Bench Talk

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The power delivery capability of Power over Ethernet (PoE) is increasing, and with the latest IEEE 802.3bt standard, it can deliver up to 90W per channel. In this blog, we will examine the basic concepts behind the 802.3bt power sourcing and power delivery standards and how they have evolved to deliver the power requirements of today’s developments. We will also discuss why PoE is ideal for the built environment, bringing data and power to smart office lighting, room access, and heating controls.

The concept of delivering power over network cabling dates back to 2003 when the IEEE ratified the first Power over Ethernet (PoE) standard, an amendment to the core IEEE 802.3bt standard. The first standard, titled Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI), was IEEE 802.3af. The premise of delivering power in this way was based around cost-reduction initiatives when deploying the growing number of Ethernet-connected devices. Any networked device, whether a security camera, wireless access points, or an industrial sensor, requires power to operate. Typically, this would involve placing a line power socket in the vicinity of the device for its associated plugin power adapter, even though the low-voltage power requirement is relatively small, often no more than 10W. In most cases, the device might be the only reason a line power socket is required, and clearly, a wide-spread deployment might be associated with a significant electrical installation cost in advance of connecting everything up. Another benefit of PoE is to provide backup power to essential infrastructure, such as IP phones and security cameras, in the event of a line power failure.

Before delving more into PoE, let’s explain some of the terminology involved. Power over Ethernet is a means of delivering power from power sourcing equipment (PSE) via a link—the Ethernet cable—to a powered device (PD). The standard defines different types of devices based on the power delivery requirement. Type 1, for example, refers to the initial IEEE 802.3af standard that is specified to deliver up to 15.4W fewer cable losses to the PD load (Table 1). The IEEE 802.3at standard subsequently increased that to 30W in 2009, also known as PoE+, and introduced the Type 2 device. The voltage used for each pairset is in the range of 44V_DC to 52V_AC and is type-dependent.

Table 1: Power over Ethernet capabilities by type, class, and standard. (Source: Ethernet Alliance)

The power is delivered, along with the data, through two or more pairs of Ethernet cables (See Table 2). There are two kinds of PSEs defined:

• Endpoint PSE: An endpoint PSE is typically a network switch that has a PoE power-supply capability designed in as standard.
• Midspan PSE: A midspan PSE provides a way of injecting power into an Ethernet link segment, for example, the connection between a switch and a PD. In this way, any network that did not previously have PoE capability can be retrofitted to include a PD device such as a pan, tilt and zoom (PTZ) IP camera.

In addition to type, a class parameter defines the eight classes, from 1 to 8, where 8 is the highest power, denoting the maximum power sourced or capable of being supplied across any PoE-based system.

The power interface (PI) is perhaps the most crucial aspect of any PoE interface (Figure 1). Two of the eight conductor cable pairs are always powered together, with two pairs—a positive and a negative pair—forming a pairset. The initial standard termed each pairset either Alternative A/Mode A or Alternative B/Mode B.

Figure 1: The architecture of the power interface of an eight-wire Ethernet cable. (Source: Ethernet Alliance)

As PoE becomes a viable way of powering wired networked devices, so did the limitation on the amount of power that could be delivered in this way. Type 1 and Type 2, while providing enough power for many simple devices, could not adequately supply sufficient power for PD applications such as point-of-sale terminals, intelligent IP-based PTZ cameras, and IP videophones. Also, the emergence of smart factory and smart office products are going to take advantage of the combined power and data capabilities that PoE yields.

In 2013, the IEEE 802.3 Working Group responded to the industry’s need for an increased power capability with the goal to deliver power over all four pairsets. This work culminated in the ratification and launch of the 802.3bt standard in September 2018, which introduced Type 3 and Type 4 devices and four power classes from Class 5 (45W from PSE) up to Class 8 (90W from PSE). The four pairsets are always used for Class 5 to Class 8 802.3bt compliant devices. The 802.3bt standard is also backward compatible for Type 3 PDs for two pairsets.

In the four pairset configurations, the 802.3bt standard also introduces two ways of powering one or more circuits within the same PD. Termed a single-signature or a dual-signature PD, the way the four pairsets are rectified and supplied to the load is illustrated in Figure 2.

Figure 2: Single-signature and dual-signature PD architecture. (Source: Microchip Technology)

If you are in the process of incorporating PoE capability into your next design, Microchip Technology offers a comprehensive portfolio of PSE and PD devices. Example products include the Microchip Microsemi PD69208, a PSE Ethernet IC series that integrates 802.3at, or 803.bt power delivery and analog data in a single QFN package (Figure 3). Supporting Ethernet supply voltage from 32V to 57V, they are suitable for use in both Ethernet switches and midspan devices. For power delivery applications, the PD70224EVB development board aids in the evaluation and prototyping of PD-based designs.

In addition to providing ICs, Microchip/Microsemi also provides a series of integrated midspan power injectors such as the PD-96xxGC series. Available in 1-, 6-, 12- and 24-port versions capable of delivering 90W to the IEEE 802.3bt standard, the midspans can power a wide variety of IP PTZ cameras, IP videophones, and thin client workstations.

Figure 3: Microchip Technology’s PSE and PD devices deliver up to 90W per channel and are ideal for PoE 802.3bt applications. (Source: Microchip Technology)

As the power delivery capability of Power over Ethernet grows, the scope widens to support more and more applications. Many of the new applications take PoE outside the traditional networked device arena to encompass the control and management of the built environment, such as occupancy-aware office lighting, heating and ventilation, and access security.