Ten Reasons For LED Driver Failure

- Apr 22, 2018-

    Basically speaking, the main function of the LED driver is to convert the input AC voltage source into a current source whose output voltage can change 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 with related technologies such as LED driving and customer application experience, and analyzes and analyzes many failures in the design and application of lamps and lanterns:

    1, did not consider the LED lamp beads Vf range of changes, resulting in low efficiency lamps, or even unstable

    The LED luminaire load terminal is generally composed of a number of LED strings connected in parallel. The operating voltage is Vo=Vf*Ns, where Ns represents the number of LEDs connected in series. The Vf of the LED fluctuates with temperature. Generally, Vf becomes lower at high temperatures and Vf becomes higher at low temperatures when constant current is caused. Therefore, the LED lamp load operating voltage corresponds to VoL at a high temperature, and the LED lamp load operating voltage corresponds to VoH at a low temperature. When selecting the 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, considering the overall 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, did not consider the power margin and derating requirements

    Under normal circumstances, the nominal power of the LED driver refers to the data measured under rated conditions and rated voltages. 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 versus input voltage derating curves).

    As shown in Figure 1, the red curve represents the power derating curve of the load of the LED driver under the input of 120Vac, which 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 of the load of the LED driver when the input 230Vac or 277Vac is input, and the principle is the same.

    As shown in Figure 2, the blue curve represents the derating curve of the LED driver's output power as a function of the input voltage at an ambient temperature of 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 input loss to increase, the efficiency to decrease, and the device temperature rising. Individual temperature points will 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, comprehensively consider and select, and leave derating allowance appropriately.

    3, do not understand the operating characteristics of the LED

    There were customers who requested that the lamp input power be a fixed value, fixed 5% error, and the output current can only be adjusted to the specified power for each lamp. 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 varies with the LED load's series voltage Vo, and the input power varies with Vo when the driver's overall efficiency is basically constant.

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

    Therefore, when designing the demand, the LED driver's application 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 avoid indicators that far exceed the actual requirements to avoid excessive quality and cost.

    4, failure in the test

    There were customers who had purchased many brands of LED drivers, but all samples failed during the test. Later, after analyzing the scene, it was found that 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 working voltage of the LED driver.

    This test operation makes it easy for the LED driver to start up and load with a small input voltage. This situation can result in input currents much larger than the rated values. Internal input-side devices such as fuses, bridge rectifiers, etc. 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 startup 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, different test results

    When the LED driver is tested with an LED light, the result is normal and when the test is conducted with an electronic load, the result 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 power of the load, otherwise the load may momentarily overpower protection during loading, causing the drive to fail to work or load.)

    (2) The characteristics of the electronic load meter used are not suitable for measuring the constant current source, and the jump of the load voltage level 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 to meet the operating characteristics of the LED lamp, the test method closest to the actual and real application should be to use the LED lamp beads as the load, and the ammeter and voltage meter on the string to test.

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

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

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

    (3) The constant current output is connected with the dimming light, causing the driver to fail;

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

    7, the wrong phase

    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 neutral 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 and the product fails.

    As shown in the above figure, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 represent the LED drivers 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. The fluctuation range of the power grid exceeds a reasonable range

    When the wiring of the same transformer network branch is too long and there are large-scale power equipment in the branch, when the large-scale equipment is started or stopped, the voltage of the power grid will fluctuate violently and even lead to instability of the power grid. 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 of 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, line tripping

    Too many lights on the same branch lead to overload on one phase of electricity, and uneven power distribution between the phases, resulting in frequent tripping of the line.

    10, drive cooling

    When the driver is installed in a non-ventilated environment, the driver housing should be contacted with the lamp housing as much as possible. When conditions permit, heat-conductive adhesive or thermal pad is applied on the contact surface between the housing and the lamp housing to improve the heat dissipation performance of the driver, thereby ensuring the driver's performance. Life and reliability.

    To sum up, there are many details in the practical application of the LED driver, and many problems need to be analyzed and adjusted in advance to avoid unnecessary failures and losses!

Dalian Xinghai Technology Co., Ltd.