BU-411: Charging from a USB Port

The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices. Compaq, DEC, IBM, Intel, NEC and Nortel contributed to the developments with the goal of simplifying the interconnection of peripheral devices to a PC, as well as to allow a greater data transfer rate than was feasible with earlier interfaces. The USB port can also be used to charge personal devices, but with a current limit of 500mA on the original design, this might have been an afterthought.

A typical USB network consists of a host that is often a PC and peripherals such as a printer, smartphone or camera. Data streams in both directions but the power is unidirectional and always flows from the host to the device. The host cannot take power from an outside source.

With 5V and 500mA available on version USB 1.0 and 2.0, and 900mA on USB 3.0, the USB can charge a small single-cell Li-ion pack. There is, however, a danger of overloading a USB hub when attaching too many gadgets. Charging a device that draws 500mA connected together with other loads will exceed the port’s current limit, leading to a voltage drop and a possible system failure. To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn.

The original USB port can only charge a small single-cell Li-ion battery. Charging a 3.6V pack begins by applying a constant current to a voltage peak of 4.20V/cell, at which point the voltage peaks and the current begins to taper off. (See BU-409: Charging Lithium-ion) Due to the voltage drop in the cable and connectors, which is about 350mV, as well as losses in the charging circuit, the 5V supply may not be high enough to fully charge the battery. This is a minor problem; the battery will only charge to about 70 percent state-of-charge and deliver a slightly shorter runtime than with a fully saturated charge. The advantage: Li-ion will last longer if not fully charged.

Standard A and B USB plugs, as illustrated in Figure 1, feature four pins and a shield. Pin 1 delivers +5VDC and pin 4 forms the ground that also connects to the shield. The two shorter pins, 2 and 3, are marked D- and D+ and carry data. When charging a battery, these pins have no other function than to negotiate current.

Pin 1 carries +5VDC (red wire) and 4 is ground (black wire). The housing connects to the ground and provides shielding. Pin 2 (D-, white wire) and pin 3 (D+, green wire) carry data

Besides the standard type-A and type-B configurations with 4 pins, there are also the USB Mini-A, Mini-B, Micro-A and Micro-B that include an ID pin to permit detection of which cable end is plugged in. The outer pin-1 is positive and pin-4 is negative. USB cables are generally standard type-A on one end and either type-B, Mini-B or Micro-B on the other. The new type-C connector described later features 24 pins and runs on the USB 3.1 standard.

Power Delivery

USB 2.0 with a current of 500mA has limitations when charging a larger smartphone or tablet battery. Keeping the smartphone running on a bright screen during charge could result in a net discharge of the battery as the USB cannot satisfy both. Connecting a high-speed disk drive requires more than 500mA and this can create a power issue with the original USB port.

In 2008, USB 3.0 relieved the power shortage by upping the current to 900mA. This current ceiling was chosen to prevent the thin ground wire from interfering with high-speed data transfer when drawing a full load.

With the need for more power, the USB Implementers Forum released the Battery Charging Specification in 2007 that enables a faster way to charge off a USB host. This led to the dedicated charger port (DCP) serving as a USB charger, delivering currents of 1,500mA and higher by connecting the DCP to an AC outlet or a vehicle. To activate the DCP, the D- and D+ pins are internally connected by a resistor of 200 ohms or less. This distinguishes the DCP from the original USB ports that carry data. Some Apple products limit the charge current by connecting different resistor values to the D+ and D- pins.

To support charging and data communication when using the DCP, a Y-shaped cable is offered that connects to the original USB port for data streaming and to the DCP port to satisfy charging needs. This appears like a logical solution but the USB compliance specification states that the “use of a Y-cable 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.” The Y-cables and the so-called accessory charging adapters (ACA) are being used without apparent difficulties.

The question is asked: “Can I cause damage by plugging my device into a USB charger that delivers more current than 500mA and 900mA?” The answer is no. The device only draws what it requires and no more. An analogy is plugging in a lamp or a toaster into an AC wall plug. The lamp requires little current while the toaster goes to the maximum. More power from the USB charger will shorten the charge time.

Sleep-and-charge Mode

In most cases, turning the computer off also shuts down the USB. Some PCs feature the sleep-and-charge USB port that remains powered on and can be used to charge electronic devices when the computer is off. Sleep-and-charge USB ports might be colored in red or yellow, but no standard exists. Dell adds a lightning bolt icon and calls it the “PowerShare” while Toshiba uses the term “USB Sleep-and-Charge.” The sleep-and-charge USB ports may also be marked with the acronym USB over the drawing of a battery.

USB 3.1 – Type-C Connector

As with most other successful technologies, USB has spawned several versions of connectors and cables over the years. USB chargers do not always work as advertised and charge times are slow. Incompatibilities between competitive systems exist, willingly or by oversight.

Companies overseeing USB standards are aware of the shortcomings and brought out the type-C connector and cable based on the USB 3.1 standard. Rather than using four-pins as in the classic type-A and type-B, the type-C connector has 24 pins and is reversible, meaning it can be plugged in either way. It supports 900mA and, on command, delivers 1.5A and 3.0A over a 5V power bus while streaming data. This results in 7.5 and 15 watt power consumption respectively, as opposed to 2.5W using the original USB (current times voltage = wattage). The type-C can go up to 5A at 12V or 20V, providing 60W and 100W respectively. Figure 2 shows the pinout of the USB Type-C connector.

Side A and B are mirror images. Some pins are connected in parallel to gain higher power and more reliable connections.

New devices come with the USB-C connector and USB 3.1, but consumers beg for two or three regular USB 3.0 ports on their gadgets to support what worked so well in the past. USB 3.1 is backward compatible with USB 2.0 and USB 3.0 and the classic type-A and type-B connectors. While in transition to the type-C, adaptors are available to convert, but expect lower data transfer speeds with adapters than what USB 3.1 offers.

With the availability of higher currents and voltages on the Type-C system as compared to the Standard A and B connectors, damage to a device can be afflicted when giving a wrong digital command. The commands may come from a device or an adapter requesting modified power demands. It is advised to only use compatible or trustworthy brands when experimenting with higher voltages and currents in USB connectors.