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10 may lead to LED driver failure reasons

Basically, it can be said LED driver's main role is to input the AC voltage source into the output voltage can be LED
Vf (positive turn-on voltage drop of the current source.As a key component of LED lighting, LED drive quality directly affects the reliability and stability of the overall lighting.This article from the LED driver and other related technology and customer application experience, Finishing analysis of lighting design and application of many failures:
1, does not take into account the LED lamp beads Vf range of change, resulting in low efficiency lamps, and even work is not stable
LED lamp load side, usually by a number of LED strings in parallel composition, the operating voltage Vo = Vf * Ns, where Ns that the number of LED series. LED Vf with temperature changes and changes, under normal circumstances, at a constant current, high temperature Vf becomes low, low temperature Vf becomes high. Therefore, the high-temperature LED lamp load operating voltage corresponds to VoL, low temperature LED lamp load working voltage corresponds to VoH. In the choice of LED driver should consider the drive output voltage range 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 can not reach the required power. If the minimum voltage of the selected LED driver is higher than VoL, the driver output may exceed the operating range at high temperature. Instability, there will be flashing lights and so on.
But the overall cost and efficiency considerations, LED drivers can not blindly pursue ultra-wide output voltage range: because the drive voltage is only in a certain interval, the drive efficiency is the highest. Over the range of efficiency, power factor (PF) will be worse, while the driver output voltage range design is too wide, resulting in higher costs, efficiency can not be optimized.
2, does not consider the power margin and derating requirements
Under normal circumstances, the nominal power LED driver is the rated environment, rated voltage measured data. Considering that different customers will have different applications, most of the LED driver suppliers in their own product specifications to provide power derating curve (common with the load vs. ambient temperature derating curve and load vs. input voltage derating curve).
Figure 1 Load versus ambient temperature derating curve
As shown in Figure 1, the red curve shows the LED driver in the case of input 120Vac, its load changes with the ambient temperature power derating curve. When the ambient temperature is below 50 ℃, the driver allows 100% full load, when the ambient temperature up to 70 ℃, the drive can only derate to 60% of the load, when the ambient temperature between 50-70 ℃ change, the driver load Temperature rise and linear decline.
Blue curve shows the LED driver in the input 230Vac or 277Vac case, the load changes with the ambient temperature power derating curve, the principle of similar.
Figure 2 shows the power derating curve for the load vs. input voltage
As shown in Figure 2, the blue curve shows the LED driver at an ambient temperature of 55 ° C, the output power of the input voltage with the derating curve. When the input voltage is 140Vac, the driver's load allows 100% full load, with the input voltage down; if the output power is constant, the input current will rise, resulting in increased input loss, efficiency, device temperature rise, the individual temperature point May be excessive, and may even lead to device failure.
Therefore, as shown in Figure 2 when the input voltage is less than 140Vac, requires the output load of the driver decreases linearly with the input voltage decreases. Read the derating curve and the corresponding requirements, the choice of LED driver should be based on the actual use of the ambient temperature and input voltage conditions, consider and select, and appropriate to set aside the amount of margin.
3, do not understand the work of LED characteristics
There was a customer request lamps fixed power input power, fixed 5% error, only for each lamp to adjust the output current to the specified power. Due to different working environment temperature, and lighting time is different, each lamp power or there will be a big difference.
Customers to make such a request, although its marketing and business considerations. However, the LED's voltage-ampere characteristic determines the LED driver is a constant current source, its output voltage changes with the LED load series voltage Vo, and the input power varies with Vo when the overall efficiency of the driver is almost constant.
At the same time, the overall efficiency of the LED driver in thermal equilibrium will be increased, under the same output power conditions, compared to the boot time, the input power will drop.
Therefore, LED drive applications in the development of demand, we should first understand the work of LED characteristics, to avoid some of the indicators do not meet the work characteristics of the principle, while avoiding far more than the actual demand indicators, to avoid excess quality and cost waste.
4, the test failure
There have been a lot of brand LED driver, but all the samples in the test process failure. Later to the scene analysis and found that customers use self-dual voltage regulator directly to the LED driver power supply for testing, power regulator will be gradually increased from 0Vac LED driver rated operating voltage.
