Basically, it can be said that 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 LED Vf forward voltage drop.
As a key component in LED lighting, the quality of the LED driver directly affects the reliability and stability of the overall lamp. This article starts from related technologies such as LED drivers and customer application experience, and organizes and analyzes many failures in lamp design and application:
1. The range of LED lamp bead Vf is not considered, resulting in low efficiency of the lamp and even unstable operation
The load end of LED lamps is generally composed of a number of LED strings connected in parallel, and its operating voltage Vo=Vf*Ns, where Ns represents the number of LEDs connected in series. The Vf of an LED varies with temperature. Generally, when the cause is a constant current, Vf becomes low at high temperatures and Vf becomes high at low temperatures. Therefore, the working voltage of LED lamps at high temperature corresponds to VoL, and the working voltage of LED lamps at low temperature corresponds to VoH. When choosing an LED driver, it is necessary to consider that the output voltage range of the driver 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 at low temperature may not reach the actual required power. 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 temperature. Unstable, the lamp will flicker and so on.
However, considering the overall cost and efficiency, the ultra-wide output voltage range of the LED driver cannot be pursued blindly: because the driver voltage is only in a certain range, the driver efficiency is the highest. After exceeding the range, the efficiency and power factor (PF) will deteriorate. At the same time, the design of the driver’s output voltage range is too wide, which will lead to increased costs and the efficiency cannot be optimized.
2. No consideration of power margin and derating requirements
In general, the nominal power of the LED driver refers to the data measured under the rated environment and rated voltage. Considering that different customers will have different applications, most LED driver suppliers will provide power derating curves in their own product specifications (common load vs. ambient temperature derating curve and load vs. input voltage derating curve).
As shown in Figure 1, the red curve represents the power derating curve of the LED driver’s load with the ambient temperature when the 120Vac input is applied. When the ambient temperature is lower than 50℃, the driver allows 100% full load. When the ambient temperature is up to 70℃, the driver can only be derated to 60% load. When the ambient temperature changes between 50-70℃, the driver load The temperature rises and decreases linearly.
The blue curve represents the power derating curve of the LED driver with 230Vac or 277Vac input, and its load changes with the ambient temperature. The principle is similar.
As shown in Figure 2, the blue curve represents the derating curve of the LED driver’s output power with the input voltage when the ambient temperature is 55℃. When the input voltage is 140Vac, the load of the driver is allowed to be 100% full, and the input voltage will be adjusted down with the input voltage. If the output power is unchanged, the input current will increase, resulting in increased input loss, reduced efficiency, increased device temperature, and individual temperature points. May exceed the standard, and may even cause device failure.
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 as the input voltage decreases. After understanding the above derating curve and the corresponding requirements, when selecting the LED driver, comprehensive consideration and selection should be made according to the actual ambient temperature and input voltage conditions, and the derating margin should be appropriately reserved.
3. I don’t understand the working characteristics of LED
Some customers have requested that the input power of lamps be a fixed value with a fixed 5% error, and the output current can only be adjusted for each lamp to reach the specified power. Due to different working environment temperature and different lighting time, the power of each lamp will still be quite different.
The client made such a request, although there are considerations for its marketing and business factors. However, the LED’s volt-ampere characteristics determine that the LED driver is a constant current source. Its output voltage varies with the LED load series voltage Vo. When the overall efficiency of the driver is basically unchanged, its input power varies with Vo.
At the same time, the overall efficiency of the LED driver will increase after thermal equilibrium. Under the same output power, the input power will decrease compared to the start-up time.
Therefore, when formulating requirements, LED driver users should first understand the operating characteristics of LEDs, avoid putting forward some indicators that do not meet the principle of operating characteristics, and avoid indicators that far exceed the actual requirements, to avoid excess quality and cost waste.
4. Failure during testing
Once customers have purchased many brands of LED drivers, but all samples failed during the test. Later, after on-site analysis, it was found that the customer used a self-coupled voltage regulator to directly supply power to the LED driver for testing. After power-up, the voltage regulator was gradually increased 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 work with load at a small input voltage, and this situation will cause the input current to be much greater than the rated value. The internal input related devices, such as fuses, rectifier bridges, Thermistors, etc., fail due to excessive current or overheating, resulting in drive failure.
