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INDUSTRY 4.0/SMART FACTORIES
POE STANDARDS The supply of power via a Cat-5 cable is defined in the IEEE 802.3af Power over Ethernet standard. The PoE standards used to be limited to a few watts, but newer PoE technologies enable even higher power. For example, PoE+ allows power up to 25W per port and PoE++ (a four-pair Power over Ethernet system) ranges from 70W to 100W by using all of the wires of the existing cable. In parallel to this PoE standard, Analog Devices has defined the proprietary standard LTPoE++, which defines the specifications up to 90W powered device (PD) power (see Table 1). LTPoE++ reduces the technical complexity of
the PoE system in relation to comparable solutions. Plug and play capability, easy implementation, and a safe, robust power supply are further features of LTPoE++. Moreover, LTPoE++ is interoperable and backward- compatible with the standard PoE specifications of the IEEE. However, the actually usable power is somewhat lower than the specified PD power because of losses in the system as well as cable losses, as is also the case with PoE+ and PoE++.
POE COMPONENTS Essentially two components are necessary to supply devices over the Ethernet cable: the powered device and the power sourcing equipment (PSE).
POWER OVER ETHERNET:
SUPPLY OF
ETHERNET DEVICES VIA DATA LINES
Figure 1. Block diagram showing the main components of a PoE system.
The PSE has the task of delivering the power
like a power supply, whereas the PD receives the power and uses it (load). PSE devices have a signature process while powering up to protect incompatible devices from damage when they
In process automation systems, important parameters such as temperature, pressure, flow rate, humidity, and many others must be monitored and measured. In the era of Industry 4.0, Ethernet is a popular communication standard. Because Ethernet is wired and transmitters and sensors typically require a power supply, the question arises: why not use the Ethernet cable for both data transmission and supply? Here, Thomas Brand, field applications engineer at Analog Devices, describes how Ethernet devices can use the cable simultaneously for transmitting data and for supplying power. Power over Ethernet (PoE) systems are widely used in the industry and will play an important role in the future.
TABLE 1. ANALOG DEVICES’ LATEST POE STANDARDS PoE Standard PoE (IEEE 802.3af) PoE+ (IEEE 802.3at) ADI’s LTPoE++ PoE Type Type 1 Type 2 LTPoE++ LTPoE++ LTPoE++ LTPoE++ PD Power (W) Usable Power (W) 13 25 38.7 52.7 70 90 11.25 22 32.9 44.8 60.8 76.8
are connected. This involves first checking the signature resistance of the PD. The PD will only be supplied with power if this value is correct (25kΩ). If the PSE detects a PD, it starts with the classification; that is, with the determination of the power requirements of the connected device. For this, the PSE applies a defined voltage and measures the resulting current. The PD is assigned to a power class on the basis of the current level. The full voltage and current will be supplied if everything is correct. As soon as the PD is supplied, it has the task of converting the PoE voltage of –48V to a supply voltage suitable for the terminal devices. In typical PD designs, an additional dc-to-dc converter (diode bridge controller) is used. It has the task of adjusting or covering the power requirements of the components supplied by the PD. Newer ICs already offer the possibility of integrating the interface and the dc-to-dc converter into a single component for low power classes, which simplifies the design.
14 MARCH 2024 | FACTORY&HANDLINGSOLUTIONS
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