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10 LED drive failure could cause problems Detailed - (AddTime:2015-03-31)

Basically you can say that the main role of the AC voltage source is LED driver converts the input to the output voltage with LED Vf (forward voltage drop varying current sources. As a key component of LED lighting, LED drive directly affect the quality of By the reliability and stability of the overall lighting of this article from the relevant technology and application experience in LED drivers and other customers starting analyzed with lighting design and application in the case of many failures:
1. LED lamp beads Vf not consider changes in scope, resulting in low lighting efficiency, and even instability
LED lighting load side, usually by a certain number of LED strings in parallel, and its working voltage Vo = Vf * Ns, where Ns is the number of LED in series. LED's Vf fluctuates with temperature change, in general, at constant current, Vf becomes low at high temperatures, low temperatures Vf becomes high. Therefore, the high temperature LED lighting load operating voltage corresponding to VoL, when low voltage LED lamps work load corresponding to VoH. In the selection of LED drivers need to be considered when the driver output voltage range greater than VoL ~ VoH.
If you use the LED driver output voltage is below the maximum VoH, could cause the low power lamps reach the actual maximum power required, if the choice of the LED drive voltage is higher than the minimum VoL, you may drive the output beyond the scope of work at high temperatures, working unstable, there will be flashing lamps and so on.
But the overall cost and efficiency considerations, the LED driver can not pursue wide output voltage range: Because the drive voltage only when a certain range, drive efficiency is highest. After exceeding the scope of the efficiency, power factor (PF) will be deteriorated, while the design driver output voltage range is too wide, resulting in higher costs, efficiency can not be optimized.
2. not consider power margin and derating requirements
Under normal circumstances, LED drive nominal power is the data measured at the nominal environment, rated voltage conditions. Taking into account the different customers have different applications, most vendors offer LED driver power derating curve in its own product specifications (common with load vs ambient temperature derating curves vs input voltage and load derating curve).
Shown in Figure 1, the red curve shows the LED drive at 120Vac input case, the load power derating curve with the ambient temperature changes. When the ambient temperature is below 50 ℃, drive allows 100% full load, when the ambient temperature up to 70 ℃, the drive can only be derated to 60% of the load, when the ambient temperature is between 50-70 ℃ change, drive the load with decreases linearly with increase in temperature.
Figure 1 Load vs ambient temperature derating curve
The blue curve indicates that the LED driver input 230Vac or 277Vac in the case, the load derating curve with the ambient temperature changes, the principle similar.
Shown in Figure 2, the blue curve represents the LED drive at ambient temperature 55 ℃, the output power derating curve with the input voltage changes. When the input voltage is 140Vac, the drive load allows 100% of full load, as the input voltage down; if the output power constant, the input current will rise, leading to greater loss inputs, reduced efficiency, the device temperature rises, the temperature of individual points may be excessive, and may even lead to device failure.
Figure 2 Load vs input voltage derating curve
Therefore, as shown when the input voltage is less than 140Vac, requires the drive output load decreases linearly with the input voltage decreases. Read the above derating curve and the corresponding requirements, use LED driver when it should be based on the ambient temperature and input voltage conditions of actual use, considering and selecting and leaving appropriate derating margin.
3. To understand the operating characteristics of the LED
Customer input power lamps had a fixed value, fixed 5% error, only for each lamp to adjust the output current reaches the specified power. Different due to different ambient temperature, and lighting time, each lamp's power will still be a big difference.
Customers make such a request, although its marketing and business factors to consider. However, the VA determines the characteristics of LED driver for LED constant current source, the output voltage in series with the LED load voltage Vo change, drive the overall efficiency under the same basic conditions, the input power with Vo changed.
Meanwhile, LED drive after thermal equilibrium has increased overall efficiency, power output at the same conditions, compared to the boot time, the input power will decline.
So, LED driver applications in formulating those needs, you should know LED operating characteristics, avoiding some of the indicators proposed does not meet the operating characteristics of the principle, while avoiding indicators appear far more than the actual demand, excess capacity and quality to avoid the cost of waste.
4. Test Failure
There was a lot of customers purchasing brand LED driver, but all samples failed during testing. Later, the site analysis found that customers using self-dual regulator directly to the LED driver power supply for testing, the power regulator will gradually upwards from 0Vac rated working voltage of the LED driver.
