MOSFET enhances voltage regulator's overcurrent protection

The classic LM317 adjustable-output linear voltage regulator offers a relatively high, if package-dependent, current-handling capability. In addition, the LM317 features current limiting and thermal-overload protection. With the addition of a few components, you can enhance an LM317-based voltage regulator by adding a high-speed short-circuit current limiter (Figure 1). Under normal operation, resistors R₂ and R₃ apply V_GS bias to power MOSFET Q₁, an IRF4905S, which fully conducts and presents an on-resistance of a few milliohms. The voltage drop across current-sampling resistor R₁ is proportional to IC₁'s input current and provides base drive for bipolar transistor Q₂.

As load current increases, the voltage across R₁ increases, biasing Q₂ into conduction and decreasing Q₁'s gate bias. As Q₁'s gate bias decreases, its on-resistance increases, limiting the current into IC₁, according to I_MAX=V_BEQ₂/R₁, or approximately 0.6V/1Ω.

Resistors R₅ and R₆ set IC₁'s output voltage, as the LM317's application notes describe. By varying the value of R₁, you can adjust the circuit's limiting current from milliamperes to the LM317's maximum current-handling capability. Diodes D₁ and D₂, respectively, protect against capacitive-load discharge and polarity reversal. Depending on the circuit's requirements, IC₁ and Q₁ may require heat dissipators.

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