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Ten reasons why LED drivers fail

Feb 19, 2019

Ten reasons why LED drivers fail

Basically, the main function of the LED driver is to convert the input AC voltage source into a current source whose output voltage can vary with the forward voltage drop of the LED Vf.

 

As a key component in LED lighting, the quality of the LED driver directly affects the reliability and stability of the overall luminaire. This article starts from LED driver and other related technologies and customer application experience, and analyzes many failures in lamp design and application:

1. The range of variation of LED lamp bead Vf is not considered, resulting in low efficiency of the lamp and even unstable operation.

The load end of the LED luminaire is generally composed of a number of LED strings in parallel, and its working voltage is Vo=Vf*Ns, where Ns represents the number of LEDs connected in series. The Vf of the LED fluctuates with temperature fluctuations. In general, Vf becomes low at high temperatures and Vf becomes high at low temperatures when a constant current is caused. Therefore, the operating voltage of the LED luminaire at high temperature corresponds to VoL, and the operating voltage of the LED luminaire at low temperature corresponds to VoH. When selecting an LED driver, consider that the driver 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 luminaire may not reach the actual power required 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, the lamp will flash and so on.

However, considering the overall cost and efficiency considerations, the LED driver's ultra-wide output voltage range cannot be pursued: because the driver voltage is only in a certain interval, the driver efficiency is the highest. After the range is exceeded, the efficiency and power factor (PF) will be worse. At the same time, the output voltage range of the driver is too wide, which leads to cost increase and efficiency cannot be optimized.

2. Lack of consideration of Power reserve and derating requirements

In general, the nominal power of an LED driver is the measured data at rated ambient and rated voltage. Given the different applications that different customers have, most LED driver suppliers will provide power derating curves on their own product specifications (common load versus ambient temperature derating curve and load vs. input voltage derating curve).

3. Do not understand the working characteristics of LED

Some customers have requested that the input power of the lamp be a fixed value, fixed by 5% error, and the output current can only be adjusted to the specified power for each lamp. Due to different working environment temperatures and lighting times, the power of each lamp will vary greatly.

Customers make such requests, despite their marketing and business factor considerations. However, the volt-ampere characteristics of the LED determine that the LED driver is a constant current source, and its output voltage varies with the LED load series voltage Vo. The input power varies with Vo when the overall efficiency of the driver is substantially constant.

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 to the startup time.

Therefore, when the LED driver application needs to formulate the requirements, it should first understand the working characteristics of the LED, avoid introducing some indicators that do not conform to the principle of the working characteristics, and avoid the indicators far exceeding the actual demand, and avoid excessive quality and waste of cost.

4. Invalid during test

There have been customers who have purchased many brands of LED drivers, but all samples failed during the test. Later, after on-site analysis, the customer used the self-adjusting voltage regulator to directly test the power supply of the LED driver. After power-on, the regulator was gradually upgraded from 0Vac to the rated operating voltage of the LED driver.

Such a test operation makes it easy for the LED driver to start and load at a small input voltage, which would cause the input current to be much larger than the rated value, and the internal input related devices such as fuses, rectifier bridges, The thermistor and the like fail due to excessive current or overheating, causing the drive to fail.

Therefore, the correct test method is to adjust the voltage regulator to the rated operating voltage range of the LED driver, and then connect the driver to the power-on test.

Of course, technically improving the design can also avoid the failure caused by such test misoperation: setting the startup voltage limiting circuit and the input undervoltage protection circuit at the input of the driver. When the input does not reach the startup 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 if the self-recommended regulator operation steps are still used during the customer test, the drive has self-protection function and does not fail. However, customers must carefully understand whether the LED driver products purchased have this protection function before testing (taking into account the actual application environment of the LED driver, most LED drivers do not have this protection function).

5. Different loads, different test results

When the LED driver is tested with LED light, the result is normal, and with the electronic load test, the result may be abnormal. Usually this phenomenon has the following reasons:

(1) The output voltage or power of the output 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 over-power protected during loading, causing the drive to not work or load.

(2) The characteristics of the electronic load meter used are not suitable for measuring the constant current source, and the load voltage position jump occurs, resulting in the drive not working or loading.

(3) Because the input of the electronic load meter will have a large internal capacitance, the test is equivalent to a large capacitor connected in parallel with the output of the driver, which may cause unstable current sampling of the driver.

Because the LED driver is designed to meet the operating characteristics of LED luminaires, the closest test to actual and real-world applications should be to use LED bead as the load, string on the ammeter and voltmeter to test.

6. The following conditions that often occur can cause damage to the LED driver:

(1) The AC is connected to the DC output of the driver, causing the drive to fail;

(2) The AC is connected to the input or output of the DCs/DC drive, causing the drive to fail;

(3) The constant current output end and the tuned light are connected together, resulting in the drive failure;

(4) The phase line is connected to the ground wire, resulting in the drive without output and the shell charged;

7. Wrong connection of Phase line

Usually outdoor engineering applications are 3-phase four-wire system, with the national standard as an example, each phase line and 0 line between the rated operating voltage is 220VAC, phase line and phase line between the voltage is 380VAC. If the construction worker connects the drive input to two phase lines, the LED driver's input voltage is exceeded after the power is powered on, causing the product to fail.

 

8. The power grid fluctuation range beyond the reasonable range

When the same transformer grid branch wiring is too long, there are large power equipment in the branch, when the large equipment start and stop, the power grid voltage will fluctuate wildly, and even lead to instability of the power grid. When the instantaneous voltage of the grid exceeds 310VAC, it is possible to damage the drive (even if there is a lightning protection device is not effective, because the lightning protection device is to cope with dozens of uS level pulse spikes, while the power grid fluctuation may reach dozens of MS, or even hundreds of ms).

Therefore, street lighting branch power Grid has a large power machinery to pay special attention to, it is best to monitor the extent of the power grid fluctuations, or separate power grid transformer power supply.

 

9. Frequent tripping of lines

The lamp on the same road is connected too much, which leads to the overload of the load on a certain phase, and the uneven distribution of power between the facies, which causes the line to trip frequently.

10. Drive Heat Dissipation

When the drive is installed in a non-ventilated environment, the drive housing should be as far as possible in contact with the luminaire housing, if conditions permit, in the shell and the lamp shell on the contact surface coated with heat conduction glue or affixed heat conduction pad, improve the heat dissipation performance of the drive, thus ensuring the life and reliability of the drive.

 

To sum up, LED drivers in the actual application of a lot of details to pay attention to, many problems need to be analyzed in advance, adjust, to avoid unnecessary failure and loss!