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 change with the LED Vf (forward voltage drop. As a key component in LED lighting, the quality of the LED drive power supply is directly Affect the reliability and stability of the overall luminaire. Starting from the LED driver and other related technologies and customer application experience, this article sorts out and analyzes many failures in the design and application of the luminaire:
▲The Vf variation range of LED lamp beads 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 LED strings in parallel, and its working voltage Vo=Vf*Ns, where Ns represents the number of LEDs connected in series. The Vf of the LED changes with temperature. Generally, at a constant current, the Vf becomes lower at high temperatures, and Vf becomes higher at low temperatures. 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 the LED drive power supply, it is necessary to consider that the output voltage range of the drive power supply is greater than VoL~VoH.
If the maximum output voltage of the selected LED drive power is lower than VoH, the maximum power of the lamp may not reach the actual power required at low temperatures. If the minimum voltage of the selected LED drive power is higher than VoL, the output of the drive power may exceed the working temperature at high temperatures. Range, work is unstable, lamps will flicker, etc.
However, considering the overall cost and efficiency, it is impossible to blindly pursue the ultra-wide output voltage range of the LED driving power supply: because the driving power supply voltage is only in a certain range, the driving power supply efficiency is the highest. When 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
There were customers who required the input power of the lamps to be a fixed value, with a fixed error of 5%, and only for each lamp to adjust the output current to reach the specified power. Due to different working environment temperatures and different lighting times, the power of each lamp will still vary greatly.
The customer puts forward such a request, although it has its marketing and commercial considerations. 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 change of the LED load series voltage Vo. When the 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 same output power condition, the input power will decrease compared to the moment of startup.
Therefore, when formulating requirements, the application of LED drive power should first understand the working characteristics of the LED, avoid putting forward some indicators that do not conform to the principle of working characteristics, and avoid indicators that far exceed actual requirements, and avoid excess quality and cost waste.
▲Failed during the test
There have been customers who purchased many brands of LED driver power supplies, but all samples failed during the test. Later on-site analysis found that the customer used a self-dual voltage regulator to directly power the LED drive power for testing. After power on, the voltage regulator was gradually increased from 0Vac to the rated operating voltage of the LED drive power.
This kind of test operation can easily make the LED driving power supply start and work with load when the input voltage is very small, and this situation will cause the input current to be far greater than the rated value, and the internal input terminal related components, such as fuses, rectifier bridges , Thermistors, etc., fail due to excessive current or overheating, causing the drive power supply to fail.
Therefore, the correct test method is to adjust the voltage regulator to the rated operating voltage range of the LED drive power supply, and then connect the drive power supply for power-on test.
Of course, technically improving the design can also avoid the failure problem caused by such misoperation of the test: a starting voltage limit circuit and an input undervoltage protection circuit are set at the input end of the drive power supply. When the input does not reach the starting voltage set by the drive power supply, the drive power supply does not work; when the input voltage drops to the input undervoltage protection point, the drive power supply enters the protection state.
Therefore, even if the operation steps of the self-dual voltage regulator are still used in the customer's test process, the driving power supply has the self-protection function and will not fail. However, before testing, customers must carefully understand whether the purchased LED drive power product has this protection function (considering the actual application environment of the LED drive power supply, most LED drive power supplies currently do not have this protection function).
▲Different load, different test results
When the LED drive power supply is tested with LED lights, the result is normal, and when the test is performed with an electronic load, the result may be abnormal. Usually this phenomenon has the following reasons:
(1) The output instantaneous voltage or power of the drive power supply exceeds the working range of the electronic load instrument. (Especially in the CV mode, the maximum test power should not exceed 70% of the maximum power of the load, otherwise the load may instantaneously overpower protection when loading, causing the drive power supply to fail to work or load.)
(2) The characteristics of the electronic load meter used are not suitable for measuring constant current sources, and the load voltage level jumps, causing the drive power supply to fail to work or load normally.
(3) Because there is 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 drive power supply, which may cause the current sampling of the drive power supply to be unstable.
Because the LED drive power supply is designed to meet the operating characteristics of LED lamps, the closest test method to actual and real applications should be to use LED lamp beads as a load, and string an ammeter and a voltmeter to test.