Downstream USB connectors supply power at a nominal via the V_BUS pin to upstream USB devices.
Voltage tolerance and limits The tolerance on V_BUS at an upstream (or host) connector was originally ±5% (i.e. could lie anywhere in the range 4.75 V to 5.25 V). With the release of the
USB Type-C specification in 2014 and its 3 A power capability, the
USB-IF elected to increase the upper voltage limit to 5.5 V to combat
voltage drop at higher currents. The USB 2.0 specification (and therefore implicitly also the USB 3.
x specifications) was also updated to reflect this change at that time. A number of extensions to the USB Specifications have progressively further increased the maximum allowable V_BUS voltage: starting with 6.0 V with USB BC 1.2, to 21.5 V with USB PD 2.0 and 50.9 V with USB PD 3.1, A bus-powered hub is a high-power device providing low-power ports. It draws one unit load for itself and one unit load for each of at most four ports. The hub may also have some non-removable devices in place of ports, a common example being a keyboard with two low-power A ports included, sufficient for pointing devices such as mice. (Such a keyboard is, in USB terms, one hub and one peripheral device.) A self-powered hub is a device that provides high-power ports by supplementing the power supply from the host with its own external supply. Optionally, the hub controller may draw power for its operation as a low-power device, but all high-power ports must draw from the hub's self-power. Where devices (for example, high-speed disk drives) require more power than a high-power device can draw, they function erratically, if at all, from bus power of a single port. USB provides for these devices as being self-powered. However, such devices may come with a Y-shaped cable that has two USB plugs (one for power and data, the other for only power), so as to draw power as two devices. Such a cable is non-standard, with the specification stating that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral", meaning that "if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered." Since
USB 2.0, USB standardized overcurrent protection mechanisms for USB hosts and USB devices.
USB battery charging USB Battery Charging (
BC) defines a
charging port, which may be a
charging downstream port (CDP), with data, or a
dedicated charging port (DCP), without data. Dedicated charging ports can be found on USB power adapters to run and charge attached devices and charge battery packs. Charging ports on a host with both kinds will be labeled. The charging device identifies a charging port by non-data signaling on the D+ and D− terminals. A dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals. Per the base specification, any device attached to a
standard downstream port (SDP) must initially be a low-power device, with high-power mode contingent on later USB configuration by the host. Charging ports, however, can immediately supply between 0.5 and 1.5 A of current. The charging port must not apply current limiting below 0.5 A, and must not shut down below 1.5 A or before the voltage drops to 2 V. 1.2 specifies that charging devices and ports must be designed to tolerate the higher ground voltage difference in High Speed signaling. Revision 1.2 of the specification was released in 2010. It made several changes and increased limits, including allowing 1.5 A on charging downstream ports for unconfigured devices—allowing High Speed communication while having a current up to 1.5 A. Also, support was removed for charging-port detection via resistive mechanisms. Before the Battery Charging Specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's
iPod and
iPhone chargers indicate the available current by voltages on the D− and D+ lines (sometimes also called "Apple Brick ID"). When D+ = D− = 2.0 V, the device may pull up to 900 mA. When D+ = 2.0 V and D− = 2.8 V, the device may pull up to 1 A of current. When D+ = 2.8 V and D− = 2.0 V, the device may pull up to 2 A of current. The maximum power delivered with this method was 12.48 W (5.2 V, 2.4 A), with D+ = D- = 2.7 V.