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 power-on test.
Of course, technically improving the design can also avoid the failure problem caused by such test misoperation: setting the starting voltage limiting circuit and the input undervoltage protection circuit at the input end of the driver. When the input does not reach the starting 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 customer still adopts the operation steps of the auto-regulator, the driver has self-protection function and will not fail. However, customers must carefully understand whether the purchased LED driver products have this protection function before testing (considering the actual application environment of LED drivers, most LED drivers currently do not have this protection function).
5. Different loads, different test results
LED driver with LED lamp test results are normal, with electronic load test, the results may be abnormal. Usually this phenomenon has the following reasons:
(1) The output voltage or power 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 power of the load, otherwise the load may momentarily overpower the protection while 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 load voltage level jump occurs, which causes the drive to fail to work or load.
(3) Because there will be a large capacitor inside the input of the electronic load meter, the test is equivalent to a large capacitor connected in parallel to the output of the driver, which may cause the current sampling of the driver to be unstable.
Because the design of the LED driver is to comply with the working characteristics of the LED luminaire, the closest test method to the actual and real application should be to use the LED lamp beads as a load, and connect the ammeter and voltmeter to test.
6. The following conditions often occur will cause damage to the LED driver:
(1) Connect AC to the DC output terminal of the driver, causing the driver to fail;
(2) Connect the AC to the input or output of the DC/DC driver, causing the driver to fail;
(3) Connected the constant current output terminal with the dimming light, which caused the driver to fail;
(4) Connected the phase wire to the ground wire, resulting in no output of the driver and energization of the housing;
7. The phase wire is wrong
Usually outdoor engineering applications are 3-phase four-wire system. Taking the national standard as an example, the rated operating voltage between each phase and neutral is 220Vac, and the voltage between phase and phase is 380Vac. If the construction worker connects the driver input to the two phase wires, after the power is turned on, the LED driver input voltage exceeds the standard and the product fails.
As shown in the figure above, V1 represents the voltage of the first phase, V2 represents the voltage of the second phase, and R1 and R2 respectively represent the LED drivers normally installed in the circuit. When the neutral line (N) on the line is disconnected as shown, the drivers R1 and R2 on the two branches are equivalent to 380Vac after they are connected in series. Because of the difference in input internal resistance, when one of the drivers is charged to startup, the internal resistance becomes smaller, and most of the voltage may be applied to the other driver, causing its overvoltage damage to fail.
Therefore, it is recommended that switches or circuit breakers should be broken together on the same distribution branch circuit, not only the zero line. Do not put the distribution fuse on the neutral line, and avoid poor contact of the neutral line on the line.
8. The fluctuation range of the power grid exceeds the reasonable range
When the wiring of the same transformer network branch is too long and there are large power equipment in the branch, the grid voltage will fluctuate drastically when the large equipment starts and stops, or even cause the grid to be unstable. When the instantaneous voltage of the grid exceeds 310Vac, the drive may be damaged (even if there is a lightning protection device, it is invalid, because the lightning protection device is to deal with pulse spikes of tens of uS level, and the grid fluctuation may reach tens of mS or even hundreds of mS).
Therefore, special attention should be paid when there are large electric machinery on the street lighting branch network. It is best to monitor the fluctuation range of the power grid or supply it by a separate power grid transformer.
9. Line trips frequently
Too many lights on the same branch lead to overload on a certain phase, and uneven power distribution among the phases, resulting in frequent tripping of the line.
When the driver is installed in a non-ventilated environment, the driver housing should be in contact with the lamp housing as much as possible. If possible, apply thermal adhesive or paste a thermal pad on the contact surface of the housing and the lamp housing to improve the heat dissipation performance of the driver, thereby ensuring the driver’s heat dissipation performance. Life and reliability.
In summary, there are many details of the LED driver in actual application that need attention. Many problems need to be analyzed and adjusted in advance to avoid unnecessary failures and losses!