The fundamental reason why micro fuses are difficult to carry high currents is that their physical structure and material properties have a series of bottlenecks that are difficult to overcome when dealing with high currents. The following is a detailed analysis:
Core limitation: Thermal equilibrium equation (I ² t law)
Principle of circuit breaker: When a fuse passes current, it heats up due to resistance (Joule heating Q=I ² Rt). When heat accumulates to the melting point of the molten material, the melt melts and disconnects the circuit.
The cost of miniaturization:
The cross-sectional area of the melt (A) is extremely small: miniaturization inevitably requires the melt (conductive part) to be very thin. The resistance R is inversely proportional to the cross-sectional area A (R=ρ L/A). The sharp decrease in cross-sectional area leads to a significant increase in resistance R.
Resistance (R) increases: According to the Joule heating formula Q=I ² Rt, at the same current I and time t, the greater the resistance R, the more heat Q is generated.
Small heat capacity: The material volume and mass of the micro melt itself are extremely small, and the amount of heat (heat capacity) that can be stored is very limited. A small accumulation of heat can rapidly increase its temperature.
Difficulty in heat dissipation: A small volume means that the ratio of its surface area to volume is relatively large (larger specific surface area), which theoretically benefits heat dissipation. But the key point is that the heat rate (I ² R) generated by the micro melt is much greater than the rate at which it dissipates heat through its small surface area. Especially in enclosed spaces or under sudden impacts of high currents, heat cannot be dissipated in a timely manner.
Result: Even a relatively small current I can cause a sharp rise in melt temperature and rapid melting due to high resistance R, low heat capacity, and poor heat dissipation. To carry high currents, extremely low resistance and strong heat dissipation are required, while microstructures require the opposite.
Constraints on material properties
High resistivity requirement: In order to achieve effective melting characteristics at a small size (i.e., to melt within a reasonable time under certain overload current), micro fuses usually require the use of alloy materials with relatively high resistivity (such as tin/lead/antimony alloys, etc.). This contradicts the low resistivity materials required to carry high currents, such as copper and silver.
Balance between melting point and mechanical strength: The melting point of the material should not be too high, otherwise the protective circuit cannot be melted in a timely manner when overloaded; But it also requires a certain level of mechanical strength to support small structures. This limits the range of materials that can be selected, and it is often difficult to find materials that are both easily melted and can withstand significant electrical and thermal stresses to cope with high currents.
Arc and Breaking Ability Challenge
Arc generation: When the melt melts, the circuit voltage will generate an arc at both ends of the melting gap. The arc temperature is extremely high, maintaining the current path, which may burn out the fuse body or surrounding components, and even cause short circuits or fires.
Disadvantages of microstructure: The internal space of micro fuses is extremely limited, with almost no physical space to set up effective arc extinguishing structures (such as arc extinguishing sand, arc extinguishing grid, slit arc extinguishing, etc.). The melt gap is also very small, making it difficult to reliably elongate and cool the arc.
Insufficient breaking capacity: Breaking capacity refers to the maximum fault current that a fuse can safely cut off. Due to its small structure, limited arc extinguishing ability, and material tolerance limit, micro fuses have much lower breaking capacity than large fuses. The enormous energy and strong arc generated by high current faults can easily cause micro fuses to explode, splash molten metal, or fail to completely disconnect the circuit, losing their protective function and causing secondary accidents.
Limitations on physical strength and reliability
Electric force shock: When a large current flows through a conductor, a strong electromagnetic force (electric force) is generated. For extremely thin micro melts, this electromagnetic force may cause vibration, deformation, and even mechanical fracture, affecting normal operation or leading to misoperation.
Thermal stress shock: The severe temperature rise and cooling during the moment of high current switching can generate enormous thermal stress. The material volume of microstructures is small, and their ability to withstand periodic thermal stress is weak, making them prone to fatigue, cracking, and reducing long-term reliability.
The manufacturing tolerance has a significant impact: small manufacturing deviations (a few micrometers) in the size of the melt have a significant effect on the resistance value and melting characteristics of micro fuses. To stably manufacture micro melts that can accurately carry high currents, the technical difficulty and cost are extremely high.
The bottleneck of heat dissipation cannot be broken through
This is the most fundamental physical limitation:
Heat dissipation area/volume ratio: Although micro objects have a larger surface area, their absolute heat dissipation area is too small. The rate of heat generation (proportional to the sum of the square of the current and the resistance) is extremely high in micro high current scenarios, while the heat dissipation rate of small areas (proportional to the area and temperature difference) has an upper limit.
The heat conduction path is short and limited: heat needs to pass through the melt itself, the end caps/pins at both ends, and then be transferred to the PCB board or air. The thermal conduction path of microstructures is short and has a small cross-section, resulting in high thermal resistance. The heat dissipation capacity of the end cap and PCB ultimately becomes a bottleneck.
Highly concentrated heat: The huge heat generated by high current is concentrated in a small volume of molten material, which can easily lead to local overheating, melting, or material performance degradation.
Summary: The physical contradiction between micro and high current
The design goal of miniature fuses is to provide precise and fast overcurrent protection in extremely small spaces. This determines that it must adopt a structure with small cross-section, high resistivity, and melting materials. The core requirements for carrying high current are extremely low resistance (large cross-section, low resistivity material), super strong heat dissipation ability, sturdy structure to withstand electrical/thermal stress, and strong arc extinguishing ability. There is a fundamental contradiction between these two in terms of physical principles:
'Micro' requires a small cross-section and high resistance, which can lead to severe heating.
'Micro' leads to small heat capacity and poor absolute heat dissipation ability, making it unable to withstand severe heating.
The 'micro' cannot accommodate effective arc extinguishing structures and cannot safely cut off high current faults.
Micro materials are difficult to withstand the high electrical and thermal stresses of large currents, resulting in low reliability.
Therefore, the physical nature of micro fuses determines that they cannot safely and reliably carry and break large currents. For high current applications, medium to large fuses or circuit breakers with larger volume, stronger structure, low resistivity materials, specialized heat dissipation design, and powerful arc extinguishing devices must be used.