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:
First, the range of variation of the LED lamp bead Vf is not considered, resulting in low efficiency of the lamp and even unstable operation.
The load end of the LED lamp is generally composed of a number of LED strings connected 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 fluctuation. In general, when a constant current is caused, Vf becomes low at a high temperature, and Vf becomes high at a low temperature. Therefore, the operating voltage of the LED lamp load corresponds to VoL at high temperature, and the operating voltage of the LED lamp 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 comprehensive cost and efficiency considerations, the ultra-wide output voltage range of the LED driver cannot be pursued blindly: the driver efficiency is the highest because the driver voltage is only in a certain interval. 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.
Second, the power headroom and derating requirements are not considered
In general, the nominal power of the LED driver refers to the measured data under the rated environment and rated voltage. Considering that different customers have different applications, most LED driver suppliers will provide power derating curves (common load vs ambient temperature derating curve and load vs input voltage derating curve) on their own product specifications.
Third, do not understand the working characteristics of LED
Some customers have requested that the input power of the lamp be a fixed value, fixed 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 characteristic of the LED determines 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.
Fourth, failure in the 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 supply power to 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, and this situation causes 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. (In view of the actual application environment of LED drivers, most LED drivers do not have this protection function at present.)
Five, different loads, different test results
When the LED driver is tested with an LED lamp, the result is normal. When the test is carried out with an electronic load, the result may be abnormal. Usually this phenomenon has the following reasons:
(1) The instantaneous 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 capacitance inside, the test is equivalent to a large capacitor connected in parallel with the output of the driver, which may cause instability of the current sampling operation of the driver.
Because the LED driver design is to meet the working characteristics of LED lamps, the closest test method to actual and real applications should be to use LED lamp beads 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 DC/DC driver, causing the drive to fail;
(3) The constant current output terminal is connected with the modulating light, which causes the driver to fail;
(4) The phase line is connected to the ground line, resulting in no output of the driver and charging of the outer casing;
Seven, the phase line is wrong
Generally, the outdoor engineering application is a 3-phase four-wire system. Taking the national standard as an example, the rated working voltage between each phase line and the neutral line is 220Vac, and the voltage between the phase line and the phase line is 380Vac. If the construction worker connects the input of the drive to the two phase lines, the power input of the LED driver exceeds the standard and the product fails.
Eight, the grid fluctuation range is beyond the reasonable range
When the wiring of the same transformer grid branch is too long and there are large power equipment in the branch, when the large equipment starts and stops, the grid voltage will fluctuate drastically, and even the grid will be unstable. When the instantaneous voltage of the power grid exceeds 310Vac, it may damage the driver (even if there is a lightning protection device, because the lightning protection device is dealing with pulse spikes of several tens of uS level, and the power grid fluctuation may reach several tens of mS, even several hundred mS).
Therefore, special attention should be paid to the large-scale electric machinery on the street lighting branch network. It is best to monitor the fluctuation range of the power grid or the power supply of the separate grid transformer.
Nine, the line frequently trips
The lamps on the same branch are connected too much, resulting in overloading of the load on one phase and uneven distribution of power between the phases, causing the lines to trip frequently.
Ten, drive cooling
When the drive is installed in a non-ventilated environment, the drive housing should be in contact with the lamp housing as much as possible. If possible, apply thermal conductive adhesive or thermal pad on the contact surface between the outer casing and the lamp housing to improve the heat dissipation performance of the driver, thus ensuring the driver's Life and reliability.
In summary, the LED driver has many details to be noted in practical applications. Many problems need to be analyzed and adjusted in advance to avoid unnecessary failures and losses!
Editor: Yingzi
Tungsten Molybdenum Fabricated
Shaanxi Xinlong Metal Electro-mechanical Co., Ltd. , https://www.cnxlalloys.com