Related Instructions For Fuses

1. When operating at a normal operating current of 25 ℃, the rated current of the fuse should usually be reduced by 25% to avoid harmful melting. Most traditional fuses use materials with lower melting temperatures. Therefore, this type of fuse is more sensitive to changes in ambient temperature. For example, a fuse with a rated current of 10A is usually not recommended to operate at an ambient temperature of 25 ℃ and a current greater than 7.5A.
2. Voltage Rating The voltage rating of a fuse must be equal to or greater than the effective circuit voltage. The general standard voltage rating series are 32V, 125V, 250V, and 600V.
3. The resistance of a resistor fuse is not important in the entire circuit. Due to the fact that the resistance of a fuse with an amperage less than 1 is only a few ohms, this issue should be considered when using a fuse in low-voltage circuits. Most fuses are made of materials with a positive temperature coefficient, so there is a distinction between cold resistance and hot resistance.
4. The current carrying capacity of the ambient temperature fuse was tested under 25 ℃ ambient temperature conditions, which are influenced by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, and the shorter its lifespan. On the contrary, operating at lower temperatures will prolong the lifespan of the fuse.
5. The rated capacity of the fuse is also known as the breaking capacity. The rated fuse capacity is the maximum allowable current that a fuse can truly fuse at rated voltage. When there is a short circuit, the fuse will experience multiple instantaneous overload currents greater than the normal operating current. Safe operation requires the fuse to remain intact (without bursting or breaking) and eliminate short circuits.
intelligence
For most asynchronous rectifier boost switching converters that use inductors, there is a DC path between the input and output. The existence of this path can cause two adverse consequences: firstly, once the output short circuit or severe overload time exceeds a few hundred milliseconds, it will cause the diode (usually Schottky diode) to overheat and damage; Secondly, when for some reason, such as human shutdown, the switching oscillation circuit stops working, and there is still voltage at the load end, only one diode lower than the input voltage drop, the output will still consume energy. In addition, if the residual voltage is lower than the steady-state operating voltage range of the load, it will put the circuit in an uncertain state.
For applications with relatively small output currents (less than 5A), both of the above problems can be effectively solved using a single-chip current mode controller and high-end current sampling technology. In these circuits, the diode is replaced by a synchronous rectifier switch transistor, so the input-output path can be cut off by turning off the internal switch transistor. As a result, the load end is in a high resistance state to the input end, which is the desired result. In normal working conditions, the high-end sampling resistor inside the circuit periodically samples the load current, thus avoiding catastrophic consequences caused by overcurrent. Therefore, the internal overheating protection circuit provides a safe operating area (SAO) for the converter.
MAX668 is a switch controller that completes the boost function. The current feedback boost controller (MAX668) drives a low-end logic level N-channel enhanced MOSFET, which is grounded through a low-end current sampling resistor. The high-end switch is a Schottky diode, chosen mainly because it has a low forward conduction voltage drop. As shown in Figure 7, the basic topology structure of the boost converter has not been disrupted. In this application, MAX668 converts 3.3V voltage to 5V, and the load current can reach 3A.
Among them, the P-channel enhanced MOSFET-Q1 is the key component for achieving load interruption. When MAX668 is in off mode, diode D1 still conducts, resulting in a voltage of 3.3V at the power terminal of MAX810L minus the voltage drop of diode D1. Due to the reset threshold level of MAX810L being 4.65V, its RESET output is at a high level, forcing Q1 to turn off, thereby disconnecting the load from the input power supply. MAX668 sets a 5V output voltage through an external feedback resistor network. When the output voltage exceeds the reset threshold level of MAX810L, the internal monostable circuit begins to operate with a delay of about 240ms. Afterwards, the output of MAX810L decreased, causing Q1 to conduct.
