10 Reasons May Lead to LED Driver Failure

Basically, it can be said that the main function of the LED Driver is to convert the input AC voltage source to an output voltage that can follow the LEDVf (current source with a positive conduction voltage drop. As a key component in LED lighting, the quality of the LED driver directly affects The reliability and stability of the overall luminaire.From the LED drive and other related technologies and customer application experience, this article collates and analyzes many failures in the luminaire design and application:

1, did not consider the LED lamp beads Vf range of changes, resulting in low efficiency lamps, or even unstable LED lighting load terminals, generally consists of a number of LED strings connected in parallel, its operating voltage Vo = Vf * Ns, where Ns represents the number of LED series . The Vf of the LED fluctuates with temperature. Generally, at a constant current, Vf becomes lower at a high temperature, and Vf becomes higher at a low temperature. Therefore, the LED lamp load operating voltage corresponds to VoL at high temperatures, and the LED lamp load operating voltage corresponds to VoH at low temperatures. When choosing 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 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 temperatures. Unstable, lighting fixtures may flicker.

However, in consideration of comprehensive cost and efficiency, it is impossible to blindly pursue the ultra-wide output voltage range of LED drivers: because the driver voltage is only in a certain range, the driver efficiency is the highest. After the range is exceeded, the efficiency and power factor (PF) will be degraded. At the same time, the output voltage range of the driver is designed to be too wide. As a result, the cost is increased and the efficiency cannot be optimized.

2. Power margin and derating requirements are not considered Under normal circumstances, the nominal power of the LED driver refers to the data measured under rated environment and rated voltage. Considering that different customers have different applications, most LED driver suppliers will provide power derating curves on their own product specifications (common load versus ambient temperature derating curves and load vs input voltage derating curves).

The power derating curve of load vs. ambient temperature in Figure 1 is shown in Figure 1. The red curve represents the power derating curve of the LED driver when the input is 120Vac and the load changes with the ambient temperature. When the ambient temperature is lower than 50°C, the driver is allowed 100% full load. When the ambient temperature reaches 70°C, the driver can only derate to 60% of the load. When the ambient temperature changes between 50-70°C, the driver load varies. The temperature rises linearly.

The blue curve represents the power derating curve where the load of the LED driver changes with the ambient temperature when the input is 230Vac or 277Vac. The principle is the same.

The power derating curve of load vs. input voltage of Figure 2 is shown in Figure 2. The blue curve represents the derating curve of the output power of the LED driver when the ambient temperature is 55°C. When the input voltage is 140Vac, the load of the driver is allowed to be 100% full load, and the input voltage is adjusted downward; if the output power is constant, the input current will increase, causing the loss of the input end to increase, the efficiency to decrease, the temperature of the device to rise, and individual temperature points will be It may exceed the standard and may even cause the device to fail.

Therefore, as shown in Figure 2, when the input voltage is less than 140Vac, the output load of the driver is required to decrease linearly with decreasing input voltage. After understanding the above derating curves and corresponding requirements, when selecting an LED driver, it should be based on the actual ambient temperature conditions and the input voltage conditions, comprehensive consideration and selection, and leave allowances properly.

3, do not understand the characteristics of the LED's work had customer requirements lamp input power is a fixed value, a fixed 5% error, can only be adjusted for each lamp output current to achieve the specified power. Due to the different working environment temperatures and different lighting times, the power of each lamp will be quite different.

The client makes such a request, although there are considerations for its marketing and business factors. However, the LED's volt-ampere characteristic determines that the LED driver is a constant current source and its output voltage changes with the LED load's series voltage Vo. With the driver's overall efficiency basically unchanged, the input power varies with Vo.

At the same time, the overall efficiency of the LED driver will increase after thermal equilibrium. At the same output power, the input power will drop compared to the startup time.

Therefore, when designing a demand, an LED driver user should first understand the operating characteristics of the LED, avoid suggesting some indicators that do not conform to the principle of the working characteristics, and at the same time avoid indicators that far exceed the actual requirements and avoid excessive quality and cost.

