Power sourcing equipment having bipolar junction transistor for controlling power supply and supporting AC disconnect-detection function

A system for supplying power to a powered device over a communication link, comprising:

• a power supply device having a controller and an output port coupled to the communication link,
• a bipolar junction transistor (BJT) controlled by the controller to provide power to the output port,
• the BJT being controlled to increase impedance of the BJT so as to enable the controller to detect an AC impedance at the output port, wherein
• the power supply device further comprises an AC disconnect-detection pin for detecting the AC impedance at the output port,
• the BJT is configured to turn off to support detection of the AC impedance at the output port, and
• sensing capacitance is coupled to the AC disconnect-detection pin for supporting detection of the AC impedance at the output port, the sensing capacitor further being coupled to a first electrode of the BJT to turn the BJT off in response to a probing signal supplied to the sensing capacitance.