Molex Mini-Fit Jr – Know your Power, and Don’t Get Confused – “PCIe” and “Xilinx Not PCIe” Power Connector

To move packets you need power, but how much can you draw from an interface?

PCI Express Cards Consumption Limit

PCIe has been around for a long time, since 2003, and many people know the maximum power draw of 75W per PCIe slot, but is it that simple? The short answer is no, but let’s have a look into the spec. It states that all cards can consume up to 3A on the 3.3 V power rail with the following restrictions applying:

• x1 cards: 0,5A on 12V, but overall consumption limit is 10W
• x4 – x16 cards: 2,1A on 12V, but overall consumption limit is 25W 

But where does the 75W come from? Well, exceptions apply:

High power devices can draw more power after the initialization and configuration:

• x1 cards can consume 25W 
• x16 cards can consume 75W

Using Additional Connectors for Higher PCIe Card Consumption

With additional connectors (6pin and 8 pin) cards can consume up to 300W. A 6 pin adds 75W while a 8 pin adds 150W.

But be careful, in the FPGA world there are two versions of the 6/8 pin Molex Mini-Fit Jr connectors, the Xilinx and the PCIe option. 

The 6 Pin option has 2 pins (3 and 4) swapped. If you mix up the connectors 12V will directly connect to Ground, this can destroy your device.

6 Pin Power Connector
Pin	PCIe Spec	AMD/Xilinx Dev Boards
1	+12 V	+12 V
2	+12 V	+12 V
3	+12 V	Ground
4	Ground	+12 V
5	Sense	Sense
6	Ground	Ground

The 8 Pin option the changes are less subtle, all pins have a different function and cause an electrical short.

8 Pin Power Connector
Pin	PCIe Spec	AMD/Xilinx Alveo Cards
1	+12 V	Ground
2	+12 V	Ground
3	+12 V	Ground
4	Sense1	Ground
5	Ground	+12 V
6	Sense0	+12 V
7	Ground	+12 V
8	Ground	+12 V

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