Accessory Charger Adapter A
USB On-The-Go (OTG) device has a single Micro-AB port (or, formerly, a Mini-AB port) for charging as well as for connecting either to a host or to peripheral devices. An Accessory Charger Adapter (ACA) allows simultaneous connection to a charger and either to a host or to peripheral devices, with the charger providing power to both the OTG device and any connected peripheral devices. For example, a keyboard can connect to a smartphone, or a printer, a keyboard, and a flash drive can connect to a smartphone through a
USB hub, with the ACA capable of charging the smartphone and powering the keyboard, flash drive, and hub; or the smartphone can connect to a computer (host) that does not provide full power for charging, while the ACA provides full charging power. An Accessory Charger Adapter has three ports:
OTG,
Charger, and
Accessory. The OTG port connects to the On-The-Go device through a permanently-attached (
captive) cable with a (mechanically) Micro-A plug. The Charger port is visibly marked
Charger Only and does not support USB communication with the OTG device. It is either a Micro-B receptacle or a captive cable; such a captive cable either has a Standard-A plug or is permanently attached to a charger. The Accessory port is either a Micro-AB or Standard-A receptacle. An
A receptacle by definition can only connect to peripheral devices; the Micro-AB receptacle can be used to connect either a host or peripheral devices. The captive plug of the OTG port is unusual in that, unlike a normal Micro-A plug, which is not only mechanically identifiable as an
A plug but also electrically marked as such (causing an OTG device to behave as a host), the Micro-A plug of the Accessory Charger Adapter electrically becomes
B when a Micro-B plug is connected to the (Micro-AB) Accessory port, causing the OTG device to behave as a peripheral.
USB Power Delivery 20 Gbps port) In July 2012, the USB Promoters Group announced the finalization of the
USB Power Delivery (
USB-PD) specification (USB PD rev. 1), an extension that specifies using certified
PD aware USB cables with standard USB Type-A and Type-B connectors to deliver increased power (more than the 7.5 W maximum allowed by the previous
USB Battery Charging specification) to devices with greater power demands. (USB-PD A and B plugs have a mechanical mark while Micro plugs have a resistor or capacitor attached to the ID pin indicating the cable capability.) USB-PD Devices can request higher currents and supply voltages from compliant hosts—up to 2 A at 5 V (for a power consumption of up to 10 W), and optionally up to 3 A or 5 A at either 12 V (36 W or 60 W) or 20 V (60 W or 100 W). In all cases, both host-to-device and device-to-host configurations are supported. The intent is to permit uniformly charging laptops, tablets, USB-powered disks and similarly higher-power consumer electronics, as a natural extension of existing European and Chinese mobile telephone charging standards. This may also affect the way electric power used for small devices is transmitted and used in both residential and public buildings. The standard is designed to coexist with the previous
USB Battery Charging specification. The first Power Delivery specification (Rev. 1.0) defined six fixed power profiles for the power sources. PD-aware devices implement a flexible power management scheme by interfacing with the power source through a bidirectional data channel and requesting a certain level of electrical power, variable up to 5 A and 20 V depending on supported profile. The power configuration protocol can use
BMC coding over the configuration channel (CC) wire if one is present, or a 24 MHz
BFSK-coded transmission channel on the VBUS line. It covers the
USB-C cable and connector with a separate configuration channel, which now hosts a
DC coupled low-frequency
BMC-coded data channel that reduces the possibilities for
RF interference. Power Delivery protocols have been updated to facilitate USB-C features such as cable ID function, Alternate Mode negotiation, increased VBUS currents, and VCONN-powered accessories. As of specification revision 2.0, version 1.2, the six fixed power profiles for power sources have been deprecated. USB PD Power Rules replace power profiles, defining four normative voltage levels at 5, 9, 15, and 20 V. Instead of six fixed profiles, power supplies may support any maximum source output power from 0.5 W to 100 W. The USB Power Delivery specification revision 3.0 defines an optional Programmable Power Supply (PPS) protocol that allows granular control over VBUS output, allowing a voltage range of 3.3 to 21 V in 20 mV steps, and a current specified in 50 mA steps, to facilitate constant-voltage and constant-current charging. Revision 3.0 also adds extended configuration messages and fast role swap and deprecates the BFSK protocol. On January 8, 2018, USB-IF announced the Certified USB Fast Charger logo for chargers that use the Programmable Power Supply (PPS) protocol from the USB Power Delivery 3.0 specification. In May 2021, the USB PD promoter group launched revision 3.1 of the specification. Revision 3.1 adds Extended Power Range (EPR) mode which allows higher voltages of 28, 36, and 48 V, providing up to 240 W of power (48 V at 5 A), and the "Adjustable Voltage Supply" (AVS) protocol which allows specifying the voltage from a range of 15 to 48 V in 100 mV steps. Higher voltages require electronically marked EPR cables that support 5 A operation and incorporate mechanical improvements required by the USB Type-C standard revision 2.1; existing power modes are retroactively renamed Standard Power Range (SPR). In October 2021 Apple introduced a 140 W (28 V 5 A)
GaN USB PD charger with new MacBooks, and in June 2023
Framework introduced a 180 W (36 V 5 A) GaN USB PD charger with the Framework 16. In October 2023, the USB PD promoter group launched revision 3.2 of the specification. The AVS protocol now works with the old standard power range (SPR), down to a minimum of 9 V. Prior to Power Delivery, mobile phone vendors used custom protocols to exceed the 7.5 W cap on the
USB Battery Charging Specification (BCS). For example, Qualcomm's
Quick Charge 2.0 is able to deliver 18 W at a higher voltage, and
VOOC delivers 20 W at the normal 5 V. Some of these technologies, such as Quick Charge 4, eventually became compatible with USB PD again.
