1 Introduction to Class D Amplifier Overcurrent Protection Principles

First, a brief introduction to the overcurrent protection mechanism of Class D amplifiers. In the half-bridge output stage of a Class D amplifier, when the MOSFET (M1) is turned on, its drain-to-source path (Vds) is equivalent to a small resistance, Rdson, and the inductor current flowing through it produces a voltage drop across its terminals. An internal comparator continuously monitors this voltage drop. Once it exceeds a preset threshold, an overcurrent condition is determined to have occurred, and the PWM drive to M1/M2 is immediately shut off to protect the device. This protection mechanism is similar to that of conventional switching power supplies. The entire scheme requires no external sense resistor and directly reuses the MOSFET itself as the sensing element, offering both low loss and a simple structure. In addition, the sensing of the high-side transistor M1 is typically implemented with the aid of the BST bootstrap capacitor, which provides a floating reference potential. To prevent current spikes at the moment of PWM switching from causing false triggering, the circuit also incorporates a brief blanking time window.