When it comes to USB 3.0 extension cards, using the PCI Express bus, only some come with an additional header, that allows connecting a power plug from the power supply (PSU). Why is that?
There are several reasons, actually. Usually a x1 PCIe slot (the small ones) only delivers up to 25 W, per specification. The longer slots (x16 PCIe) should provide up to 75 W. USB 3.0 ports on the other hand can deliver up to 900 mA (0.9 A) at 5 Volts. If the card provides four ports, this sums up to a theoretical maximum of 18 W (4.5 W per port). So it should be enough, shouldn’t it? Well, yes…
Another import thing to note is, that PCIe ports actually do not deliver 5 V, only 3.3 V and 12 V. So the 5 V USB voltage needs to be generated by regulators on the card. These usually only employ the 12 V rail for this, which limits the maximum consumption to 66 W on a x16 slot. Still, this should be more than enough…
The cards coming with the power plug, obviously don’t need these additional regulators, because they draw the current directly from (one of) the 5V line(s) of the PSU, which is also a little bit more efficient, since the PSU provides 5 V anyway.
In fact, when looking at the cards, there are usually two types: One with a power plug, but only one switching voltage regulator (probably for the IC itself), easily recognizable by an inductor — or multiple regulators (and multiple inductors), usually one per port. These ones don’t need the additional power plug.
So which one to get? It depends! When the additional ports will (also) be used to charge devices or power 2.5″ devices like hard disks or SSDs, the power consumption often goes (far) beyond what the specifications allow. So in this case it’s better to be safe and get one with a power plug. Also: If you already have such a card, always use it with the power plug! Because if not, it will either not work at all or the ports will all be powered by only a single voltage regulator (which almost certainly won’t deliver up to 18 W or even more). But if you only plan to connect devices like USB drives or external storage devices, that come with their own power supply, a 2-port or 4-port card without the power plug will be fine. These should also be able to deliver 4.5 W at 5 V per port, maybe even a little bit more.
If the card provides more than 4 USB-ports, usually with multiple ICs, a direct connections to the PSU seems definitely more adequate. Usually there isn’t even a choice anymore, since these larger cards only come with power connectors anyway.
One last thing to consider is also the motherboard itself. A good brand will most likely have no issues delivering the needed power through the PCIe slots, even if the card exceeds the limit a little. A cheap motherboard or small form factor office PC on the other hand, may be always better of with a direct connection to the power supply.
- When in doubt and planing to buy a new one, get one with a connector to your power supply.
- When only self-powered devices or small devices like USB-Drives will be used, one without power connector will be fine.
- If you already have one without power connector, it should be fine, but avoid heavy loads, like charging larger devices or gizmos like USB ventilators and USB soldering irons. 😉