Basically, it can be said that the main function of the LED drive power supply is to convert the input AC voltage source into a current source whose output voltage can vary with the LED Vf (forward conduction voltage drop. As a key component in LED lighting, the quality of the LED drive power supply directly Affects the reliability and stability of the overall luminaire. Starting from the LED driver and other related technologies and customer application experience, this article organizes and analyzes many failure situations in the design and application of luminaires:
▲The variation range of LED lamp bead Vf is not considered, resulting in low lamp efficiency and even unstable operation
The load end of the LED lamp is generally composed of a number of LEDs in series and parallel, and its working voltage Vo=Vf*Ns, where Ns represents the number of LEDs in series. The Vf of the LED fluctuates with temperature. Generally, at a constant current, Vf becomes lower at high temperature, and Vf becomes higher at low temperature. Therefore, the LED lamp load working voltage corresponds to VoL at high temperature, and the LED lamp load working voltage corresponds to VoH at low temperature. When selecting an LED drive power supply, it should be considered that the output voltage range of the drive power supply is greater than VoL~VoH.
If the maximum output voltage of the selected LED driving power supply 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 driving power supply is higher than VoL, the output of the driving power supply may exceed the working power at high temperature. range, unstable operation, and flickering of lamps and lanterns.
However, considering the overall cost and efficiency, we cannot blindly pursue the ultra-wide output voltage range of the LED drive power supply: because the drive power supply voltage is only in a certain range, the drive power supply efficiency is the highest. After the range is exceeded, the efficiency and power factor (PF) will deteriorate. At the same time, the design of the output voltage range of the driving power supply is too wide, which will lead to higher costs and the efficiency cannot be optimized.
▲Do not understand the working characteristics of LED
A customer once required the input power of the lamps to be a fixed value, with a fixed 5% error, and the output current could only be adjusted for each lamp to reach the specified power. Due to different working environment temperatures and different lighting times, the power of each lamp will still be quite different.
The client put forward such a request, although there are considerations of its marketing and business factors. However, the volt-ampere characteristics of the LED determine that the LED drive power supply is a constant current source, and its output voltage changes with the LED load series voltage Vo. Under the condition that the overall efficiency of the drive power supply is basically unchanged, its input power changes with Vo.
At the same time, the overall efficiency of the LED drive power supply will increase after thermal equilibrium. Under the condition of the same output power, the input power will decrease compared to the power-on time.
Therefore, when making demands for LED drive power, users should first understand the working characteristics of LEDs, avoid putting forward some indicators that do not conform to the principle of working characteristics, and at the same time avoid indicators that far exceed actual needs, and avoid excess quality and cost waste.
▲Failure 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, it was found that the customer used an auto-coupled voltage regulator to directly supply power to the LED drive power supply for testing. After power-on, the voltage regulator was gradually increased from 0Vac to the rated working voltage of the LED drive power supply.
Such a test operation can easily make the LED drive power supply start and work with load at a small input voltage, and this situation will cause the input current to be much larger than the rated value, and internal input related devices, such as fuses, rectifier bridges , thermistor, etc. fail due to excessive current or overheating, resulting in the failure of the drive power supply.
Therefore, the correct test method is to adjust the voltage regulator to the rated working voltage range of the LED driving power supply, and then connect the driving power supply for power-on test.
Of course, technically improving the design can also avoid the failure problem caused by this kind of test misoperation: set up a startup voltage limit circuit and an input undervoltage protection circuit at the input end of the drive power supply. When the input does not reach the starting voltage set by the driving power supply, the driving power supply does not work; when the input voltage drops to the input undervoltage protection point, the driving power supply enters the protection state.
Therefore, even if the operation steps of the auto-voltage regulator are still used in the customer's test process, the driving power supply has a self-protection function and will not fail. However, before testing, customers must carefully understand whether the purchased LED drive power products have this protection function (considering the actual application environment of LED drive power supplies, most LED drive power supplies currently do not have this protection function).
▲ Different loads, different test results
When the LED driver is tested with LED lights, the result is normal, but when it is tested 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 driving power supply exceeds the working range of the electronic load instrument. (Especially in CV mode, the maximum test power should not exceed 70% of the maximum power of the load, otherwise the load may be temporarily over-power protected during loading, causing the drive power to fail to work or load normally.)
(2) The characteristics of the electronic load meter used are not suitable for measuring the constant current source, and the load voltage gear jumps, causing the drive power to not work or load normally.
(3) Because there will be a large capacitor inside the input of the electronic load instrument, the test is equivalent to connecting a large capacitor in parallel with the output of the driving power supply, which may cause the current sampling of the driving power supply to be unstable.
Because the LED driver power supply is designed to meet the working characteristics of LED lamps, the test method closest to the actual and real application should be to use the LED lamp beads as the load, and connect the ammeter and voltmeter to test.
▲The following conditions that often occur will cause damage to the LED drive power supply:
The AC is connected to the DC output terminal of the driving power supply, resulting in the failure of the driving power supply;
The AC is connected to the input or output of the DC/DC drive power supply, resulting in the failure of the drive power supply;
The constant current output terminal and the dimming light are connected together, resulting in the failure of the driving power supply;
·The phase wire is connected to the ground wire, resulting in no output of the drive power supply and the shell is charged;