Controllers for Power Over Ethernet (PoE) Circuit Design

Created: June 4, 2021
Updated: July 1, 2024

Follow the 802.3af/at/bt standards for power over ethernet (PoE) circuit design.

Anytime you have a large number of power adapters packed into a small space, you have an all-too-familiar problem of excess heat and bulky transformers coming out of a wall or power strip. In a data center, industrial, or office environment, power over Ethernet (PoE) is one way to provide power from a hub, switch, or other controller to multiple downstream devices. Example applications include security systems, networking equipment, wireless access points, kiosks, and much more.

Every PoE circuit design, whether a power supplier or power receiver, needs a controller IC to provide power to downstream devices. These controllers follow specific standards that govern power output and allowed power inputs at each end of a link. Delivered power at the downstream end of a link can then be regulated with an isolated or non-isolated power supply, depending on the amount of power being delivered.

PoE Circuit Design and Standards

PoE systems have a simple topology that governs link design and power delivery. The power supply equipment (PSE) end of the link delivers power down to a power device (PD), often over multiple ports. Power is delivered over an Ethernet cable from 44 and 57 V (48 V nominal). This relatively high voltage allows efficient power transfer without creating an excessive safety hazard.

The IEEE 802.3 working group is in charge of all Ethernet standards and is in charge of overseeing PoE evolution. The table below lists the main PoE specifications for different controllers and devices. These standards govern the PSE and PD ends of a PoE link, and they’ll form the starting point for any controller you need to select for a PoE system. These specifications are valid for power provided over Cat5e cable up to 100 m.

The responsibility of these controllers is to negotiate with each other so that the PSE end can determine how much power (current) to supply to the PD end of the PoE link. Once power is delivered at the desired level, a standard DC-DC converter at the PD end of the link can be used to step down the voltage to a lower level and provide power to end devices. The converter on the output of the PD controller can be isolated or non-isolated, depending on the amount of current drawn through the PoE link.

Selecting Power Supply and Power Device Controllers

The power supply and power device controller are the cornerstone of your PoE circuit design and should be chosen to match the power requirements per port for your ideal environment. Controllers that are designed to one of the above standards will have current and voltage ratings that ensure reliability within the required power limits, but there are other standards to consider, such as integrated power or thermal protection circuits, number of outputs (for PSE controllers), and programmability.

Maxim Integrated, MAX5969A/B

The MAX5969A/B from Maxim Integrated is a powered device interface controller that is compliant to the IEEE 802.3af/at standards for PoE circuit design (Classification 0 to 5). For power transfer, the MAX5969A/B can withstand 100 V at the input and limits inrush current below 180mA before switching to its higher current limit between (720 and 880 mA). The device features integrated power MOSFETs and an input UVLO with wide hysteresis to prevent any power glitches when switching states. The rated temperature range spans up to 85 °C.

MAX5969A/B power over Ethernet application circuit. From the MAX5969A/B datasheet.

ON Semiconductor, NCP1096

ON Semiconductor’s NCP1096 power device interface controller is compliant to the IEEE 802.3af/at/bt specification and supports proprietary 100 W+ applications. The rated junction temperature spans up to 125 °C, making this component ideal for harsh environments requiring PoE. Other integrated features include overcurrent protection and over-temperature protection. The Class requested by the NCP1096 during device classification can be set through the resistors connected to the CLA and CLB pins. An example is shown in the application circuit below.

Application circuit for the NCP1096 power device interface controller. From the NCP1096 datasheet.

Analog Devices, LTC4266A/C

On the source side of a PoE system, the LTC4266A/C power sourcing equipment (PSE) controller provides power for up to 4 devices; this component comes in different grades based on power output level (13, 38.7, 57.2, 70, and 90 W). The variants of this component are ideal for powering a range of lower power devices. Analog Devices has other PSE controllers compatible with up to Type 4 PDs.

Example application circuit with the LTC4266 and LTC4265 PoE circuit design components. From the LTC4266 datasheet.

Other Components for PoE CircuitDesign

The example components shown above address the important controller interfaces required on both sides of a PoE link. However, other components are needed to provide power delivery to the PD end of the link. For example, if you’re using an RJ45 without integrated magnetics, you’ll need common-mode chokes and other passives to provide discrete magnetics and Bob Smith termination. Another common protection component on the input of the PD side is a TVS diode. Both are examples of components needed for high-power industrial applications. In addition to these components, PoE systems need a number of passives and other power management components to provide regulated power to downstream components. Here are some of the other components you’ll need for PoE circuit design:

When you need to select components for PoE circuit design, you can find all the parts you need with the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to a complete set of distributor data and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our power management integrated circuits page to find the components you need.

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