4. Failure in testing Once customers had purchased many brands of LED drivers, but all samples failed during the test. Later, after field analysis, the customer used a self-coupler voltage regulator to directly supply power to the LED driver. After power-on, the voltage regulator was gradually increased from 0 Vac to the rated operating voltage of the LED driver.

This test operation makes it easy for the LED driver to start up and carry the load at a very small input voltage. This situation will cause the input current to be much larger than the rated value. The internal input-related devices, such as fuses, rectifier bridges, Thermistors, etc., fail due to excessive current or overheating, causing the driver to fail.

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

Of course, technically improved design can also avoid the failure problem caused by this type of test misoperation: set the start voltage limit 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 operate; when the input voltage drops to the input undervoltage protection point, the driver enters the protection state.

Therefore, even if the customer's test process is still using the operating steps of the self-coupling regulator, the driver is self-protecting and does not fail. However, customers must carefully understand whether the purchased LED driver products have this protection function before testing (taking into account the actual application environment of LED drivers, most LED drivers currently do not have this protection function).

5, different loads, test results different LED driver with LED lamp test, the result is 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. (In CV mode in particular, the maximum test power should not exceed 70% of the maximum power of the load, otherwise the load may momentarily overpower protection during loading, causing the drive to fail to operate or load.)

(2) The characteristics of the electronic load meter used are not suitable for measuring the constant current source, and the load voltage level jump occurs, causing the driver to fail to work or load.

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

Because the LED driver design is in order to meet the LED lamp operating characteristics, the test method closest to the actual and real application should be to use LED lamp beads as the load, string ammeter and voltmeter to test.

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

• AC is connected to the DC output of the driver, causing the driver to fail;

• AC is connected to the input or output of the DC/DC driver, causing the driver to fail;

• The constant current output is connected to the dimming light, causing the driver to fail;

· Connect the phase wire to the ground wire, resulting in no output of the driver and energized shell;

7. The phase line connection error Usually, 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 220Vac, and the voltage between the phase line and phase line is 380Vac. If the construction worker connects the driver input to the two phases, after the power is turned on, the LED driver input voltage exceeds the standard, causing the product to fail.

Fig. 3 Open circuit of neutral line As shown in Fig. 3, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 respectively represent the LED driver normally installed in the line. When the neutral line (N) on the line is disconnected as shown in the figure, the drivers R1 and R2 on the two branches are connected in series and connected to the 380Vac voltage. Because of the difference in input resistance, when one of the drivers is charged to startup, the internal resistance becomes smaller, and the voltage may be mostly applied to another driver, resulting in failure of its overvoltage damage. Therefore, it is recommended that the switches or circuit breakers on the same distribution branch should be broken together, and it is not possible to disconnect only the neutral line. Do not place the power distribution fuse on the neutral line, and avoid contact with the neutral wire on the line.

8. When the power grid fluctuating range exceeds the reasonable range When the wiring of the same transformer power grid branch is too long, and there are large-scale power equipment in the branch, when the large-scale equipment is started or stopped, the power grid voltage will fluctuate violently and even cause the power grid to be unstable. When the instantaneous voltage of the power grid exceeds 310Vac, the driver may be damaged (even if there is a lightning protection device, since the lightning protection device responds to pulse spikes of several tens uS level, the power grid fluctuation may reach tens of mS or even several hundred mS). Therefore, special attention should be paid to the large-scale electric machinery on the street lighting branch power network. It is better to monitor the amplitude of power grid fluctuations, or to supply power to separate grid transformers.

9. Frequent tripping of the lights on the same branch of the circuit leads to overloading of the load on one phase and uneven distribution of power between the phases, resulting in frequent line tripping.

10, driver heat dissipation When the driver is installed in a non-ventilated environment, it should try to contact the driver shell and the lamp shell, if conditions permit, the shell and the lamp shell contact surface coated with thermal paste or thermal pad to improve the drive's thermal performance, This guarantees the longevity and reliability of the drive.

In summary, there are many details to be noted in practical applications of the LED driver. Many problems need to be analyzed and adjusted in advance to avoid unnecessary failures and losses! Text / Shenzhen LED base