Views: 1 Author: Site Editor Publish Time: 2020-04-29 Origin: Site
What’s new in USB Power Delivery 3.0
Since its official debut in 1996, the Universal Serial Bus (USB) protocol has seen numerous revisions spanning three generations of the standard, with USB 4.0 waiting in the wings for later in 2019. Meanwhile, in parallel, the USB Implementer’s Forum (USB-IF), which oversees the USB standard, has maintained parallel development tracks for USB interconnects and USB Power Delivery (USB PD) technology.
The USB Type-C interconnect (2014) brought a smaller, slimmer plug that’s reversible, eliminating the fumbling around to properly align the plug with the jack. USB Type-C also delivered higher data transfer rates and higher charging voltages/currents. When USB 3.1 arrived, so did USB PD 2.0. Now, USB 3.2 has been released as has PD 3.0. It is useful to review some of the changes from PD 2.0 to the current 3.0 revision of the standard as well as elements of PD compliance testing.
USB PD is a specification for handling higher power on USB and allows a range of devices to charge quickly over a USB connection. It facilitates negotiation between two devices for a power contract, so they can determine how much power can be pulled from the charger. Power Delivery starts at the 5-V setting and is configurable up to 20 V. Using a standard USB-C cable, it can handle up to 60 W and will go up to 100 W using a full-featured USB Type-C (ECMA) cable.
Another point of interest regarding Power Delivery is that it allows for power to flow both ways, with no set direction based on circuit or connection. For example, if you were to connect two phones that support Power Delivery with a USB-C charging cable, one phone could charge the other and vice versa.
A newly introduced PPS (Programmable Power Supply) protocol requires that a PPS-capable sink be able to request adjustments of source output voltage in increments as small as 20 mV. Likewise, the sink can request source current limits in 50-mA steps.
The sink should check in with the source at least every 10 seconds with a voltage/current request. Sources operate in constant-voltage or current-limit modes depending on the load; current-limit mode happens only when the sink attempts to draw more than theload; current-limit mode happens only when the sink attempts to draw more than the negotiated current.
Power Delivery 3.0 vs 2.0
There are a number of significant differences between the PD 2.0 specification and the latest 3.0 specification:
Power Delivery Profiles (PDPs): Since the PD 1.0 specification, USB PD’s Power Profiles were a way to communicate power-supply capabilities to a USB end user. PDPs are a revamp of Power Profiles that now tell the user what the power supply can deliver in Watts; a higher value means more power. A PDP is the highest nominal voltage times the nominal current reported in fixed Power Delivery Objects (PDOs).
Standards-Based Charging: For years, the USB Implementer’ Forum (USB-IF), which oversees the USB standard, has been working to globally standardize around USB for charging. The International Electro-Technical Commission (IEC) has long since formally adopted USB, and more recently, USB Type-C and PD.USB PD Multi-Port Chargers: PD 3.0 needed a way to communicate PDPs to multi-port chargers. The standard settled on two categories of charger ports—Assured-Capacity Ports and Shared-Capacity Ports. In the former case, each port is able to deliver its rated/labeled power capacity independent of all other ports. In the latter, each port is able to deliver its rated/labeled capacity depending on the remaining available capacity that’s shared among multiple ports. The total available power capacity of the multiple ports is indicated to the user, and all ports on the charger are capable of delivering the same power rating. So, for example, as long as one port on the charger can maintain 30 W, charger packaging can claim a PDP of 30 W.
From the USB Implementors Forum. At left, an example of an Assured-Capacity Charger with a total capacity of 60 W and a USB Charger certification of 30 W. At right, an example of Shared-Capacity Charger with a total capability of 60 Wand a USB Charger certification of 27 W.
PPS: Another outcome of the USB-IF’s efforts to create an international standard that uses the USB Type-C interconnect is the concept of PPSs. There are two usage models: Powering devices in use with a fixed source and charging of battery-powered devices. The fixed-voltage features were in place with PD 2.0; PD 3.0 introduces PPS, which attempts to make battery charging more efficient.
The USB Type-C compliance test matrix, created by the USB Implementors Forum, indicates a large amount of testing for products to be certified as compliant with the standard. Essentially, everything but cables must be subjected to the Type-C test suite for compliance purposes. If a device is “PD enabled,” it must also pass the PD compliance suite. There is also an interoperability test suite as well as several tests specifically for power sources.
For power delivery to devices, the key characteristics include a fixed-voltage supply, stable current, and the ability to handle dynamic loads. For battery charging, the source must deliver variable voltage and current and account for charging rate and thermal constraints. Most battery manufacturers recommend beginning with a constant current and gradually increasing voltage followed by constant voltage and gradual reduction in current.
With fixed PDOs, the charger advertises its maximum output and the sink side chooses the closest fit.Fixed-voltage chargers are typically unable to charge quickly without generating excessive heat. In the PPS methodology, however, the sink “micro-manages” its own charging regime by requesting that the source make voltage changes in small steps. As a result, charging completes in less time while keeping heat under control, which can also help extend the battery’s lifespan. Once the source reaches predetermined voltage level, the sink re-negotiates by requesting a constant voltage.
New PD 3.0 terminology for PPS mode: Many different devices might be plugged into a given PD charger, but PD aspires to universality. Thus, the PPS approach demands that the sink side be smarter, given that the sink knows how much power it needs. This gives rise to the new term Augmented PDO (APDO) to define chargers that can deliver a range of voltages. Yet, a maximum current still must be defined. PPS sinks will now use what are termed Request Data Objects (RDOs) to make these real-time dynamic adjustments to power.
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