Smart electronic fuse

Smart electronic fuse

For most non-synchronous rectification boost switching converters using inductors, there is a DC path between the input and output, as shown in Figure 1. The existence of this path will cause two adverse consequences: first, once the output is short-circuited or severely overloaded for more than a few hundred milliseconds, the diode (usually a Schottky diode) will be overheated and damaged; second, when the switching oscillation circuit stops working for some reason, such as manual shutdown, there is still voltage at the load end, which is only a diode voltage drop lower than the input end. At this time, the output will still consume energy. In addition, if the residual voltage is lower than the load steady-state operating voltage range, the circuit will be in an uncertain state.

For applications with relatively low output currents (less than 5A), both of these problems can be solved using monolithic current mode controllers and high-side current sampling techniques. In these circuits, the diode is replaced by a synchronous rectifier switching transistor, so the input-output path is cut off by turning off the internal switching transistor, so that the load terminal presents a high impedance state to the input terminal, which is exactly the desired result. In normal operation, the high-side sampling resistor inside the circuit periodically samples the load current, thus avoiding catastrophic consequences due to overcurrent. Therefore, the internal overheat protection circuit provides a safe operating area (SAO) for the converter.

Figure 2 shows a simple solution where the MAX668 is a switch controller that performs the boost function. The current feedback boost controller (MAX668) drives a low-side logic-level N-channel enhancement-mode MOSFET that is connected to ground through a low-side current-sampling resistor. The high-side switch is a Schottky diode, chosen primarily for its low forward voltage drop. As can be seen from the figure, the basic structure of the boost converter topology is intact. In this application, the MAX668 converts 3.3V to 5V, with a load current of up to 3A.

---- Among them, the P-channel enhancement MOSFET, Q1, is the key component to achieve load disconnection. When the MAX668 is in shutdown mode, the diode D1 is still turned on, so that the voltage at the power supply end of the MAX810L is 3.3V minus the tube voltage drop of the diode D1. Since the reset threshold level of the MAX810L is 4.65V, its RESET terminal output is high, forcing Q1 to turn off, thereby disconnecting the load from the input power supply. The MAX668 sets the 5V output voltage through an external feedback resistor network. When the output voltage exceeds the reset threshold level of the MAX810L, its internal monostable circuit starts to work and delays for about 240ms. After that, the output of the MAX810L becomes low, turning on Q1.

After Q1 is turned on, the MAX810L monitors the output voltage to determine if the output is overcurrent. Overload will cause the output voltage to drop. When it is lower than the MAX810L threshold level, the MAX810L output changes from high to low after a 20μs delay, thereby turning off Q1 and disconnecting the load. Due to the boost effect of the MAX668, the voltage at the MAX810 power supply end will be higher than its threshold level again. After a reset delay of 240ms, the MAX810L output changes from high to low again, turning on Q1 and automatically connecting the load again. The above process will be repeated cyclically unless the excess load is removed or the MAX668 is turned off to stop working. Therefore, the MAX810L and switch Q1 together form a solid-state switch (electronic fuse).

---- The MAX810L (micropower device) has an unbalanced push-pull output stage. When outputting current, it is equivalent to a 6kΩ resistor; when drawing current from the outside, it is equivalent to a 125Ω resistor. When turning Q1 on or off, the MAX810L's resistance prevents the rapid charging and discharging of Q1's Miller capacitance and gate-source capacitance, thus slowing down the switching transient process. Assuming that Q1's total equivalent capacitance is 5000pF, the time constant of the RC circuit of the high-current transistor when the MAX810 draws current (equivalent to a 125Ω resistor) is about 0.6μs. The voltage transient response time of the entire turn-on process is about 10RC=6μs. The time to completely turn off the same switch Q1 is about 48 times the time to fully turn on.

---- When the external load or C2 draws a large current at the moment of startup, the rapid turn-on of Q1 may cause the MAX810 input voltage to be lower than its reset threshold voltage, resulting in a reset. Therefore, an RC network is added to Figure 2 to slow down its turn-on process (as shown in Figure 3). Proper selection of R and C can extend the load connection process to 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 extend the turn-on time of Q1, they also extend the turn-off time, which is an undesirable result. Therefore, a Schottky diode needs to be connected in parallel with the resistor to speed up the process of turning off Q1 when the load is overloaded.

---- In order to obtain an enhanced channel and lower on-resistance, the above circuits all require a logic-level controlled P-channel MOSFET. If the on-resistance of Q1 is large and a large voltage drop occurs across it (especially in low output voltage applications or when the load is far from the power supply), the output should be regulated by feeding back the voltage from the drain of Q1. When designing the circuit, parasitic parameters must be minimized and the circuit layout must be carefully considered. The above remote regulation can be achieved using a low-voltage analog switch (MAX4544) in a SOT23 package, which is controlled by the output of the MAX810L, as shown in Figure 4.

---- According to the MAX4544 product parameters, its minimum operating voltage is 2.7V. Since the input voltage is 3.3V and the forward voltage drop of Schottky is 0.3V, the MAX4544 (and MAX810) are in operation even if the boost converter is in shutdown mode. At this time, the MAX810 outputs a high level, and the common terminal COM of the MAX4544 is connected to its normally open terminal NO (source of Q1). When the MAX668 is enabled, the resistor network connected to the common terminal of the MAX4544 provides feedback voltage for the MAX668. Since the on-resistance of the MAX4544 can reach up to 60Ω at 5V, the value of the feedback resistor should be large in order to obtain the minimum output voltage error. Since the on-resistance of the MAX4544 is only 120Ω at 3V operating voltage, the error voltage introduced by the switch MAX4544 is very small, even at low output voltages.

---- When the boost converter is enabled and its output voltage exceeds the reset threshold level of the MAX810 and after the reset delay, the output of the MAX810 will change from high to low, turning on Q1 and connecting the load. At the same time, the low level of the MAX810 output connects the COM terminal of the MAX4544 to the NC terminal (normally closed terminal), causing the feedback resistor to switch from the source of Q1 to the drain of Q1, thereby allowing the output voltage to be adjusted from the load end far away from the converter.

The switching process of the MAX4544 also switches the input of the MAX810 from the source of Q1 to the drain of Q1. In this way, the MAX810 can be used to monitor whether the load is overloaded.