Many products require an auxiliary dc output to power external devices or subsystems. If such subsystems are to be hot-connected, the auxiliary output must be protected against short circuits. Schemes using fuses are slow and will cause the internal dc-rail to drop, possibly affecting the main system.

The circuit shown in the figure provides pulse current limiting at a very low cost. It can handle momentary or continuous short circuits at the output. The only effect on the input rail is a small glitch (hundreds of millivolts for hundreds of microseconds). U1 is a Schmitt trigger inverter (74HC14), Q2 is the switch, and R_SENSE is the current-sense resistor. For this circuit, V_IN = 12 V, and the circuit is designed for load currents up to 0.6 A.

Under normal conditions—a load current under 500 mA—Q1 is off, V₁ = 0 V, V₂ = 0 V, C1 is discharged, and V₃ = 5 V. Q3 is on, Q2 is on, and V_OUT = 12 V.

If the load current goes above 0.6 A, Q1 turns on, V₁ increases, and C1 charges through D1 with a small time constant (C1 × R1). When V₂ increases above the upper trip point of 74HC14, V₃ goes low and Q3 turns off, Q2 is switched off, and the load current goes to zero. Then Q1 switches off, V₁ goes low, and C1 discharges with a large time constant (C1 × R2). After a longer duration (depending on C1 and R2), V₂ goes low, V₃ switches high, and the series switching transistor (Q2) turns on.

If the short circuit is continuous, this pulsed switching continues, on and off. For high-current applications, power dissipation in RSENSE becomes a problem. Therefore, Q1 can be substituted with a high-side current sensor that has amplification (like a Zetex ZXCT1021), with suitable circuit modification. D2 is a protection diode for discharging C1 when the power is switched off. Q2 should have a sufficient current rating (preferably 4 to 5 A). Designers should also consider Schmitt-trigger trip point tolerances. Q2 can also be substituted with a Pchannel MOSFET for a lower forward drop. For higher voltages (e.g., 24 V), the MOSFET gate source should be protected: It must not exceed the breakdown voltage of a Zener diode.

When the output was shorted with a 1-Ω resistor, the resulting V₂ was a sawtooth waveform between 2 and 3.2 V, with a rise time of 500 μs and a decay time of 1 s. The amplitude of the output current pulse was approximately 1.5 A for 500 μs, and the glitch at the input rail was 0.2 V for 500 μs. C1 can be a lower value (say, 0.47 μF) to reduce the shortcircuit current pulse width.

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