I've always wondered why computer graphic cards use an 8 pin (4 positive and 4 negative wires) connector instead of a connector with only a single positive and negative wire.
This allows multiple cheap connectors and wires to be used, instead of single thick wires and more expensive high current connectors. Multiple thin wires are also more flexible than thick wires. Multiple pins on the circuit board ease the problem of tracking high currents on a PCB.
It's not just about power handling, it's the voltage drop on the cable that needs to be well controlled.
According to the PCIe Electromechanical Specifications, power connectors are specified to deliver +12 V, and a 2x3 connector can deliver 75 W, or 150 W for a 2x4. High power PCIe devices (GPUs generally) can pull up to 300 W through a combination of PCIe power connectors and 25 W from the edge connector.
In chapter 3 of the document, it mentions:
- The +12V delivered from the standard x16 edge connector and the additional +12V(s) delivered via the dedicated 2 x 3 and/or 2 x 4 auxiliary power connector(s) must be treated as coming from independent separate system power supply rails.
- The different +12V input potentials from different connectors must not be electrically shorted at any point on a PCI Express 225 W/300 W add-in card.
- The power pins of a single 2 x 3 or 2 x 4 auxiliary power connector can be shorted together.
So per-connector there may be no difference according to spec if all the wires were combined in the harness, thought splitting it out to all the pins of the Molex connector would be costly.
I don't know how the wiring is actually done in the power supplies, but the supplies are generally split into a few rails. Combining them closer to the point of load might help with some regulation.
On the GPU there's usually several (10-20) voltage phases that combined can deliver ~200 A @ 1 V into the GPU core and memory. As an example, the Nvidia Titan V has a 2x3 and 2x4 connector and can produce up to 250 W of heat. GamersNexus did a teardown of one and looked at the power delivery circuitry:
Several reasons, all stemming from the fact that GPUs draw a lot of current.
A GPU might draw 100 W through one of those connectors. At 12 volts, that's 8 amps. High current through a wire will produce a voltage drop from one end of the wire to the other, proportional to the resistance of the wire. This results in a lower voltage at the GPU and lost power in the wire, resulting in less efficiency in the power transfer and more heat in the case.
The voltage drop can be mitigated in two main ways: one is by lowering the resistance of the wiring, either by using larger wires or by placing multiple wires in parallel. Lowering the resistance means less voltage drop and more efficiency. Another solution is to add an additional voltage sense wire that does not carry current so the power supply can sense and adjust the voltage at the load, compensating for voltage drop in the wires.
Another factor in play is mechanical concerns. Thick wires are less flexible than thin wires, resulting in more difficult cable routing, bend radius concerns, and increased stress on connectors and circuit boards. Using multiple pins also increases contact surface area, decreasing contact resistance. Parallel current paths also provides some level of redundancy.
The 8 pin PCIe power connector uses both of these solutions: three parallel power pins, three parallel ground pins, and a pair of voltage sense pins. Using three parallel power pins lowers the voltage drop through the cable while also providing good flexibility while the sense pins ensure that the load is actually receiving 12 volts.
The answer is more mundane, the 2x3 and 2x4 connectors are available in "all" systems as they are the default output on the power supply. And as they are available they don't need special power supplies to have 75W or 100W outputs.
Also the lower individual power might simplify the transformers on the GPU card, using smaller components.