Power over Ethernet (PoE), which is outlined in IEEE Std 802.3™-2005 clause 33 (the PoE standard), refers to a technique for delivering power and data to an electronic device via a network cable, such as a twisted pair Ethernet cable. In a PoE system, power sourcing equipment (PSE) provides a power supply via an Ethernet cable to electronic devices, which may be referred to as powered devices. PoE eliminates the need for a separate power source to deliver power to attached powered devices. Such powered devices may include Voice over Internet Protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and other electronic devices.
The PoE standard specifies a detection process for detecting a PoE powered device that is coupled to the PSE before supplying power via the Ethernet cable. To perform the detection process, the PSE provides a voltage level that is within a range of 2.8 to 10 Volts DC on pairs of wires of the Ethernet cable and monitors a received current (Amps) or a received voltage (V) to detect a resistance within an expected range (approximately 25 K-ohms). The PSE determines the powered device's presence using a measured Volt-Amp (VA) slope related to the powered device's voltage/current signature. If the PSE does not detect a valid resistance, the PSE does not apply power to the Ethernet port assigned to the electronic device.
Typically, a powered device includes a diode bridge to rectify power received from the PSE and includes a precision external resistor to provide the 25 k-ohm impedance. However, each of the diodes of the diode bridge also provides an impedance that is related to the current and that is in series with the precision external resistor. In particular, as the current increases, the voltage drop across each diode decreases. For example, the diodes may each have an impedance of approximately 928 ohms at 28 uA and 74.4 ohms at 348 uA. Moreover, the impedance of the diodes may vary with temperature.
In general, the diodes in the diode bridge may alter the effective impedance of the powered device and may affect the device signature. In general, the effective impedance of the powered device refers to a vector sum of the resistance and reactance presented by the device in response to an applied signal. If the resistance of the precision resistor is at approximately 24 k-ohms, the diodes of the diode bridge may cause the effective impedance of the powered device to vary. For example, if the impedance is measured in ohms, the impedance may vary from approximately 25.86 k-ohms to 24.14 k-ohms. Moreover, the impedance of the diode bridge may vary with temperature, which may also alter the effective impedance and affect the device signature. Hence, there is a need for an improved device detection impedance circuit.