For containing large amounts of bulk capacitance, controlling inrush currents poses problems. The simplest approach involves placing an inrush-limiting resistor in series with the capacitor bank, but a resistor wastes power and adds a voltage drop. The circuit in Figure 1 addresses these problems and provides an additional benefit. At start-up, bipolar PNP transistor Q2 holds N-channel power MOSFET Q1 off until the voltage across capacitor C1 reaches a high enough level to turn off Q2. During this interval, resistor R1 supplies C1 and the rest of the circuit with start-up current. When Q2 turns off, Q1 turns on and provides a low-resistance path across R1. When you shut off external power, the circuit resets as C1 discharges.
 |
|
| Figure 1. | A power-MOSFET-limiting circuit restricts inrush current and protects against output short circuits. |
As an additional benefit, this circuit provides protection against short-circuited loads. As current through Q1 increases, the voltage drop across Q1 increases due to Q1’s internal on-resistance. When the voltage drop across Q1 reaches approximately 0.6 V (Q2’s VBE(ON) voltage), Q2 turns on, turning off Q1 and forcing load current through R1. Removing the short circuit restores normal operation, allowing Q2 to turn off and Q1 to turn on. Note that, because Q1’s on-resistance acts as a current-sense resistor for this function, the short-circuit trip point may vary depending on ambient temperature and Q1’s characteristics. You can adjust Q1’s turn-on and -off threshold by selecting R1 and Q1’s on-resistance characteristic. Adding a conventional or zener diode in series with Q2’s emitter increases the short-circuit trip current.
The components and values for constructing this circuit depend on the application. Depending on the design requirements, you may need to select a high-power resistor for R1 or add a heat dissipater to Q1, but, for many applications, the circuit saves power over a conventional approach.