This test operation, it is easy to make the LED driver in a very small input voltage to start and load work, and this situation will lead to input current is much greater than the rated value, the internal input related devices such as fuses, rectifier bridge, Thermistors, etc. due to excessive current or over-current failure, leading to drive failure.
Therefore, the correct test method is to adjust the voltage regulator to the LED driver rated voltage range, then connected to the drive power test.
Of course, from the technical design can also improve the design to avoid such a test error caused by the failure of the problem: the input to set the start-up voltage limiter circuit and input under-voltage protection circuit. The driver does not operate when the input voltage does not reach the set voltage of the driver. When the input voltage drops to the input undervoltage protection point, the driver enters the protection state.
Therefore, even if the customer testing process is still used self-dual voltage regulator steps, the drive with self-protection function and will not fail. But customers must be carefully tested before the purchase of the LED driver products are equipped with this protection
(Taking into account the practical application of LED driver environment, most of the LED driver does not have this protection).
5, different load, test results are different
LED driver with LED lamp test, the results are normal, with electronic load test, the results may be abnormal. Usually this phenomenon has the following reasons:
(1) The instantaneous output voltage or power of the driver exceeds the operating 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 momentarily overpower the load, causing the drive to not work or load.)
(2) The characteristics of the electronic load meter used is not suitable for measuring the constant current source, the load voltage stalls, resulting in the drive does not work or load.
(3) because the electronic load meter input will have a large internal capacitance, the test is equivalent to the drive output in parallel with a large capacitor, the drive may lead to current sampling instability.
Because the LED driver is designed to meet the working characteristics of LED lamps, the closest to the actual and practical application of the test method should be used as a load LED lamp beads, string ammeter and voltmeter to test.
6, often the following conditions will lead to LED driver damage:
• Connect the AC to the DC output of the drive, causing the drive to fail;
• AC is connected to the input or output of the DC / DC drive, causing the drive to fail;
· The constant current output terminal and the dimming line are connected together, leading to the driver failure;
· The phase line is connected to the ground line, resulting in no output drive and housing charged;
7, phase line is wrong
Usually outdoor engineering applications are three-phase four-wire system to the national standard, for example, each phase line and the rated voltage between the zero line is 220Vac, phase line and phase voltage is 380Vac. If the construction worker connects the input terminal of the driver to the two phase lines, the LED driver input voltage exceeds the standard after the power is turned on, resulting in product failure.
Figure 3 open-circuit diagram of the zero line
As shown in Fig. 3, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 denote the LED drivers normally mounted in the line, respectively. When the zero line on the line (N) shown in Figure 2, the two branches of the drive R1, R2 is equivalent to the series connected to the 380Vac voltage. Because the input impedance difference, when one of the drivers to start charging, the resistance becomes smaller, the voltage may be added to the other drive, resulting in over-voltage damage to failure. It is recommended that the same distribution branch, the switch or circuit breaker to be broken, not only disconnect the zero line. Do not place the power distribution fuse on the zero line, the line should avoid bad contact with the zero line.
8, the scope of fluctuations in the power grid beyond a reasonable range
When the same transformer power line branch wiring is too long, there is a large power equipment in the branch, the large-scale equipment start and stop, the grid voltage will fluctuate violently, and even lead to instability. When the instantaneous voltage exceeds 310Vac grid may damage the drive (even if there is no lightning protection device is also ineffective, because the lightning protection device is to deal with dozens of uS level pulse spikes, and network fluctuations may reach tens of mS, or even hundreds of mS). Therefore, the street lighting branch network of large electric power machinery to pay special attention to the best monitoring of the fluctuation range of the power grid, or a separate power grid transformer.
9, frequent line trip
Too many lights on the same branch, resulting in a phase overload on the electrical load, and uneven distribution of power between the phases, resulting in frequent line trip.
10, the drive heat
When the driver is installed in a non-ventilated environment, the drive housing should be contacted with the lamp housing as much as possible. If necessary, the thermal plastic or thermal pad should be coated on the contact surface between the shell and the lamp housing to improve the heat dissipation performance of the driver. Life and reliability.
In summary, the LED driver in practical applications, there are many details to note that many problems need to be analyzed in advance to adjust to avoid unnecessary failure and loss!

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