Such a test operation, it is easy to make the LED driver when the input voltage to start small and work load, but this situation will lead to far greater than the rated input current, internal inputs related devices such as fuses, rectifier bridge, thermistor or overheating due to excessive current failure, resulting in drive failure.
Therefore, the correct test is transferred to the regulator LED driver rated working voltage range, connect the drive power-on test.
Of course, you can also improve the design technically circumvent failures caused by misuse of this test: the driver input voltage limiting circuit is set to start and input undervoltage protection circuit. When the input is set to start the drive voltage is not reached, the drive does not work; when the input voltage drops to the input undervoltage protection point, drive into the protected status.
Therefore, even if the customer testing process is still using self-dual regulator steps, the drive for self-protection function and not fail. However, customers must carefully before testing understanding purchased LED driver products whether they have this protection (taking into account the practical application of LED driver environment, for most LED driver does not have this protection).
5. different loads and different test results
When the LED driver with LED lamp test, the results of normal, with electronic load test, the results may be abnormal. This phenomenon is usually for the following reasons:
(1) drives the output instantaneous voltage or power beyond the scope of electronic load meter. (Especially in the CV mode, the maximum test load power should not exceed 70% of maximum power, otherwise the load when the load may be transient over-power protection, resulting in the drive does not work or loaded.)
(2) the characteristics of the electronic load instrument used measure does not apply to a constant current source, load voltage gear transitions occur, resulting in the drive inoperable or load properly.
(3) because the electronic load instrument input inside there will be a large capacitance, the test is equivalent to the output of the driver of a large capacitor in parallel may cause the drive current sampling work instability.
Because LED driver design is to meet the performance characteristics of LED lamps, the closest to the actual real-world applications and test methods should be used as a load LED lamp beads, string on the ammeter and voltmeter to test.
6. The following conditions often occur will cause damage to the LED driver:
· The AC received the DC output of the driver, causing the drive failure;
· The AC received a DC / DC drive input or output, leading to failure of the drive;
· The constant current output and adjust the light received together, cause the drive to fail;
· The phase wire to the ground line, resulting in no output drive and live shell;
7. phase lines are wrong
Outdoor engineering applications are usually three-phase four-wire system, with the national standard, for example, rated working voltage of each phase line with the zero line between the 220Vac, the voltage between the phase line and phase line is 380Vac. If construction workers will drive the input end to the two-phase line, then after power, LED driver input voltage exceeding cause product failure.
3, V1 represents the first phase voltage, V2 represents the second phase voltage, R1 and R2 each represent a normal installation to the LED driver circuit. When the neutral line (N) as shown disconnected, the two branches that drives R1, R2 connected in series after receiving the equivalent voltage 380Vac. Because the input resistance differences, when one of the drives to start charging, the resistance becomes smaller, the voltage may be applied to another majority on a drive, leading to its failure overvoltage damage. Therefore recommended that the same power support on the road, along with a switch or circuit breaker to be broken, not just disconnect the zero line. Distribution fuse not on the zero line, the line to avoid the zero line is bad.
8. power fluctuations beyond the scope of a reasonable range
When the grid with a transformer branch wiring too long branches when a large power plant, in the large equipment stops, the grid voltage volatility, instability and even lead to the power grid. There may be damage to the drive when the grid instantaneous voltage exceeds 310Vac (even if there is lightning protection device is also invalid because lightning protection device is to deal with tens of uS level spikes, and power fluctuations may reach tens of mS, even hundreds mS). Therefore, street lighting power line with a large branch electrical machinery to pay special attention, power fluctuations, or a separate power supply transformer monitoring at best.
9. Line frequent trips
With an access road lights too much, resulting in a phase electrical load on overload, uneven distribution of power between the various phases, thus causing frequent tripping circuit.
10. Drive cooling
When the drive is installed in a non-ventilated environment, you should try to drive housing in contact with the lamp housing, conditions permitting, at the contact surface of the housing and lamp shell coated thermal glue or paste thermal pad to improve the thermal performance of the drive to ensure the drive life and reliability.
In summary LED driver has a lot of details need to pay attention to a lot of questions need to advance the analysis, adjustments, failure and avoid unnecessary losses in practical applications!