After Q1 conduction, MAX810L continuously monitors the output voltage to determine if the output is overcurrent. Overloading will cause a decrease in output voltage. When it is below the threshold level of MAX810L, the output of MAX810L changes from high to low after a delay of 20 μ s, thereby turning off Q1 and causing the load to disconnect. Due to the boost effect of MAX668, the voltage at the power supply terminal of MAX810 will be higher than its threshold level. After a reset delay time of 240ms, the output of MAX810L will change from high to low again, and Q1 will be turned on and automatically reconnected to the load. The above process will be repeated periodically, unless excess load is removed or MAX668 is turned off to stop working. Therefore, MAX810L and switch Q1 together form a solid-state switch (electronic fuse).
MAX810L (Micro Power Device) has an unbalanced push-pull output stage. When outputting current externally, it is equivalent to a 6k Ω resistor; When drawing current from outside, it is equivalent to a 125 Ω resistor. When Q1 is turned on or off, the resistance of MAX810L prevents the rapid charging and discharging of the Miller capacitor and gate source capacitor in Q1, thus slowing down the transient process of the switch. Assuming the total equivalent capacitance of Q1 is 5000pF, the time constant of the RC circuit of the high current transistor when MAX810 draws current (equivalent to a 125 Ω resistor) is about 0.6 μ s. The transient voltage response time during the entire conduction process is approximately 10RC=6 μ s. The time to completely turn off the same switch Q1 is approximately 48 times the time for complete conduction.
When an external load or C2 needs to draw a large current at the moment of startup, quickly conducting Q1 may cause the input voltage of MAX810 to be lower than its reset threshold voltage, resulting in a reset. Therefore, adding an RC network to slow down its turn-on process. Choosing R and C appropriately can prolong the load connection process until several MAX668 switch cycles, so that the output voltage of MAX668 is always higher than the reset threshold voltage of MAX810. If R and C prolong the conduction time of Q1 and also extend the shutdown time. Therefore, it is necessary to parallel a Schottky diode on the resistor to accelerate the process of turning off Q1 when the load is overloaded.
In order to obtain enhanced channels and lower conduction resistance, the above circuits require the use of logic level controlled P-channel MOSFETs. If the conduction resistance value of Q1 is large and there is a large voltage drop at both ends (especially in low output voltage applications or when the load is far from the power supply), the output should be adjusted by feedback voltage from Q1 drain. When designing circuits, it is necessary to minimize parasitic parameters while carefully considering the circuit layout. The above remote adjustment can be achieved using a low voltage analog switch (MAX4544) packaged in SOT23, which is controlled by the output of MAX810L.
According to the MAX4544 product parameters, its minimum operating voltage is 2.7V. Due to the input voltage being 3.3V and the forward voltage drop of Schottky being 0.3V, even if the boost converter is in off mode, MAX4544 (and MAX810) is still in operation. At this point, MAX810 outputs a high level, and the common terminal COM of MAX4544 is connected to its normally open NO (source of Q1). When MAX668 is enabled, the resistor network connected to the common terminal of MAX4544 provides feedback voltage to MAX668. Due to the maximum conduction resistance of MAX4544 reaching 60 Ω at a voltage of 5V, in order to obtain the minimum output voltage error, the value of the feedback resistance should be large. Due to the conduction resistance of MAX4544 being only 120 Ω at a working voltage of 3V, the error voltage introduced by switch MAX4544 is very small, even at low output voltage.
When the boost converter is enabled and its output voltage exceeds the reset threshold level of MAX810 and undergoes a reset delay, the output of MAX810 will change from high to low, causing Q1 to conduct and connect to the load. At the same time, the low level output of MAX810 connects the COM end and NC end (normally closed end) of MAX4544, causing the feedback resistor to switch from the source of Q1 to the drain of Q1, allowing the output voltage to be adjusted from the load end far away from the converter.
The switching process of MAX4544 also switches the input terminal of MAX810 from Q1's source to Q1's drain, so that MAX810 can be used to monitor whether the load is overloaded.
High voltage insurance
High voltage fuses are often used on high-voltage towers and poles, with fuse switches that also have the function of on-off switches, as overcurrent or short circuit protection for transmission and distribution systems.