Midspan power delivery system for reduced emissions
A midspan power adapter is provided for use in a balanced communication system, such as an Ethernet system, where differential signals are transmitted between two devices. The adapter passes data signals through, and provides power to an end device, such as an IP phone, with significantly less contribution to the electromagnetic emissions of the system than known midspan adapters. Balance and symmetry inherent in a structured cable can be maintained by a termination network connecting center taps of a transformer and the spare pairs of both the incoming and outgoing cables to a common point. The adapter can provide power through unused wires (known as spare pair power) or by combining power with data signals (known as phantom feed). A common mode choke is included in the power feed to prevent noise currents from flowing through the power supply connections to earth ground, helping to minimize radiated noise.
1. A midspan adapter to power a powered device in a balanced cable system, the power adapter having a device side including device connectors and a system side including system connectors, and a power source provided between two spare pairs of the device connectors, the adapter comprising:
a common mode choke connected inline between the power source and the spare pairs of the device connectors;
a transformer to isolate the device connectors and the system connectors, the transformer having device side terminals connected to signal pairs of the device connectors and system side terminals connected to signal pairs of the system connectors, and having first center taps and second center taps; and
a termination network connected between non-signal connectors and a common point to provide common mode termination.
2. The midspan power adapter of claim 1 wherein the non-signal connectors comprise the first center taps of the transformer, spare pairs of the device connectors and spare pairs of the system connectors.
3. The midspan power adapter of claim 2 wherein the non signal connectors further comprise the second center taps of the transformer.
4. The midspan power adapter of claim 1 further comprising a phantom power common mode choke to provide power from a power supply to the second center taps on the transformer.
5. The midspan power adapter of claim 1 wherein the termination network includes a plurality of common mode terminations each having substantially similar resistance and capacitance values.
6. The midspan power adapter of claim 1 wherein the termination network includes a Bob Smith Termination.
7. The midspan power adapter of claim 5 wherein the resistance values are determined in accordance with pair-to-pair common mode impedance within a connector cable.
8. The midspan power adapter of claim 1 wherein the transformer is a 1:1 isolation transformer.
9. The midspan power adapter of claim 1 wherein the balanced cable system is an Ethernet system.
10. The midspan power adapter of claim 1 where the end device is an internet protocol (IP) device.