Protection of auxiliary output power circuit diagram with automatic reset circuit
Many devices require an auxiliary DC output to power external components or subsystems. When these subsystems are thermally connected, it's crucial to protect the auxiliary outputs from short circuits. Traditional methods like fuses are slow and can cause a voltage drop on the internal DC rail, potentially disrupting the main system.
To address this, a pulse current-based solution is used, which is cost-effective and capable of handling both transient and continuous shorts. The impact on the input is minimal—a small glitch lasting a few hundred microseconds and only a few hundred millivolts in amplitude. In this design, U1 is a Schmitt trigger inverter (74HC14), Q2 acts as a switch, and RSENSE is a current-sensing resistor. The circuit operates with VIN = 12 V and is designed for a maximum load current of 0.6 A.
Under normal conditions, when the load current is below 500 mA, Q1 remains off, V1 is at 0 V, V2 is also 0 V, C1 is discharged, and V3 is at 5 V. At this point, Q3 and Q2 are on, and the output voltage VOUT stays at 12 V.
If the load current exceeds 0.6 A, Q1 turns on, causing V1 to rise. C1 then charges through D1 at a small time constant (C1 × R1). Once V2 crosses the upper trip threshold of the 74HC14, V3 drops, turning off Q3 and Q2, which cuts off the load current. After that, Q1 turns off, V1 decreases, and C1 discharges through a larger time constant (C1 × R2). After a delay determined by C1 and R2, V2 falls again, allowing V3 to rise and the switch (Q2) to open.
If the short persists, the pulse switch cycles on and off continuously. For high-current applications, the power dissipation on RSENSE can be significant. To mitigate this, Q1 can be replaced with a high-side current sensor that includes amplification, such as the ZXCT1021 from Zetex, with appropriate circuit modifications. D2 serves as a protection diode to discharge C1 when the power is turned off. Q2 has a sufficient current rating (4–5 A), and designers should consider the tolerance of the Schmitt trigger’s trip point. Alternatively, Q2 can be replaced with a P-channel MOSFET to reduce the forward voltage drop.
For higher voltages, like 24 V, the gate-source voltage of the MOSFET must be protected to avoid exceeding the Zener breakdown voltage. When a 1 Ω resistor causes a short, V2 generates a sawtooth waveform between 2 V and 3.2 V, with a rise time of 500 μs and a delay of 1 s. The peak current pulse reaches about 1.5 A for 500 μs, while the input disturbance is around 0.2 V for the same duration. Reducing C1 to 0.47 μF can shorten the pulse width of the short-circuit current.
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