POE PD
A powered-device interface receives power over Ethernet. It presents the required detection signature and classification behaviour, manages startup and works with a converter that supplies the application. The allowed power depends on the negotiated class and the complete design.
The PSE first checks for a valid PD signature, then performs the applicable classification process before applying operating power. Input rectification lets the PD accept the permitted pair assignments and polarities. The bridges, switching FET and converter may be internal or external depending on the controller; two bridges are not automatically built into every PD IC.
In plain terms
Like a guest at a club door — the bouncer (the switch's PSE) flashes the guest with a 25 kΩ probe to confirm they belong, asks the guest's class, and only then lifts the velvet rope of 48 V power.
Why designers use it
- Combining Ethernet data and power in a compatible installation.
- Managing the PD detection, classification and startup functions.
- Supporting centralized power management while leaving system-level compliance to the complete design.
Best for
- IP cameras
- Wi-Fi APs
- VoIP phones
Key specifications
- Role: PD receives power; PSE supplies it
- Detection: Nominal 25 kΩ PD signature (Implement with the selected controller's specified circuit.)
- Power budget: Use negotiated PD power, cable losses and converter efficiency
- Integration: Check bridges, FETs, converter and isolation requirements individually
When not to use it
- When the PSE, PD or cable cannot support the required power and PoE type.
- When only passive power injection is present and the equipment's requirements have not been checked.
Common mistakes
- Confusing the detection-signature resistor with a classification-setting resistor.
- Assuming the controller includes input bridges, a power FET and a complete DC/DC converter without checking its block diagram.
- Treating power at the PSE port as the same as usable load power after cable and converter losses.
- Ignoring inrush, maintain-power requirements, isolation, creepage or the exact supported PoE type.
Where you will find it
- A PoE camera or sensor can receive both data and power through one Ethernet connection. Its PD interface and converter must support the actual load, including startup and peak demand.
- ADI's LT4276 example supports an external power MOSFET and a converter controller, illustrating why not every PD has the same internal power path.
A short history
PoE separates the equipment supplying power from the device receiving it. Detection and classification help prevent full operating power being applied to an incompatible load. Standards-compliant PoE and a manufacturer's proprietary extensions must be distinguished when checking compatibility.
Good to know
- Ethernet data support alone does not mean a device can accept PoE power.
- A PD interface is only one part of the load's power supply.