Physical vs Virtual/digital phases

[TechyInAZ]

Ok I'm getting confused. So my Z97-A motherboard has 8 digital phases so claims Asus. However, I looked on overclock.net and my motherboard only has 4 true phases. Could somebody please explain why Asus says 8 phases when there are technically only 4?

Thank you,

 

[InvalidError]

From what I have read, the controller used on the board has four output phases and each phase drives two pairs of MOSFETs. Each MOSFET pair and inductor still only sees 1/8th of the total current, which is the main reason for splitting power delivery across more devices in the first place.

 

[TechyInAZ]

From what I have read, the controller used on the board has four output phases and each phase drives two pairs of MOSFETs. Each MOSFET pair and inductor still only sees 1/8th of the total current, which is the main reason for splitting power delivery across more devices in the first place.

Ah ok. So that is still better than 4 phases and 4 Mosfets since usually it's the more the better (less load when there's more)?

 

[InvalidError]

Ah ok. So that is still better than 4 phases and 4 Mosfets since usually it's the more the better (less load when there's more)?

Four phases = 8 MOSFETs: one for switching the source in, the other for the synchronous rectifier.

The Z97 has 8 MOSFET pairs, that's 16 MOSFETs total. Eight of them (four pairs, one pair per phase) are directly around the controller IC under one heatsink and the eight others (four more pairs) are under the other VRM heatsink.

Here's an image of what is under one of the heatsinks:

Four inductors, eight FETs, one tiny driver IC and a handful of passives.

 

[TechyInAZ]

Ah ok, thanks for the info. So that's still plenty for 4.5Ghz at a 1.235 vcore (I'm very sure it is, just double checking)?

At least that part of my motherboard is extremely cold, as one of my radiator fans is touching the heatsink...

 

[InvalidError]

Ah ok, thanks for the info. So that's still plenty for 4.5Ghz at a 1.235 vcore (I'm very sure it is, just double checking)?

Technically speaking, there is nothing wrong with a plain four phases VRM either: the CPU draws around 80A peak, divided between four phases, that's 20A average, and with FETs that have 4mOhm Rdson, that's 1.6W worth of loss per phase. If you use two pairs of FETs per channel, average current through each pair drops to 10A, allowing you to use cheaper FETs with 6mOhm Rdson while still reducing power dissipation to 0.6W per pair (1.2W per phase when driving two pairs per phase output) and making the VRM 1.6W (0.4W per phase, x4) easier to cool.

Those VRM heatsinks are mostly decorative: they are clamped on top of the VRM's plastic/epoxy encapsulation. Most of the real VRM heatsinking is provided by the motherboard's power and ground planes.