Buck Converter Reverse Bias Problem
A Buck converter operating in forced PWM mode exhibits the behavior of a Boost converter when reverse biased via the output. If the output of the converter is powered by an external power supply higher than the preset output voltage, the low-side MOSFET inside the IC will sink current from the output and form a boost converter together with the high-side MOSFET. As shown, the output of this circuit is powered by a slowly rising 5V supply, and its input voltage will rise and eventually break down its ESD cell. Power supply reverse biasing like this doesn’t happen very often, but it can easily occur in systems where batteries are present. And if dynamic voltage regulation technology is used in some designs (the output voltage is adjusted through the feedback network), if the output capacitance is large, and the setting of the output voltage suddenly becomes low, the Boost action will occur.