature, the work is unstable and the lamp will flicker.
However, considering the cost and efficiency, we can not blindly pursue the ultra-wide output voltage range of the LED driver: because when the driver voltage is only in a certain range, the driver efficiency is the highest. Over the range, efficiency and power factor (PF) will become worse. At the same time, the design of the driver output voltage range is too wide, which will lead to higher cost and inefficiency can not be optimized.
2. Failure to consider power margin and reduction requirements
Generally, the nominal power of LED driver refers to the data measured under rated environment and voltage. Considering that different customers will have different applications, most LED driver suppliers will provide power reduction curves on their product specifications (common load vs environment temperature reduction curves and load vs input voltage reduction curves).
As shown in Figure 1, the red curve represents the power drop curve of the LED driver when the load varies with the ambient temperature under the input of 120 Vac. When the ambient temperature is lower than 50 C, the driver is allowed to be 100% full load. When the ambient temperature is as high as 70 C, the load of the driver can only be reduced to 60%. When the ambient temperature changes between 50 and 70 C, the load of the driver decreases linearly with the temperature rising.
The blue curve represents the power drop curve of the LED driver when the input voltage is 230 Vac or 277 Vac. The principle is similar.
As shown in Figure 2, the blue curve represents the drop-off curve of the output power of the LED driver with the input voltage at the ambient temperature of 55 C. When the input voltage is 140 Vac, the load of the driver is allowed to be 100% full, with the input voltage decreasing; if the output power remains unchanged, the input current will rise, resulting in increased input loss, lower efficiency, device temperature rising, individual temperature points may exceed the standard, or even lead to device failure.
Therefore, as shown in Figure 2, when the input voltage is less than 140 Vac, the output load of the driver is required to decrease linearly with the decrease of the input voltage. After understanding the up-down curve and the corresponding requirements, the selection of LED driver should be based on the actual use of environmental temperature and input voltage, comprehensive consideration and selection, and appropriate allowance for the down balance.
3. Understanding the working characteristics of LED
Customers have asked that the input power of the lamps be a fixed value with a fixed error of 5%. The output current can only be adjusted for each lamp to reach the specified power. Due to the different working environment temperature and lighting time, the power of each lamp will still vary greatly.
Customers make such requests, although they have considerations of marketing and business factors. However, the voltage-ampere characteristic of the LED determines that the LED driver is a constant current source. Its output voltage varies with the series voltage Vo of the LED load, and its input power varies with Vo when the efficiency of the driver is basically unchanged.
At the same time, the overall efficiency of the LED driver will increase after thermal balance. Under the same output power, the input power will decrease compared with the start-up time.
Therefore, the application of LED driver should first understand the working characteristics of LED, avoid putting forward some indicators that do not conform to the principle of working characteristics, and avoid indicators that far exceed the actual demand, so as to avoid excessive quality and waste of cost.
4. Failure in testing
Customers have purchased many brands of LED drives, but all samples failed in the testing process. Later, after on-site analysis, it was found that customers used self-dual voltage regulators to directly supply power to the LED driver for testing. After power-on, the voltage regulator was gradually raised from 0Vac to the rated working voltage of the LED driver.
This kind of test operation can easily make the LED driver start up and work on load when the input voltage is very small, which will lead to the input current far greater than the rated value. The internal input devices, such as fuses, rectifier bridges, thermistors and so on, will fail due to the current exceeding the standard or overheating, leading to the failure of the driver.
Therefore, the correct test method is to adjust the voltage regulator to the rated working voltage range of the LED driver, and then connect the driver to power-on test.
Of course, the technical improvement of the design can also avoid the failure problems caused by the misoperation of the test: setting up the start voltage limiting circuit and the input undervoltage protection circuit at the input end of the driver. When the input voltage does not reach the starting voltage set by the driver, the driver does not work; when the input voltage drops to the input undervoltage protection point, the driver enters the protection state.