Charge controllers mainstream USB PD charging controllers support up to 100 W through a single port, with a few up to 140 W and custom built up to 180 W. However the maximal power values are barely used in practice, especially for charging of mobile phones: The used standard or used USB-C chargers still remains at 1,5A@5V (=7.5W), which is the same as for USB-A. Only few chargers (sources) and charged devices (phones, clients) can really communicate, and fit the supply/demand combinations, so charging often remains just slow, at the
USB-A standard. Some do comply together to use some negotiated
PDO (
power delivery objects), but their values are static. And because there can be at most only seven
PPSs (
preprogrammed power supply), only a few devices can really use the effectiveness of
APDOs (
Augmented Power Data Object, intervals of values, dynamic by increments of 20/50 mV/mA), usually only two these dynamic offered, leaving five options for some static ones. And even if overheating is prevented, e.g. with 4.1V@2.8A (11.48W), the charging is mostly not even close to the upper power limits of the devices, mostly 27W (9V@3A). That is because the voltages supported by phones are usually 5V or luckily also 9V (3.3-11V on APDO), and 3A at most, often 2.22A only, for voltages over 9V, so 20W is even lower upper limit for power delivery in practice, and still as a peak optimal value. Another reason for differences in speed of charging, the power delivered to mobile phones, are different approachs of producers: E.g. Samsung prefers max charging speed, accepting power even out of USB-C/PPS specifications, so 5A @4.2V (21W) are measurable. On the other hand Sony Xperia shows the very opposite extreme, following strictly the PPS specifications, so when the charging status is out of the official, it quickly defaults down to the basic 1.5A (7W) charging, the "classical slow", effectively protecting the phone from overheating and prolonging lifetime of its battery. Higher powers than 27W/33W are then accomplished by higher current (5A), and higher voltage (20V), which render the upper limit 100W (or 60W @3A). But such high power is not commonly possible for mobile phones, and for currents over 3A special protocols and cables (with electronics inside connectors) are usually needed, e.g.
E-marker.
Sleep-and-charge ports Sleep-and-charge USB ports can be used to charge electronic devices even when the computer that hosts the ports is switched off. Normally, when a computer is powered off the USB ports are powered down. This feature has also been implemented on some laptop docking stations allowing device charging even when no laptop is present. On laptops, charging devices from the USB port when it is not being powered from AC drains the laptop battery; most laptops have a facility to stop charging if their own battery charge level gets too low. On Dell, HP and Toshiba laptops, sleep-and-charge USB ports are marked with the standard USB symbol with an added lightning bolt or battery icon on the right side. Dell calls this feature
PowerShare, and it needs to be enabled in the BIOS. Toshiba calls it
USB Sleep-and-Charge. On
Acer Inc. and
Packard Bell laptops, sleep-and-charge USB ports are marked with a non-standard symbol (the letters
USB over a drawing of a battery); the feature is called
Power-off USB.
Lenovo calls this feature
Always On USB.
Mobile device charger standards In China Starting in 2007, all new
mobile phones applying for a license in
China are required to use a USB port as a power port for battery charging. This was the first standard to use the convention of shorting D+ and D− in the charger.
