This section explains some of the important signals in a TI PoE PD design. First are VDD and VSS. These signals are the input power of the system, after it has been rectified off the input twisted pairs. Next is RTN (Return), which is the ground of the IC. RTN is also referred to as the primary ground because it is the ground on the primary side of the transformer. RTN is connected to VSS by an internal pass MOSFET after a successful PoE handshake is done. So VSS and RTN are not the same thing, VSS is not primary ground, and RTN is not negative input power. VC (also known as VCC and VC_in) is the input power to the IC. It is commonly believed that VDD is IC power, but VDD is input power for the whole design. In respect to the power path, VDD is the positive input power from the power sourcing equipment (PSE), and that power is what is sent through the transformer to the load. VC is taken from VDD and powers the IC.

Another important signal is VB (Bias Voltage). This is the 5-V regulator output. If the IC has an integrated DC/DC controller, it has a VB pin. Again following the power, VDD is input power, of which a portion is transformed down to VC through the auxiliary winding to power the IC, and that voltage gets bucked down to VB to produce a 5-V rail to power other functions within the IC. There is an important distinction to note here. TI has two device types for PDs: standalone PDs and PDs with integrated PWM controllers. An example of the first is the TPS2373-4 device. An example of the second is the TPS23730 device. Note that the PD does the PoE portion of operation – the detection, classification, MPS signal (if the device has Auto-MPS), and so on. Many of these functions are not dependent on VC or VB because their function comes before power is negotiated (like detection). Then there is the PWM controller (also known as the DC/DC controller), which is powered by VC and has functions like the GATE, the dead time, the switching frequency, and so forth. These functions are normally referenced to RTN for that reason.

So, problems within these designs can occur on either side: PoE or DC/DC. As previously stated, generally it is the DC/DC, but not always. Another important signal is the secondary side ground. This is also the output ground. V_OUT is the output voltage of the DC/DC converter.