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8/9/2012
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USB Poised To End Power Brick Hell

The new USB Power Deliver (USB-PD) standard and several associated standards put USB on a path to being the universal electronics connector, replacing even the power adapter for notebooks. A greater variety of power levels allow it to control and power many more devices, including HDTV, than current standard connectors.

Last month, the USB 3.0 Promoter Group announced a new 100-watt power delivery standard called USB Power Delivery (USB-PD). This is a massive improvement from the existing 2.5-watt capability and even the unsupported 10.5-watt devices on the market such as the iPad. With USB-PD, and the new Mobile High-Definition Link (MHL)-USB standard, which lets USB ports connect to high-definition monitors, USB is poised to become a universal three-in-one connector for tablets and notebooks.

The newest MHL-enabled smartphones--and soon tablets--have already implemented this three-in-one capability with a consolidated Micro-USB connector. A $10 MHL USB-HDMI adapter allows these smartphones to connect to existing HDTVs or LCDs that have HDMI ports. But until now, this capability had been missing from notebooks because of the inadequate power supplied by the old USB standard and the lack of MHL support. With USB-PD and MHL, gone will be the bulky HDMI ports and proprietary DC power connectors and everything will be consolidated to the existing USB or Micro-USB port.

To understand why the old USB standard was so stingy on power delivery, first we need to know a little bit about current (ampere), voltage (volt), and power (watt). In laymen's terms, current is the amount of electrical charge delivered per second and voltage is the energy potency of that charge. Power is the amount of energy delivered per second. When we multiply current and voltage, we get power in watts.

Many companies inflate their battery capacity by quoting massive amp-hour ratings based on some internal battery voltage of 3.7 volts. The third-generation iPad, for example, claims 11,666 milliamp-hours even though it only contains 43 watt-hours of battery energy which means it can run at something like 4.3 watts for 10 hours. (We derived 43 watt-hours by multiplying 11.666 amp-hours by 3.7 volts.) Yet a 4000 milliamp-hour battery in a notebook such as the ThinkPad X200 contains 57.6 watt-hours of energy because Lenovo bases its amp-hour rating on 14.4 volts. The moral of the story? Higher voltage means more energy bang per amp.

The old USB standard was limited to a maximum voltage of 5 volts and a maximum current of 0.5 amps. Five volts multiplied by 0.5 amps equals 2.5 watts maximum power delivery. Most smartphone makers unofficially bumped the maximum current to 2.1 amps using thicker cabling because they needed to deliver 10.5 watts to charge their power-hungry smartphones. This unsupported configuration was fine with a thicker USB cable but it could potentially cause overheating problems for thinner USB cables designed to push 0.5 amps.

The new USB-PD standard not only bumps up the maximum current to 5 amperes, it also bumps the maximum voltage up to 20 volts. Five amps times 20 volts produces an astounding 100 watts! Twenty volts was selected because nearly all notebook power connectors operate at between 16 and 20 volts. USB-PD eliminates the need for separate power connectors on notebooks and tablets. So not only will the USB port on a notebook be able to support higher-power devices such as external optical drives without the need to daisy chain two USB connectors to double up on power, the same USB port can reverse direction and pull power from an external source such as a battery or wall socket.

Because the USB-PD standard is backward compatible and also needs to be able to support a wide range of lower-power devices, it has the intelligence to detect the cabling and end-point devices to avoid fried circuits and overheated wires. Not all circuits can handle higher voltages and not all wires and connectors can handle higher amperage, so the USB-PD standard supports a wide range of current and voltages through different profiles from which system and device designers can choose:

Original
USB Spec
  • 5V @ 0.5A 2.5W
Nonstandard
USB
  • 5V @ 2.1A 10.5W
Profile 1
  • 5V @ 2A 10W
Profile 2
  • 5V @ 2A 10W
  • 12V @ 1.5A 18W
Profile 3
  • 5V @ 2A 10W
  • 12V @ 3A 36W
Profile 4
  • 5V @ 2A 10W
  • 12V @ 3A 36W
  • 20V @ 3A 60W
Profile 5
(Supports only
full-size USB
connectors)
  • 5V @ 2A 10W
  • 12V @ 5A 60W
  • 20V @ 5A 100W

As you can see, devices with Micro-USB ports can draw only a maximum of 60 watts due to smaller connectors being limited to 3 amperes. That's still a vast improvement over the existing nonstandard solutions. The third-generation iPad, for example, takes nearly seven hours to charge its relatively large 43 watt-hour battery even though the iPad claims the battery is full an hour before it's actually done charging. If future iPads or other tablets bumped up their voltage to 12 volts or more, they could easily halve the charging time.

The USB-PD standard also can solve other problems such as excessive cabling for TV sets. Set-top boxes could conceivably use a single MHL-USB connector to connect to an HDTV while drawing up to 100 watts of power. The potential flexibility means that USB could become the de facto power connector for all sorts of electronics devices and end our collective DC adapter hell. Wall plates could be replaced with combination AC sockets and USB-PD enabled ports. Many airports already are doing this with normal USB sockets for smaller devices, and you can buy wall plates and power strips with USB. But USB-PD means that the heavy notebook chargers we've lugged around for the last 30 years finally could become a thing of the past.

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