Cause Analysis of LED Driver Failure

- Nov 10, 2017 -

Basically, it can be said that the main function of the LED driver is to convert the input AC voltage source into an output voltage that can be varied with the LED

Vf (forward conduction voltage change current source.As a key component in LED lighting, the quality of LED driver directly affects the reliability and stability of the overall lighting.In this paper, starting from the LED driver and other related technology and customer experience, Finishing analysis of lighting design and application of many failures:

1, did not consider the LED lamp beads Vf range, resulting in low efficiency lamps and even unstable work

LED lamps load terminal, generally composed of a number of LED series and parallel, its operating voltage Vo = Vf * Ns, where Ns said the number of LED series. Vf LED with the temperature changes and changes, under normal circumstances, the constant current, high temperature Vf become low, low temperature Vf becomes high. Therefore, the working voltage of LED lamp load corresponds to VoL at high temperature, and the working voltage of LED lamp load corresponds to VoH at low temperature. In the selection of LED drivers need to consider the driver output voltage range 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 temperature. If the lowest voltage of the selected LED driver is higher than VoL, the driver output may exceed the working range at high temperature, Unstable, there will be flashing lamps and so on.

However, the overall cost and efficiency considerations, can not blindly pursue the LED driver Wide output voltage range: Because the drive voltage is only in a certain interval, the drive efficiency is the highest. Beyond the scope of efficiency, the power factor (PF) will be worse, while the drive output voltage range design is too wide, resulting in increased costs, efficiency can not be optimized.

2, did not consider the power margin and derating requirements

Under normal circumstances, the nominal LED driver power refers to the rated environment, rated voltage measured data. Considering different customers have different applications, most LED driver suppliers provide power derating curves (common load vs ambient temperature derating curves and load vs. input voltage derating curves) in their own product specifications.

3, do not understand the working characteristics of LED

Some customers have asked the lamp input power for a fixed value, 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, the power of each lamp or there will be a big difference.

Customers make such a request, although there are marketing and business considerations. However, the LED's volt-ampere characteristic determines that the LED driver is a constant current source. The output voltage of the LED driver varies with the series load voltage Vo of the LED load. The input power varies with Vo when the overall efficiency of the driver remains the same.

At the same time, the overall efficiency of the LED driver will increase after the thermal balance. Under the same output power, the input power will decrease compared with the startup time.

Therefore, LED driver applications in the development of demand, we should first understand the working characteristics of LED, to avoid making some indicators do not meet the characteristics of the principle of work, while avoiding the need to go far beyond the actual needs of indicators to avoid excess quality and cost waste.

4, test failure

There have been customers who have purchased many brands of LED drivers but all of them failed during the test. Later to the scene analysis found that customers use self-voltage regulator directly to the LED driver power supply test, the voltage regulator from 0Vac power gradually raised to the LED driver rated voltage.

This test operation, it is easy to make the LED driver at a small input voltage to start and carry work, and this situation will lead to the input current is much larger than the rated value, the internal input related devices such as fuses, rectifier bridge, Thermistors, etc. due to excessive current or overheating failure, leading to drive failure.

Therefore, the correct test method is to tune the regulator to the LED driver rated operating voltage range, then connect the drive power test.

Of course, the technical improvement of the design can also avoid the failure caused by misoperation of such a test: setting a start-up voltage limiting circuit and an input under-voltage protection circuit at the input end of the driver. The driver does not operate when the input voltage does not reach the startup voltage set by the driver. When the input voltage drops to the input voltage protection point, the driver enters the protection state.

Therefore, even if the customer is still using self-paced voltage regulator test procedures, the drive has self-protection function without failure. However, before testing, customers must carefully understand the purchased LED driver products have this protection

(Considering the practical application of LED driver environment, most LED drivers do not have this protection function).

5, different loads, different test results

LED driver with LED test, the result is normal, with electronic load test, the result may be abnormal. Usually this phenomenon for the following reasons:

(1) The instantaneous voltage or power output of the driver exceeds the working range of the electronic load cell. (In CV mode, in particular, 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 malfunction or load.

(2) The characteristics of the electronic load device used is not suitable for measuring the constant current source, the load voltage stall occurs, resulting in the driver can not work or load.

(3) Since there is a large capacitance inside the input of the electronic load, the test is equivalent to connecting a large capacitor in parallel with the output of the driver, which may cause instability of the current sampling of the driver.

Because LED driver is designed to meet the working characteristics of LED lamps, the closest practical and real application of the test method should be to use LED lamp beads as a load, ammeter string and voltmeter to test.

6, often the following conditions will lead to LED drive damage:

· AC connected to the drive's DC output, resulting in drive failure;

· AC connected to the DC / DC drive input or output, resulting in drive failure;

· The constant current output and dimming line received together, leading to drive failure;

· Connect the phase to ground, resulting in no output from the drive and live charge on the housing.

7, the wrong line

Usually outdoor engineering applications are 3-phase four-wire system to the national standard, for example, each phase and the zero line between the rated voltage is 220Vac, phase and phase voltage is 380Vac. If the construction workers will drive the input termination to the two phases, the power supply, LED driver input voltage exceeds the standard lead to product failure.

8, the scope of power grid fluctuations beyond the reasonable range

When the wiring of a branch of the same transformer is too long and there is a large power device in the branch circuit, the voltage of the power grid will fluctuate violently when the large equipment is started and stopped, and even the power grid may be unstable. It is possible to damage the drive when the instantaneous voltage of the power grid exceeds 310Vac (lightning protection is not effective because the lightning protection device is capable of handling spikes of several tens uS while the power grid may fluctuate by several tens mS or even hundreds of mS). Therefore, the street lighting branch power grid with large-scale electrical machinery to pay special attention, it is best to monitor fluctuations in the power grid, or a separate grid transformer power supply.

9, the line frequently trip

The lamps on the same branch are connected too much, causing the overload on one phase of the electricity and uneven distribution of power between the phases, which leads to the frequent tripping of the lines.

10, drive cooling

When the driver is installed in a non-ventilated environment, the driver housing should be kept in contact with the lamp housing as much as possible. If possible, apply thermal glue or thermal pad on the contact surface between the housing and the lamp housing to improve the heat dissipation performance of the driver Life expectancy and reliability.

In summary, the LED driver has many details to be noticed in practical application. Many problems need to be analyzed and adjusted in advance to avoid unnecessary failures and losses!

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