Low-side driver's failure mechanism in a class-D amplifier under short circuit test and a robust driver device

Abstract

The failure mechanism in a class-D audio amplifier under short-circuit test is analyzed. The damage, always in the low-side driver, is due to high current induced thermal run-away, which occurs during the shutdown after the over-current is detected. However, this high current doesn't come from the over-current itself since the current is limited to below that the transistor in the class-D amplifier can sustain. Instead, the damage is caused by the displacement current when there is a large voltage change at the output of the class-D amplifier. Although the shutdown circuit is designed to prevent the high current flowing through the transistors of the class-D amplifier, it cannot prevent the current coming from the class-D amplifier itself. To eliminate the damage, the output transistors should be designed robust enough to against the low-pass filter induced large voltage swing.