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There are now so many so-called standards for USB, Thunderbolt and Power Delivery that we have updated our guide – my how it has changed over a year.

USB, Thunderbolt and Power Delivery for dummies covers from USB 1.0 to the new USB-4 and the confusing power delivery (PD), battery charging (BC) and Quick Charging (QC) standards (and they are not the same things).

In preparing the USB, Thunderbolt and Power Delivery for Dummies, it was not a matter of looking up a Wiki (wish it were) but consolidating USB standards and battery power standards.

  • USB-A (and variants) – As USB-A should be on the way out now our best advice is to look for USB-C devices.
  • USB-C is a little easier. Well, it was more difficult this time due to the new USB-4 standards (not yet seen on any device). And it seems some manufacturers are pushing the standards as well. For example, USB-C can have a PD from 10W to 100W, but it depends on cable ratings!
  • Thunderbolt 3 is much easier – there is only one standard but several cables. Intel is the guardian of Thunderbolt and has given it’s standard free (without royalty) to the world. Basically, it means all Thunderbolt 3 devices are interoperable.
  • BC is about charging batteries (mainly USB-A)
  • PD is about using USB-C for data and power delivery
  • Fast charge is how fast you can fill the tank (Mainly USB-C)

USB, Thunderbolt and Power Delivery for dummies

USB, Thunderbolt and Power Delivery

Note 1Mbps = .125MBps = .001Gbps or .000125GBps.


Note: Half-Duplex (HD) can send or receive data but not at the same time (think of a single lane bridge where a car has to stop to let another through), so it achieves about half the speed. Full-Duplex (FD) means it can send and receive data (two lanes) at the same time only losing a small amount of speed.

USB-A only (includes micro-USB and mini-USB connectors and variants).

  • 1.0 – up to 12Mbps (HD) – black keyed port and cable up to 5 metres
  • 2.0 – 480Mbps (HD tops out at 280Mbps) – ditto
  • 3.0 – 5Gbps (FD tops out at 3.2Gbps) – blue keyed port and cable to 3 metres

USB-C (3.1 can also use some USB-A blue keyed ports)

  • 3.1 Gen 1 – 5Gbps (FD with the right SuperSpeed cable)
  • 3.1 Gen 2 – 10Gbps (FD with the right SuperSpeed+ cable)
  • 3.2 Gen 1 – 5Gbps
  • 3.2 Gen 2 – 10Gbps
  • and 3.2 Gen 2 2×2 – 20Gbps

USB-4 (based on Thunderbolt 3)

  • Gen 2×1 – 10Gbps with USB3.2, DP1.4a and PCIe tunnelling, Host to host OTG transfers
  • Gen 2×2 – 20Gbps – ditto
  • and Gen 3×2 – 40Gbps – ditto

Thunderbolt 3

USB, Thunderbolt and Power Delivery Thunderbolt 3
  • Usually, 40Gbps over .5m Thunderbolt certified USB-C cables but older implementations and longer ‘passive’ cables will see that reduce to 10 or 20Gbps.
  • Thunderbolt 2 is 10Gbps FD and 20Gbps HD.

Battery charging (BC) Specification (these are not as ‘standard’ as we would like)

Battery Charging (for USB-A only and relates to ‘dumb’ chargers). It is unwise to plug a higher rated charger into a lower-rated device as excess power converts into heat. Conversely, you can use a lower-rated charger, and it takes longer to charge.

In our experience, USB-A chargers come in 5V/.3A (1.5W), 5V/.9A or 1A (4.5-5W), 5V/2A (10A and the limit of micro-USB cables) and 5V/3A (15W and for USB-C devices).

  • 1.0 is 5V up to 500mAh (2.5W) – there is no intelligence, and the charger should match the device amperage needs, e.g. 5V 300mA (1.5A), 5V/500mA (2.5W).
  • 1.1 is 5V up to 900mA (4.5W) – ditto. Also called USB 3.1 gen 1 over USB-C
  • 1.2 is 5V up to 1.5A (7.5W) – ditto. Also called USB 3.1 gen 2 over USB-C
  • We have seen a lot of USB-A, 5V/2.4A and 5V/3A (12-15W) chargers and these are outside BC standards – more the PD 1.0 standard below. Do not use these with older USB devices.

Generally, USB-A self-powered (bus-powered) devices (using the USB-A 2.0 host or later) draw 5V and from 100mA-1000mA (.5W to 5W).

Obviously, the BC protocol ran out of ‘steam’, and USB-C PD came into play.

Power Delivery (PD) over USB-C (applies to USB-C 3.1 or higher)

PD 1.0 (5V/3A/15W)smartphones and devices USB devices to 15W: This is the most common USB-A to USB-C cables can handle that. If the device does not need the wattage, the charger delivers lower wattage.

US PD is a standard as the charger and device have some intelligence (called a power contract or handshake) to deliver the right voltage and amperage. The only catch is that you need a cable rated for the amperage. You need an EMCA (Electronically Marked Cable Assembly) cable for over 3A. And power can flow upstream (to the device) and downstream (from the device).


You will hear the term ‘fast charge’ bandied around but, it is just pouring (flooding) more wattage into the battery. It is safe, but the jury is out on whether repeated charging shortens battery life.

There are other lower PD standards like 1.5A (12-20V/1.5-3A/18-60W), 3A (12/20V/3-5A (36-60W) and 5A (12-20/5W. 60-100W).  We seldom see them.

PD 2.0 introduced the ability to use fixed voltages from 5/9/15/20V and 3 to 5A (15-100W).

But it is not an elegant standard relying on large voltage, and amperage jumps from say, 20V/3W for the first 30%, dropping to 15W/3A for the next 20% etc. PD 2.0 tops out at 60W without a special cable.