There’s a good chance that most of your electronic gadgets charge from USB, but it wasn’t initially designed to provide much juice. USB Power Delivery (PD) changed this in a major way, making power-hungry devices on USB a possibility.
Standard USB Power Is a Little Weak
Standard USB power specifications don’t offer much in the way of power. There are various USB power standards that devices can call on. Which ones are available depends on what type of USB standard the controller, cable, and device all support. For example, USB 3.2 Gen x2 offers at most 7.5W while USB 3.0 offers 4.5W.
There are special battery charging standards over USB that can pump up to 25W of power into a device and standard USB-C can deliver up to 15W of power, assuming that the charger can provide 3A of current.
Why are USB power standards so limited? The answer is somewhat complicated, but the common denominator here is that all of these standards, even the latest USB-C standard, operate at a mere 5V. Since the total wattage that can be delivered is a product of voltage and current, the only way to give more watts at 5V is to increase current. However, as the Amps go up you’ll run into various issues such as needing thicker wiring and of course, the level of current can quickly become dangerous.
USB Battery Charging (BC) 2 offers the most watts at 5V and 5A, totaling 25W. Fast-charging smartphones, tablets, and modern laptops can easily meet and exceed that number. Quite a few proprietary USB fast-charging methods have been developed to address this, and USB PD exists as a way to accommodate these growing power needs in an industry-standard way.
USB Power Delivery Adds The Volts
The first version of USB Power Delivery running over Micro-USB offered 60W at 3A, which means that it’s also pushing 20V. That’s four times as much as the base-level USB standard. PD 1.0 on USB Type-A/B upped this to 5A, offering 100W of power.
USB-C Power Delivery comes in two power levels. PD 2.0 and 3.0 over USB-C have the same 100W power rating as PD 1.0 over Type-A/B connectors. Power Delivery 3.1, however, offers a whopping 240W of power by pushing the voltage up.
USB PD never exceeds 5A of current, but the voltage can be dynamically configured to meet the needs of a device up to the maximum power limit for the standard.
The Device Handshake
When a USB PD charger connects to a device, it performs a “handshake” asking the device how much power it needs. USB PD supports seven voltage levels at 5V, 9V, 15V, 20V, 28V, 36V, and 48V.
In the latest revision of USB PD, a device can ask for an intermediate voltage starting at 15V. If there are multiple peripherals connected to a USB PD power source, then each device only gets the amount of power it needs. When a device needs more power, it gets it for the duration of that need and then drops back to the lower level of power use.
The Cable Matters
While often overlooked, the cable that sits between your device and the USB PD charger is a crucial component. Power can only flow at a rate the cable can handle. The USB PD standard has strict safety limits to prevent cables from overheating with possibly catastrophic results.
In other words, you can only get a particular level of power such as 100W or 240W if the cable you’re using supports it. This will usually be indicated in the cable’s specifications, but in practice, the charger and device will negotiate the fastest power rate that the weakest link in the chain can handle. Whether it’s the charger, cable, or device.
Be Wary of Uncertified Products
In order for a device such as a charger to claim that it’s a USB PD product, it must comply with USB PD specifications. This means that the device will display the USB Implementer Forum’s compliance logos.
It is worth doing a little research before you buy a USB-C device, this relatively new USB standard was off to a shaky start with many cables entering the market which did not comply with the USB-IF’s standards, leading to device damage in some cases.
In other words, make sure you buy a good, high-quality cable.
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