High-Side Load Driver Enhances Short-Circuit Protection

摘要

This circuit builds on a previous idea in which the addition of an optocoupler to a microcontroller output converted a "dumb" high-side driver into a "smart" one with diagnostics and short-circuit protection. The circuit adapts this concept for cases where a microcontroller is either not required or inappropriate. It also adds short-circuit load protection to an even "dumber" driver such as the output of a simple logic gate (Fig. 1). As a result, it too allows a "dumb" low-level device to control a high-power load operating at a much higher voltage without fear of destructive short-circuit conditions. Assume that C1 is initially uncharged, the driver output is at 0 V, and N-channel MOSFET Q1 is off. When the driver output transitions from a low level to a high level, the pulse couples through C1 and R2 to the gate of Q1, turning it on. Without the feedback provided via optocoupler U1, capacitor C1 would rapidly charge via R1, Q1's gate voltage would fall to zero, and the MOSFET would turn off after about a millisecond. However, the presence of the optocoupler effectively allows the circuit to monitor the voltage across the load.