A PoDL system conducts differential data and DC power over the same wire pair, and various DC coupling techniques are described that improve DC voltage coupling while attenuating AC common mode noise. Pairs of differential mode chokes (DMCs) are used to share current supplied by a power supply. In one embodiment, one DMC is coupled to the line side of a common mode choke (CMC), and one DMC is coupled to the PHY side of the CMC. The line-side DMC has windings that are loosely magnetically coupled so that DMC does not present a very low impedance to AC common mode noise on the wires. Therefore, the performance of the wires' RC termination circuitry is not adversely affected by the line-side DMC when minimizing reflections of common mode signals. The DMCs may use the same magnetic core, and the CMC may be series CMCs that used the same magnetic core.

Power over data lines system using split or coupled CMCs and DMCs for coupling DC voltage and attenuating common mode noise

In a PoDL system, where DC power and differential data are supplied over the same conductors between a PSE and a PD, a low-power test is first performed to determine the conductor resistance. The PSE and PD are connected to the conductors via DC-coupling inductors that block the data signals. Such inductors have a DC resistance (DCR). To avoid the DCR of the PD inductors being included in the resistance test of the conductors and to avoid the requirement of additional pins for kelvin sensing, the PD includes a MOSFET switch connected across the conductors on the line-side of the PD inductors. The PSE applies a current pulse of a known value to one of the conductors while the MOSFET switch is temporarily closed. The PSE measures the resulting voltage across the wires and uses Ohm's law to more accurately calculate the resistance of the conductors.

Power over data lines system with accurate and simplified cable resistance sensing

In a Power over Data Lines (PoDL) system that conducts differential data and DC power over the same wire pair, various DC coupling techniques are described that improve DC voltage coupling while attenuating AC common mode noise. Pairs of differential mode chokes (DMCs) are used to share current supplied by a single phase or multi-phase power supply. In one embodiment, one DMC is coupled to the line side of a common mode choke (CMC), and one DMC is coupled to the PHY side of the CMC. The line-side DMC has windings that are loosely magnetically coupled so that DMC does not present a very low impedance to AC common mode noise on the wires. Therefore, the performance of the wires' RC termination circuitry is not adversely affected by the line-side DMC when minimizing reflections of common mode signals.

Power over data lines system using pair of differential mode chokes for coupling DC voltage and attenuating common mode noise

A PHY is coupled across split primary windings of an isolation transformer for differential data transmission and reception between PHYs and for DC isolation. Positive and negative low impedance terminals of a DC power supply are coupled to first and second secondary windings of the transformer as split center taps of the transformer. Respective ends of the wires in the wire pair are coupled to the other ends of the secondary windings. Therefore, the power supply conducts DC current through the secondary windings, while the differential data signals also flow through the secondary windings, generating corresponding differential data signals at the inputs to the PHY. The transformer also attenuates common mode noise. Therefore, the circuit makes multi-use of the isolation transformer, allowing fewer components to be used for the DC coupling, wire termination, and common mode noise cancellation.

Termination for wire pair carrying DC and differential signals using isolation transformer with split primary and secondary windings

In a PoDL system, a PHY has its I/O terminals coupled to a wire pair via a galvanic isolation transformer and a CMC. Thus, DC power and common mode noise are blocked from the PHY inputs. One end of the secondary winding of the transformer is directly coupled to one winding of the CMC. A DC power supply has its positive voltage terminal directly coupled to the other end of the secondary winding and has its other output terminal (e.g., ground) directly coupled to the other winding of the CMC. An AC-coupling capacitor is coupled between the two outputs of the power supply. Differential signals are applied across the secondary winding to couple the differential signals to the PHY, while the secondary winding conducts the DC voltage to one of the wires (via the CMC), and the ground is coupled to the other one of the wires (via the CMC).

Gitesh Bhagwat

Papers

Termination for wire pair carrying DC and differential signals using isolation transformer with split primary and secondary windings

Power over data lines system using power supply coupled to end of winding of isolation transformer