Therefore, even though the operation steps of the self-couple voltage regulator are still used in the customer testing process, the driver has self-protection function without failure. However, before testing, customers must carefully understand whether the purchased LED driver products have this protection function (considering the actual application environment of the LED driver, most of the current LED drivers do not have this protection function).
5. Test results are different under different loads
When the LED driver is tested with the LED lamp, the results are normal. When the LED driver is tested with the electronic load, the results may be abnormal. Usually this phenomenon has the following reasons:
(1) The output instantaneous voltage or power of the driver exceeds the working range of the electronic load meter. (Especially in CV mode, the maximum test power should not exceed 70% of the maximum load power. Otherwise, the load may be instantaneously over-power protected, resulting in the driver unable to work properly or load. )
(2) The characteristics of the electronic load meter used are not suitable for measuring the constant current source, resulting in load voltage shift, which makes the driver unable to work properly or load.
(3) Because there will be a large capacitor in the input of the electronic load meter, the test is equivalent to a large capacitor in parallel with the output of the driver, which may lead to instability of the current sampling of the driver.
Because the design of the LED driver is to conform to the working characteristics of the LED lamps, the closest test way to the actual and real application should be to use the LED beads as a load, series of ammeters and voltmeters to test.
6. The following frequently occurring conditions can cause damage to the LED driver:
(1) The AC is connected to the DC output of the driver, which results in the failure of the driver.
(2) AC is connected to the input or output of DC/DC driver, resulting in driver failure;
(3) Connect the constant current output terminal with the light adjustment, which causes the driver to fail.
(4) The phase wire is connected to the ground wire, which results in no output of the driver and charged housing.
7. Phase Wiring Error
Generally, outdoor engineering applications are three-phase four-wire system. Taking the national standard as an example, the rated working voltage between each phase line and zero line is 220 Vac, and the voltage between the phase line and the phase line is 380 Vac. If the construction worker connects the input end of the driver to two phase lines, the input voltage of the LED driver will exceed the standard after power-on, which will lead to the failure of the product.
As shown in the figure above, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 represent the LED drivers normally installed in the line, respectively. When the zero line (N) on the line is disconnected as shown in the figure, the drives R1 and R2 on the two branches are equivalent to connecting in series to 380Vac voltage. Because of the difference of input internal resistance, when one driver is charged to start, the internal resistance decreases, and most of the voltage may be added to another driver, resulting in overvoltage damage and failure.
Therefore, it is suggested that switches or circuit breakers should be disconnected together in the same distribution branch, and not only zero lines should be disconnected. Distribution fuses should not be placed on the zero line, and bad contact of the zero line should be avoided on the line.
8. The fluctuation range of power grid is beyond the reasonable range
When the branch wiring of the same transformer network is too long and there are large power equipments in the branch, when the large equipments start and stop, the grid voltage will fluctuate sharply, even lead to the unstable grid. 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 is ineffective, because the lightning protection device is to deal with the pulse peak of tens of uS levels, and the fluctuation of the power grid may reach tens of mS, or even hundreds of mS).
Therefore, special attention should be paid when there are large electric machinery on the street lamp lighting branch power grid. It is better to monitor the fluctuation amplitude of the power grid or supply power by a single power transformer.
9. Frequent trip of line
Too many lights on the same branch lead to overload of load on one phase and uneven distribution of power among phases, which leads to frequent trip of lines.
10. Driver Heat Dissipation
When the driver is installed in non-ventilated environment, the driver shell should be contacted with the lamp shell as far as possible. If conditions permit, the thermal conductive glue or heat pad should be coated on the contact surface between the driver shell and the lamp shell to improve the heat dissipation performance of the driver, so as to ensure the life and reliability of the driver.
In summary, there are many details to be noted in the practical application of the above-mentioned LED driver. Many problems need to be analyzed and adjusted in advance to avoid unnecessary failure and loss.
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