Wired data connections such as USB and Ethernet can also offer an effective and convenient solution to delivering power for today’s most demanding IoT endpoints.
IoT endpoints are typically characterized as being extremely low-power devices that are also small in size, and they may also be self-powered or designed to run from a coin cell for many years. In reality, the IoT comprises a greater diversity of device types, including smart sensors, data-logging devices, smart-building controllers, security devices (such as intruder detectors and networked cameras), and many other retail, enterprise, and infrastructure applications (Figure 1).
Figure 1. IoT applications are expanding to encompass devices that require more than smaller batteries can deliver. [Source: Diodes Incorporated]
Many of these devices come with significant energy demands; needs that cannot be satisfied by a small battery. They may feature a powerful embedded processor to handle data locally to reduce network latency, add privacy, or just lower the end cost. They might include a mechanical subsystem, such as a camera with pan-tilt-zoom (PTZ) capability, which can easily add several Watts to the total system power requirements. In these applications, a small battery would simply be unable to satisfy the peak system power demand, or would provide insufficient capacity to deliver the desired runtime, resulting in periodic replacement.
A larger battery could be considered but that would also increase overall size, weight, and bill of materials. Taking power from a nearby AC line, if available, would eliminate the necessity for a battery, but this introduces an additional power cable along with the AC-DC power conversion inside the device, creating extra complexity. While this design complexity can be addressed using an external power adapter, it would not address the additional expense.
If a wired USB or Ethernet connection is included in the endpoint, the designer can take advantage of their respective power delivery features. Power over Ethernet (PoE) and USB power delivery use the same cable for power, as well as data, and thus avoid the drawbacks associated with both batteries and AC power. USB power and PoE are covered by established standards and the power is delivered at low voltages, making the equipment intrinsically safe for the end user to handle.
USB Power Specifications
An ordinary USB 2.0 connection, which contains two data lines (D+ and D-), a 5V DC supply, and ground, is normally able to supply up to 500mA. This is sufficient to power an endpoint requiring 2.5W or less. A USB 3.0 port increases this, providing up to 900mA, while the latest USB Type-C® port can be configured to supply up to 1.5A or 3.0A at 5V, increasing the maximum power to 15W.
USB Power Delivery (USB PD) is a separate standard that uses data lines in the USB Type-C connector. These allow compliant devices to negotiate with the power supplier, communicating just how much power they require. This may be 9V or 15V (3A) or 20V at 5A, raising the maximum power available from a USB connection to 100W.
Power over Ethernet Types
Existing Ethernet infrastructure can support an IoT endpoint plugged in at any point on the network, functioning as a PoE powered device (PD), while only requiring minimal new wiring, if any. The power sourcing equipment (PSE) may be a device such as a PoE switch or hub designed to supply power using one or more of the data pairs in the cable. The maximum data-transmission distance specified in the Ethernet standard is 100 meters but a PoE extender can be inserted to enable longer connections.
PoE injectors, which are designed to add power to cables coming from a non-PoE switch, are also available. Conversely, PoE splitters separate the power from the Ethernet lines to provide a dedicated power output for a non-PoE endpoint.
Several generations of PoE equipment are currently in service, summarized in Table 1. The first generation, Type 1 PoE, defined as the standard IEEE 802.3af, supports a power budget of up to 15.4W, supplied by the PSE. The later 802.3at Type-2 standard, PoE+, supports up to 30W and is backwards-compatible with Type 1 PDs.
The latest 802.3bt standard encompasses Type-3, PoE++ (or UPoE) equipment, with a maximum power budget of 60W, as well as 100W Type-4 equipment. Whereas 802.3af and 802.3at use a maximum of two pairs of wires in Cat 5/5e cable, 802.3bt calls for a Cat 6 (balanced) cable and uses all four pairs of wires. This arrangement greatly reduces the power dissipated in the cable, enabling more power to reach the PD. In addition, the minimum permitted PD standby power has been lowered, improving support for low-energy designs.
Endpoint Design for USB or PoE
|IEEE standard||PoE Type||Power to port||Maximum current|
Table 1: PoE standards.
A conventional USB 2.0, USB 3.2, or USB Type-C port provides a relatively stable 5V DC supply that can be taken directly from dedicated power lines in the connector. A DC-DC converter, such as the Diodes Incorporated AP61100 5V, 1A buck converter, is an ideal device to stabilize this power for IoT endpoints operating from a USB supply.
In a PoE powered system, a transformer is required for each cable pair, as well as a bridge rectifier, a PD controller, and a DC-DC converter to power the load. Suitable DC-DC converters for PoE secondary DC bus applications include Diodes’ AP62200 (18V/2A) and AP63200 (32V/2A) synchronous buck converters. These have a wide input-voltage range and feature high-side and low-side MOSFETs with low on-resistance to maximize energy efficiency. Constant On-Time (COT) control in the AP62200 series and peak-current mode control with integrated compensation loop in the AP63200 series offer enhanced performance while minimizing external components.
USB provides sophisticated features that help with system management, such as ensuring each device takes only the power that it requires. Negotiation protocols and power profiles defined within the standard enable communication between power sources and endpoints to optimize the available power.
On the other hand, there is no handshake or data exchange before power is applied to a device. While this contributes to the economy and plug-and-play simplicity of USB, eliminating any requirement for a secondary power source in the device being powered, there can be a risk of faults. One example is if a device tries to demand too much power or if it introduces a short-circuit.
Designers can protect against these situations by using a power switch, such as a Diodes AP22811, AP22804, or AP22814. These devices implement over-current, short-circuit and over-temperature protection with auto-recovery. Protection against reverse current and voltage is also built in. With a maximum current rating of 3A and minimum on-resistance of 50mΩ, they provide a robust and efficient means of protecting USB ports. Alternatively, the AP22652 or AP22653 allows the current limit to be adjusted up to a maximum of 2.1A and has a maximum on-resistance of 65mΩ. Another option is the AP22615 (Figure 2) or AP22815, which feature over-voltage-protection up to 28V and provide both fixed and adjustable current-limit options.
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Figure 2. The AP22615 provides integrated protection for USB-powered devices. [Source: Diodes Incorporated]
The USB switches also provide a soft-start function, which controls the output voltage rise-time to protect both the source and load. An additional safety feature protects against current surges to minimize false fault conditions.
In IoT applications that demand high power, such as access control or video devices, designers need to keep an eye on power efficiency to deliver optimal performance for a limited space and cost. It is important to choose efficient devices at each stage of the power system and take advantage of high density and integration where possible. Diodes’ ZXTR2000 family of linear regulators (Figure 3) is designed for 48V DC power systems, including PoE applications. These regulators integrate a transistor, Zener diode, and resistor into a single package, which reduces component count and board space requirements.
Figure 3. The XTR200 family delivers an integrated solution for high-power PoE applications. [Source: Diodes Incorporated]
Designers of IoT endpoints can now think beyond off-line, wireless power transfer and battery power to meet the needs of demanding applications. Wired standards such as USB and PoE offer convenient and flexible options to overcome power supply challenges, leveraging off-the-shelf DC-DC converters, regulators, and protection devices.
>> This article was originally published on our sister site, Power Electronics News.
|Benjamin Tang is Product Marketing Manager at Diodes Incorporated.|
|Mingyu Qu is Strategic Marketing Director at Diodes Incorporated.|