In this paper, a new phenomenon for the buck converter during the voltage conversion is found. It occurs at the transient that the high side driver (HS-D) is turned off in the large swing period. In order to turn on to keep the inductor current flowing, the diode of low-side driver (LS-D) is forced to turn on. This is the conventional model. From the experiment result and T-CAD simulation, we find that two parasitic bipolar transistors (npn and pnp) of high side driver (HS-D) also can turn on. Then, part of the electrons injected from the npn bipolar flow outside the HS-D, and are collected by the power-clamp device to drive it into the snapback region. Since the power-clamp device is connected to the power supply, it cannot skip this state as it is triggered on. Eventually, the high current and high voltage induced high Joule-heating leads to the thermal runaway occurs at the power-clamp device.
