Power-over-Ethernet Powered Devices (PoE-PD) Designed For The Internet Of Things (IoT)

- Sep 14, 2020-

Interconnected devices rely on two core functions, namely, communication backhaul and power supply. For Internet of Things (IoT) devices, three core issues often arise: power, communication, and security. Wireless technologies like WiFi have attracted people's attention in the market for many years, but they are troubled by these three problems. Wireless battery-powered devices need to be charged regularly, and the WiFi frequency band is saturated. These are two common problems today. Larger power requirements require mains connection, which complicates and restricts the installation point.

Power over Ethernet (PoE) solves these problems by providing flexibility, reliability, security, and power through existing Ethernet infrastructure. Since it was first published by IEEE in 2003, it has made great progress. The latest IEEE 802.3bt approval makes it a leader in the market, providing 10G-BASE-T and 60W to 90W power through Cat5/Cat6 cables.

Boosted power

Initially, Type 1 power supply equipment (or PSE) could only provide up to 15.4 W of power, and Type 2 doubled to 30 W. Now, Type 3 and Type 4 released in September 2018 have powers of 60 W and 90 W respectively. This opens the door to a world of connected devices, including wireless and cellular base stations, pan tilt zoom (PZT) and dome cameras, televisions, interactive displays, and kiosks. A single low-voltage cable is equipped with dedicated high-speed communication at the same time, so that there are fewer wiring. It is an ideal choice for the maintenance and installation of intelligent buildings for the Internet of Things (IoT) and Industrial Internet of Things (IIoT).

Power over Ethernet is a wired communication and power supply system that uses the existing Ethernet network to power endpoint devices. In these systems, the power supply equipment (PSE) provides power via eight wires, which are arranged as a four-pair twisted pair (Cat5 / Cat5e / Cat6 / Cat6a) cable and connected to the powered equipment (PD) with RJ45 connectors . The PSE provides up to 57 V to the terminal. Because the voltage is less than 60 V, it complies with the Safety Extra Low Voltage Directive (SELV), which makes electrical safety, and does not require a qualified electrician or buried cables, thus simplifying installation and maintenance. The standard limits the power of each port to 90W, making this the maximum power transmitted by an Ethernet cable.

The standard was originally scheduled to be released in 2017, and is continuously updated before its official release to ensure compatibility with previous versions. The latest update of the PoE specification, IEEE 802.3bt, introduced Type 3 and Type 4 power supply equipment (PSE) and powered equipment (PD). In order to provide higher current, the new standard allows two power modes (mode A and mode B) to be used at the same time, usually called 4 pairs or 4PPOE, powered by 4 pairs instead of Type 1 and Type 2.

The 3 categories added, namely categories 5 to 8, have an improved mutual recognition process and automatic classification functions. This update also brings lower standby power consumption and supports 10G-BASE-T and PoE.

Design with PoE

When designing a powered device, there are many functions to consider, including operating mode, PD detection, and classification. In order to avoid damage to non-PoE devices, the PSE must detect whether the powered device is connected before providing power. Use the effective feature to detect the PD mode, and use a 25kΩ resistor to implement it in the powered device. When PSE provides two continuous voltages (V1 = 2.7 V and V2 = 10.1 V) for resistance detection, it records the current value, determines the presence of PD, and then activates to power the device. Figure 1 depicts the resistance detection phase during startup.


Diagram 1: Waveform of the startup phase

In the classification stage, determine the maximum power requirements of the powered equipment. Another resistor connected to the PoE controller of the PD indicates the power range. Table 1 shows the different types of single-character PDs and their maximum average power. The category should not be confused with the Type, it refers to the specific power of the connected device. In IEEE 802.3af/at (Type 1 and Type 2 devices), a single feature PD is used. IEEE 802.3bt adds dual feature PD, where each mode or alternative (A and B) uses a separate input bridge rectifier and PD controller.

The optional classification extension is automatic classification. In "automatic classification", the PSE measures the power consumption of the connected PD within a certain period of time, so that it can determine the maximum power required by the PD. Automatic classification will never use dual feature PD to achieve.

PoE Mode

Power distribution is divided into three modes: mode A, mode B (also called alternative A and alternative B) and 4 pairs. For 10BASE-T / 100BASE-TX, in mode A, power and data pairs 1-2 and 3-6 are simultaneously transmitted. Mode B is powered by backup pairs 4-5 and 7-8. In 1000BASE-T application (four pairs), the power of mode A and mode B is also transmitted by 4 pairs at the same time. Use the center tap of a standard Ethernet transformer to extract the common-mode voltage, and then the DC-DC converter provides a stable output voltage for the system.

Figure 2 shows the mode A and mode B power supply for Type 1 and Type 2 applications. Figure 3 depicts the wiring of 4 pairs of modes in Type 3 and Type 4.


Figure 2: Mode A and Mode B PoE power transmission


Figure 3: 4 pairs of PoE power transmission

When designing devices that use PoE, it is important to consider interconnecting cables. The Ethernet cable has a maximum length of 100 m. It has a DC resistance, while reducing the voltage and dissipating power due to heat. Category 5 or Cat5 cables are twisted-pair cables used in Ethernet networks to supply power in PoE networks. It supports up to 100 Mhz and is suitable for 10/100/1000BASE-T. Category 6 or Cat6 is an improvement to Cat5 cable and supports up to 500 Mhz, suitable for 10GBASE-T Ethernet speed.

The DC resistance of a 100m Cat5 cable is 12.5Ω, and the DC resistance of Cat6 cable per 100m is 7Ω. The transmission loss increases as the current in the differential pair increases. In the case of a 25 W PD with a typical input voltage of 50 V, the current is 0.5A. The total transmission loss in Cat5 is 2.5 W, and the total transmission loss in Cat6 is 1.75 W. These losses are dissipated due to heat. For a 90 W device, this transmission loss is shared between four pairs, each pair is 930 mA, and the PSE is at least 52V. It is 17.30 W in Cat5 and 2x6.05W in Cat6. This shows that Cat5 is sufficient and safe for any application.


Wiring should be carefully considered during installation. The trade-off between cable length and equipment power must be calculated to improve energy efficiency and reduce the risk of cable damage.


Diode Bridge: Diode Bridge

Total Solution Size: total solution size

Figure 4: GreenBridge™ solution vs. diode bridge

to sum up

The newly approved IEEE 802.3bt Ethernet power supply standard has opened the market for more power-consuming devices and opened the door for higher performance applications. The increase in power consumption brings new challenges, which can be solved by HRUI's sophisticated PoE-PD solution, which integrates GreenBridge active bridge Quad MOSFETs and easy-to-implement PoE-PD controllers. These reduce the risk of new products and shorten the time to market, making Power over Ethernet an important market competition factor in the field of Internet of Things.