A PoE switch acts as the Power Sourcing Equipment (PSE), injecting DC power into Ethernet cables while simultaneously transmitting data. The switch can provide power either on the data pairs (pins 1–2 and 3–6) or the spare pairs (pins 4–5 and 7–8) depending on the PoE type and standard (IEEE 802.3af, 802.3at, or 802.3bt) ( ). Endpoint PSEs supply power directly from the switch, while midspan injectors add power between a non-PoE switch and the PD ( ).
Ethernet cables connect the PoE switch to PDs such as IP cameras, VoIP phones, or access points. Each port on the switch is typically labeled to indicate PoE capability. The wiring diagram shows the switch connected to multiple devices, with the AC power source feeding the switch's internal PSE circuitry ( ). Proper PoE-rated cables should be used to ensure safe and efficient power transmission.
The powered device (PD) receives power through a front-end circuit that includes a diode bridge for polarity protection and a transformer to separate data from power ( ). The PD may also include a DC-DC converter, often using a flyback topology, to regulate the voltage for the device's internal electronics ( ). Reference designs, such as the Si3406 or LTC4267 evaluation boards, demonstrate isolated 12V outputs and IEEE 802.3af compliance, while higher-power PDs (up to 90W) use controllers like the TPS2372-4 for IEEE 802.3bt applications ( ).
Before power is delivered, the PSE and PD perform a protocol handshake to detect the device, classify its power requirements, and initiate a soft-start to prevent inrush current ( ). The PD must generate sufficient current signals to maintain the PSE output and avoid false disconnections. The PoE circuit design also considers efficiency, electromagnetic compatibility (EMC), and thermal management to ensure reliable operation.
A PoE switch power circuit integrates PSE circuitry, proper wiring, and protocol compliance to safely deliver power over Ethernet. The PD interface includes diode bridges, transformers, and DC-DC converters to regulate and isolate power. Using PoE simplifies network deployment by eliminating separate power lines, but careful attention to wiring, cable quality, and circuit design is essential for safe and efficient operation ( ).
PoE System Design: PD Power Subsystem The power subsystem of a generic PD may look like the image shown in
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