PoE Power Supply Using BJT for AC Disconnect Detection

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Solution Overview

Problem

Conventional Power over Ethernet (PoE) systems experience substantial power loss due to the use of diodes in series with MOSFETs for AC disconnect-detection functions, which exceed the power loss caused by the MOSFETs themselves.

Innovation Solution

A Power over Ethernet system utilizing a bipolar junction transistor (BJT) controlled by a controller to detect AC impedance at the output port, eliminating the need for a diode by using sensing capacitance and a probing signal to turn the BJT off and present a high impedance, thereby supporting AC disconnect-detection without additional power loss.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a diode is used in series with MOSFET for AC disconnect-detection function, then the PSE output port can provide high impedance when port voltage is greater than PSE power supply voltage, but substantial power loss occurs due to the diode's forward voltage drop

Engineering Contradiction:

ImproveAC disconnect-detection functionVSAvoidpower loss

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The patent removes the diode from the circuit configuration. Instead of using a diode in series with the MOSFET to provide high impedance during AC disconnect detection, the invention uses the MOSFET alone by controlling its gate voltage to achieve the same high impedance state, thereby eliminating the power loss associated with diode forward voltage drop while maintaining the AC disconnect-detection function

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent changes the control parameter of the MOSFET from conventional switching control to gate voltage control that dynamically adjusts the MOSFET's on-resistance. By adjusting the gate voltage, the MOSFET can present high impedance when needed for AC disconnect detection without the continuous power loss caused by a forward-biased diode, thus resolving the contradiction between detection reliability and energy efficiency

Inventive Principle:

Principle #35Parameter changes

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This solution reduces power loss by eliminating the diode's power consumption, improving the efficiency of the PoE system's output port arrangement while maintaining compliance with IEEE 802.3af standards for AC disconnect-detection.

Implementation Method 1

detecting an AC impedance at the output port equal to or lower than 27 kΩ

Methodology Applied

Scientific EffectElectrical Impedance: Electrical Resistance

Implementation Method 2

sensing capacitance coupled to an AC disconnect-detection pin of the controller

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS8014412B2Power sourcing equipment having bipolar junction transistor for controlling power supply and supporting AC disconnect-detection function

Publication Date: 2011.09.06 ANALOG DEVICES INT UNLTD CO

AI summary

A power supply system for providing power to a powered device over a communication link includes a power supply device capable of supporting an AC disconnect-detect function. The power supply device has a controller, an output port coupled to the communication link, and a bipolar junction transistor (BJT) controlled by the controller to provide power to the output port. The BJT may be turned off to present a high impedance required to support the AC disconnect-detection function.