Many products require an auxiliary DC output to power an external device or subsystem. If these subsystems are thermally connected, it is important to protect the auxiliary outputs from short circuits. The solution using the fuse is slow and causes a drop in the internal DC rail voltage, which may affect the main system.
Pulse current is provided to limit it to very low cost. It can handle transient or continuous shorts in the output. The effect of the input range is only a small glitch (a few hundred millivolts in hundreds of microseconds). U1 is a Schmitt trigger inverter (74HC14), Q2 is a switch, and RSENSE is a current sense resistor. For this circuit, VIN = 12 V and the circuit is designed to have a load current of up to 0.6A.
Under normal conditions (load current less than 500mA) Q1 is turned off, V1 = 0 V, V2 = 0V, C1 is discharged, and V3 = 5V, Q3, Q2 are on, and VOUT = 12V.
If the load current rises above 0.6A, Q1 turns on, V1 increases, and C1 charges through D1 at a small time constant (C1 & TImes; R1). When V2 increases above the upper limit of the trip point of 74HC14, V3 falls, and Q3, Q2 turn off, and the load current becomes zero. Then, Q1 is turned off, V1 begins to fall, and C1 is discharged at a larger time constant (C1 & TImes; R2). After a longer period (depending on C1 and R2), V2 goes low, the switch of V3 goes high, and the switch (Q2) in series opens.
If the short circuit continues, the pulse switch is continuously turned on and off. For high current applications, power consumption on RSENSE is a problem. Therefore, Q1 can be replaced by a high-end current sensor that has amplification (similar to Zetex's ZXCT1021) with appropriate circuit improvements. D2 is a protection diode for discharging C1 when the power is turned off. Q2 has sufficient current rating (4 to 5A). Designers can also consider the tolerance of the Schmitt trigger short-circuit point. Q2 can also be replaced with a P-channel MOSFET to have a lower forward voltage drop. For higher voltages (eg 24V), the MOSFET gate-source should be protected: it should not exceed the breakdown voltage of the Zener diode.
When the output is shorted by a 1 Ω resistor, the resulting V2 is a sawtooth wave between 2 and 3.2 V with a rise time of 500 μs and a delay time of 1 s. The amplitude of the output current pulse is approximately 1.5A for 500μs, while the interference at the input amplitude is 0.2V for 500μs. C1 can be a lower value (such as 0.47μF) to reduce the pulse width of the short-circuit current.
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