Design Idea

Inrush limiter also provides short-circuit protection

MOSFET keeps inrush currents due to capacitive loads under control.

By Ryan Brownlee, Process Automation Division, Pepperl & Fuchs Inc, Twinsburg, OH; Edited by Brad Thompson and Fran Granville -- EDN, 6/9/2005

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 transistor Q₁ off until the voltage across capacitor C_l 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.

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 0.6V (Q₂'s V_BE(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.

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