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Application of fuse in LED lighting

2020-09-12
Application of fuse in LED lighting
For the over-current protection of LED lighting fixtures, it should be considered from the input current of the lamp body. The input current of LED lighting fixtures mainly has two basic types: DC input and grid AC input. The main difference between the two types is whether the driving power supply has an AC to DC module. For different input current types, the overcurrent protection methods are different. The application of the fuse should be considered according to the specific situation:

1. For the selection of the DC in fuse of the DC input type, special attention should be paid to the temperature reduction coefficient parameter of the fuse. Because the heat of high-power LED is relatively large, the temperature inside the LED lamp cup is relatively high, if the temperature reduction is selected A larger fuse will choose a larger current specification. Under the same working current, the protection ability of a larger current fuse will be relatively reduced; in addition, the DC in position will use capacitor filtering at the back end, which will cause comparison. Large power-on pulse current, so you need to pay attention to the pulse conditions when choosing a fuse in this part, otherwise the wrong option will easily cause the fuse to be broken by the power-on pulse, and it is difficult to go through many power-on and inrush current experiments. It is recommended here Use products with strong pulse resistance.

2. For the fuse selection of the drive output end, while paying attention to the fuse temperature reduction factor, it is also necessary to consider the fuse fusing speed index. Since the current fluctuation here is not large, it is necessary in the case of abnormal circuit or component failure. Quickly cut off the circuit to protect the LED string at the rear. It is recommended to choose a fast-acting type and reduced temperature fuse at this position
For the above two occasions, there are generally more SMD low-voltage fuses available in the market, such as AEM Technology's SolidMatrix® technology fuses, with sizes from 0402 to 1206, current specifications from 0.5 to 30A, fast-acting, fast-acting, Products with different series, different specifications and different characteristics, such as high pulse resistance, slow break, etc., are for engineers to choose.

3. For the AC in position of the AC input LED lighting, especially for LED bulbs, both the size of the fuse and the voltage withstand value of the fuse must be considered. Consider the AirMatrixTM AF2 series of chip fuses launched by AEM Technology. This series of fuses are small in size and can withstand a voltage of 250VAC. They also have the advantages of high consistency, low internal resistance, and high pulse resistance.

Double fuses provide effective protection for high-current board-level circuits

Protecting circuit board components from damage caused by increasing currents is a complicated matter because there is no fuse that meets the requirements. The method of protection can be a carefully designed double-fuse circuit, or a single fuse with sufficient rating. However, because there are no two identical fuses, there is always one fuse withstanding more current than the other. Therefore, even if the line current is within the specification range, the fuse bearing the higher load will still blow, and soon the other one will blow. How to solve this problem? The following are some guidelines for fuse matching and determining circuit ratings to provide the required protection for dual fuse solutions.

UL standard fuses usually have a 75% derating factor to ensure that they can provide the required circuit protection. The DC impedance of a fuse usually has a tolerance of 15%; therefore, in the worst case, the DC impedance of two randomly selected fuses (same rated current and from the same manufacturer) can differ by 35% (1.15 Rdc/0.85 Rdc = 1.35) , That is, a difference of 35%). If the DC impedance of the two fuses is very different, the current flowing through will also be very different, and circuit protection will be problematic. Generally speaking, one fuse bears higher current than the other, and may work close to the overcurrent limit, while the other is far below the safety limit. Therefore, using two fuses to complete a function will affect the overcurrent protection of the circuit.

In addition to DC impedance, another important consideration is the temperature difference between the locations of the two fuses. Fuses are temperature-sensitive devices, and their effective rated current will decrease as the ambient temperature rises. If the operating temperature of one of the two parallel fuses is higher than the other, it will have a smaller effective rated current and therefore enter the overload earlier than the other.

Although the use of two parallel fuses has the above uncertainties, the reliability of their work can be improved from the following four aspects:
1) The two fuses must match as closely as possible. Not only do they have the same rating, it is also a good idea to ensure that both fuses are manufactured at the same time. This ensures that the DC impedance of the two fuses match as much as possible.
2) Two fuses can never divide the current equally. Therefore, a 20% derating factor must be added to the portfolio.
3) Carefully track the thermal history of each fuse. Both fuses should be kept at the same temperature, including ambient temperature and normal operating temperature. Therefore, make sure that both fuses are exposed to the same airflow, and that there is a similar heat conduction mechanism on the leads or the fuse clip.
4) The maximum breaking current is equal to the value of a single fuse, not the sum of the maximum breaking current of two fuses. Similarly, the maximum breaking voltage is also equal to the value of a single fuse, not the sum of the breaking voltages of two fuses.

After following the above design guidelines, the currents flowing through the two parallel fuses are basically equal, and they can work well below their own overcurrent limit. In addition, when an overload event occurs, the two fuses are open at almost the same time to provide protection for the components of the circuit board.