USB Power Delivery 2.0 – End Of The Line For The Power Brick?

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The recent release of new USB specifications marks a shift in emphasis: from an interface with limited capability to supply power to a combination of data interface and power distribution system.

Traditionally, a USB port has limited power available – USB 2.0 provides a single 5V power wire with a maximum of 500 mA. That's sufficient for many smaller devices, albeit not for laptops or Hard Disk Drives (HDDs).

In fact, USB has become the sole power socket for many cell phones, solid state drives, and MP3 players. Today such devices typically charge their batteries from USB ports contained in other devices such as laptops, cars, aircraft or even wall sockets. There are also “dumb” devices such as LED lamps that are designed to run solely on USB power.

The latest USB 3.1 specification boosts the current capability to 900 mA when using traditional Type-A connectors. When paired with the new Type-C connector—with its four power/ground pairs— a USB 3.x system can deliver up to 3 A, but still at only 5 V.

That may be impressive, but it's no threat to the ubiquity of the wall wart. To do more requires a higher level of functionality—enter the USB Power Delivery Specification.

USB Power Delivery Specification: What is USB PD? 

The USB Power Delivery Specification (USB PD) enables USB to move beyond its current limited power capability to become a bi-directional power source, sinking or sourcing up to 100 W (5 A @ 20 V). USB PD is separate from USB 3.1 and USB-C; systems can be USB 3.1-compliant without meeting either USB PD or USB-C specifications.

In USB Power Delivery, pairs of USB ports negotiate voltage, current and direction of power flow over the USB cable. Power direction is no longer fixed, enabling the product with the power (host or peripheral) to provide the power; if conditions change, the system can respond.

usb figure 1

Figure 1: USB-C and USB Power Delivery specifications: one set to rule them all. (Source: NXP)

This higher power capability enables devices such as disk drives and printers to be USB-powered, which helps eliminate the need for a separate power brick for each device, and also reduces waste; by some estimates, up to 150,000 tons of battery chargers are discarded in the U.S. and Europe each year. When power management is spread across multiple peripherals, each device can take only the power it requires for the job at hand, and get more power only when needed. This enables increased flexibility in distributing power throughout a multi-device system; for example, a battery-powered device can get increased from a hub such as a laptop during charging, then supply power back when the laptop’s HDD is spinning up.

USB C Power Distribution

The USB-C connector is designed to be fully reversible and has dedicated pins for this purpose; when combined with USB-C electronically marked cable assembly (EMCA), it sets the stage for a new power distribution architecture, which is already being seen in new products.

How does it accomplish this? The USB PD makes a distinction between power sources, or hosts, and power consumers, or devices. In USB PD terminology, these functions are called DFP (Downstream Facing Port) and UFP (Upstream Facing Port) respectively.

In the simplest configuration (for example, a UFP such as a flash drive plugged into a DFP such as a laptop), 5 V power only flows in one direction—from DFP to UFP—and the orientation of the UFP Type-C connector in the DFP receptacle is all that must be determined. In this case only the DFP needs a configuration channel controller; the Vbus pin is open and the UFP includes a pulldown resistor Rd on its CC pin as shown in figure 2.

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Figure 2: Simple DFP to UFP connection. (Source: Cypress Semiconductor

The DFP has a pullup resistor Rp on both CC pins (CC1 and CC2). The value of Rp indicates the value of the current limit DFP power supply, shown in Table 1. As soon as the UFP is plugged in, the DFP can determine the connector orientation by monitoring whether CC1 or CC2 is pulled low. Figure 2 shows this situation.

usb figure 3

Table 1: Resistor values vs. DFP current limits. (Source: Cypress Semiconductor)

This simple configuration only allows for 5 V operation; if more than 3 A or 5 V is needed, then there must be a configuration controller at both ends of the link.

The next level of sophistication comes in the form of the Dual Role Port (DRP), which can be either a power provider (DFP) or a power consumer (UFP) depending on the need. The DRP now contains a configuration controller with a transmit/receive block, and includes both Rp and Rd resistors to connect to the CC pin as required.

Electronically Marked Cable Assembly (EMCA)

Most USB systems include a cable to connect one device to another. In the 5 V/3 A USB PD system, shown in Figure 2, this need only be a simple electrical connection between DFP and UFP. A more complex system, with one or more DRPs handling more than 3 A or 5 V, requires one or more EMCAs with embedded configuration controllers to provide electronic marking.

Figure 3 shows a USB connection with an EMCA; the configuration controllers are labeled “CCG1”. Since both DFP and UFP are now functioning as receptacles, they each contain CC1 and CC2 pins rather than CC and Vconn; the EMCA has two CC/Vconn pairs.

The resistor Ra in each plug signifies the presence of an EMCA to both the DFP and UFP; the Vconn supplies to the DFP and UFP are electrically isolated from each other inside the cable. Since they both must accommodate cable flipping, the DFP and UFP now include a CCG on their respective CC1 and CC2 lines.

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Figure 3: DFP to UFP connection with an EMCA. (Source: Cypress Semiconductor)

Next-Gen DC Power Distribution

Once the full USB 3.1/Type C/USB PD triumvirate is rolled out, exciting possibilities present themselves for re-imagining DC power distribution. In this vision, very few devices rely on a wall socket for power; instead DC power is routed to most devices from integrated power hubs, as shown in Figure 4.

Here, a display is connected to a standard wall socket and includes a power hub that supplies DC power to multiple devices, each with their own DRP or UFP depending on the device. In parallel with configuration and power data, normal USB serial data communication is also taking place.

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Figure 4: Potential future power distribution architecture using USB PD. (Source: USB.org)

Extending this concept one step further, futurists are predicting that low-voltage DC wiring via USB could become standard in homes and offices, providing both data and power to all connected devices. One upside is that such a network would work well with the power produced by a local solar power array, which at present, is usually fed back into the electrical grid via a complex inverter network.

In a post-Stuxnet world, though, there are obvious security implications that have yet to be satisfactorily solved. This issue of encryption and authentication over distributed networks is getting a lot of attention as part of the IoT eco-system, so it’s expected that robust and low-cost solutions will be available soon. 

AC and DC Power in Parallel

Does widespread low-voltage DC mean the end of the road for AC power to the home? Not exactly. Although USB power may be adequate for the majority of entertainment and computing devices, even small appliances use considerably more: coffee pot ~ 200 W; blender ~ 300 W; popcorn popper ~ 250 W. Many other devices—HVAC, washers and dryers, power tools—use thousands of watts or require multiphase power.

What might well happen is a parallel system of AC and DC power: DC for entertainment, computing, phones, IoT devices and perhaps lighting and alarm systems, and AC for everything else.

Arrow Electronics and the New USB standards

The USB Power delivery specification, USB 3.1, and the USB-C connector represent the new standard and the future of serial communications in numerous markets, as well as a huge opportunity for suppliers of semiconductors, connectors, cables and more. Its ramifications will affect just about every consumer market, so of course Arrow and our supplies are staying ahead of the curve.

Arrow has partnered with NXP to bring to market a comprehensive set of products, training, and support aimed at helping our customers implement world-class USB-C solutions in record time, including the industry’s first complete USB-C solution including authentication and power delivery capabilities.

In addition, other Arrow suppliers such as Microchip, TI and Cypress Semiconductor offer their own USB-C product families.



 

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