SMD fuse selection?
How to choose patch fuse?
Patch fuses are mainly used in lithium batteries, digital cameras, inverters, LED drivers, notebooks, backlights, LCD drive circuits, power tools, electric toys and other electronic products. From traditional glass tube fuses to micro fuses and patch fuses, due to the differences in product technology, their selection focuses on. It's slightly different.
The selection of patch fuses involves the following factors: ED drive, notebook, backlight, LCD drive circuit, power tools, electric toys and other electronic products. From traditional glass tube fuses to micro fuses and patch fuses, due to the differences in product technology, their selection focuses are slightly different. Impulse current, surge current, start current and circuit transient value. The patch fuse pays special attention to this point. Because of the technological reasons brought by the small size, the impact resistance of the patch fuse is much lower than that of the glass tube fuse or other larger fuses with the same rated current.
3. The magnitude of overload current and the shortest and longest time that overload current exists in the circuit. Often, it is necessary to use oscilloscope test and theoretical calculation to judge the magnitude of overload current. The basic requirement for fuses is that they should not be interrupted at any time (such as when surge current occurs), and that they should be interrupted at an appropriate time (such as when overload current needs to be cut off).
4. Resistance. The resistance of patch fuse has a certain effect on some circuits: if the fuse with too large internal resistance is installed in some circuits, it will affect the system parameters of the circuit and make the circuit not work properly.
5. The ambient temperature of the fuse. When patch fuses are used in portable devices, the temperature rise of fuses should be taken into account, that is, the reduction of rated current of fuses should be considered. The ambient temperature of the fuse should be within the prescribed working temperature range. When the ambient temperature around the fuse exceeds 25 C, it should be degraded according to the temperature reduction curve.
6. The applied voltage applied to the fuse. Usually patch fuses are used in portable devices. The working voltage of the circuit is generally not high. As long as the rated voltage of patch fuses is higher than the working voltage of the circuit, it can be safely selected.
7. Product certification. For example, export to North America must require UL or CSA certification. At present, products exported to Europe also need to comply with the EU's RoHS directive, which is commonly referred to as SGS and other environmental certification. If there are other questions, you can contact Sartre to solve them.
The common parameters and terms in fuse selection are explained below.
Normal working current: When operating at 25 C, the current rating of the fuse is usually reduced by 25% to avoid harmful fusing. Most conventional fuses use materials with lower melting temperatures. Therefore, the fuse is sensitive to the change of environmental temperature. For example, a fuse with a current rating of 1A can not normally work in a circuit with steady-state current greater than 0.75A at ambient temperature of 25 C.
Voltage rating: The voltage rating of the fuse must be equal to or greater than the effective circuit voltage. The standard voltage ratings of general patch fuses are 24V, 32V, 48V, 63V, 125V, 250V and 300V.
Resistance: The resistance of the fuse is not very important in the whole circuit. However, the resistance of fuses with less than 1 ampere will be several ohms zero, so this problem should be considered when using fuses in low voltage circuits. Most fuses are made of positive temperature coefficient materials, so there are also cold resistance and thermal resistance.
Ambient temperature: fuse is a temperature sensitive element. The current carrying capacity of fuses is tested at ambient temperature of 25 C. The higher the ambient temperature, the higher the working temperature of the fuse, the lower the current carrying capacity of the fuse, and the shorter the life of the fuse. On the contrary, the life of the fuse can be prolonged at relatively low temperatures.
Fuse rated capacity: also known as breaking capacity. Fuse rated capacity is the maximum allowable current that fuses can indeed fuse under rated voltage. When short circuit occurs, the instantaneous overload current larger than the normal working current will pass through the fuse many times. Safe operation requires the fuse to remain intact (without bursting or breaking).
Nominal melting heat energy: refers to the energy required to melt and fuse components, expressed in I2t, read as "ampere square second". Generally, in authoritative certification bodies, melting heat energy testing is required: a current increment is applied to the fuse and the melting time is measured. If no melting occurs in about 0.008 seconds or longer, the intensity of the pulse current is increased. Repeat the experiment until the fuse breaks in less than 0.008 seconds. The purpose of this test is to ensure that the heat generated does not have enough time to run away from the fuse components through heat conduction, that is to say, all the heat energy is used to fuse the fuse.
Therefore, when choosing patch fuse, normal working current, fault current, I2t value, ambient temperature, internal resistance and rated value reduction should be considered.