OMTP/GSMA Universal Charging Solution In September 2007, the
Open Mobile Terminal Platform group (a forum of mobile network operators and manufacturers such as
Nokia,
Samsung,
Motorola,
Sony Ericsson, and
LG) announced that its members had agreed on Micro-USB as the future common connector for mobile devices. The
GSM Association (GSMA) followed suit on February 17, 2009, and on April 22, 2009, this was further endorsed by the
CTIA – The Wireless Association, with the
International Telecommunication Union (ITU) announcing on October 22, 2009, that it had also embraced the Universal Charging Solution as its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."
EU smartphone power supply standard In June 2009, the
European Commission organized a voluntary Memorandum of Understanding (MoU) to adopt micro-USB as a common standard for charging smartphones marketed in the
European Union. The specification was called the
common external power supply. The MoU lasted until 2014. The common EPS specification (EN 62684:2010) references the USB Battery Charging Specification and is similar to the GSMA/OMTP and Chinese charging solutions. In January 2011, the
International Electrotechnical Commission (IEC) released its version of the (EU's) common EPS standard as IEC 62684:2011. In 2022, the
Radio Equipment Directive 2022/2380 made USB-C compulsory as a mobile phone charging standard from 2024, and for laptops from 2026.
Faster-charging standards A variety of (non-USB) standards support charging devices faster than the
USB Battery Charging standard. When a device doesn't recognize the faster-charging standard, generally the device and the charger fall back to the USB battery-charging standard of 5 V at 1.5 A (7.5 W). When a device detects it is plugged into a charger with a compatible faster-charging standard, the device pulls more current or the device tells the charger to increase the voltage or both to increase power (the details vary between standards). Such standards include: • Apple "Brick ID" 2 A and 2.4 A charging (described above, does not use BC negotiaion) •
Google fast charging •
Huawei SuperCharge •
MediaTek Pump Express •
Motorola TurboPower • Oppo Super
VOOC Flash Charge, are also known as
Dash Charge or
Warp Charge on
OnePlus devices and
Dart Charge on
Realme devices • Qualcomm
Quick Charge (QC) • Samsung Adaptive Fast Charging
Non-standard devices Some USB devices require more power than is permitted by the specifications for a single port. This is common for external hard and
optical disc drives, and generally for devices with
motors or
lamps. Such devices can use an
external power supply, which is allowed by the standard, or use a dual-input USB cable, one input of which is for power and data transfer, the other solely for power, which makes the device a non-standard USB device. Some USB ports and external hubs can, in practice, supply more power to USB devices than required by the specification but a standard-compliant device may not depend on this. In addition to limiting the total average power used by the device, the USB specification limits the
inrush current (i.e., the current used to charge decoupling and
filter capacitors) when the device is first connected. Otherwise, connecting a device could cause problems with the host's internal power. USB devices are also required to automatically enter ultra low-power suspend mode when the USB host is suspended. Nevertheless, many USB host interfaces do not cut off the power supply to USB devices when they are suspended. Some non-standard devices use the USB 5 V power supply without participating in a proper USB network, which negotiates power draw with the host interface; these devices typically violate the standards by drawing more power than is allowed without negotiation. Examples include USB-powered keyboard lights, fans, mug coolers and heaters, battery chargers, miniature
vacuum cleaners, and even miniature
lava lamps. In most cases, these items contain no digital circuitry, and thus are not standard-compliant USB devices. This may cause problems with some computers, such as drawing too much current and damaging circuitry. Prior to the USB Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and communicate their current requirements to the host, which then permits the device to switch into high-power mode. Some devices predating USB Power Delivery, when plugged into charging ports, draw even more power (10 watts) than the Battery Charging Specification allows, using proprietary methods but without violating USB standards, maintaining full compatibility—the
iPad is one such device; it negotiates the current pull with data pin voltages.
Barnes & Noble Nook Color devices also require a special charger that can provide 1.9 A.
PoweredUSB PoweredUSB is a proprietary extension, from long before USB Power Delivery, that adds four pins supplying up to 6 A at 5 V, 12 V, or 24 V. It is commonly used in
point-of-sale systems to power peripherals such as
barcode readers,
credit card terminals, and printers. ==See also==