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 RSENSE is the current-sense
resistor. For this circuit, VIN = 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 VOUT = 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.
