The most common VRM configuration is a true 8-phase VRM controlled by an IR35201 in 8+0 phase mode. These VRMs almost always use an Integrated IR PowIRstage, either the 50A IR3556 or the 60A IR3555. Some vendors have used inductors where we can find the current, or they have stated the current. The VRM on the left uses 50A power stages and 76A inductors, while the VRM on the right uses 60A power stages but we couldn't find the amperage on its inductors.
The vendor on the right has also used a different configuration for phases. Instead of a true 8-phase VRM, they used 5 PWM channels, doubled them to 10 phases. The 8-phase is more than enough, and more true phases improve performance but having ten power stages instead of 8 to spread the load across can help with powering the CPU as well. I wouldn't worry too much about VRM components, as all these boards use top-notch components, but I would focus more on cooling.
The SoC rail for the CPU also requires a VRM, and most boar4ds use the same PowIRstages and PWM controller, but a 2 or 3 phase VRM.
Some motherboards have two 8-pin power connectors, and that should provide 600W+ easily. Other vendors have one 8-pin and one 4-pin connector, and these are also good enough with up to 450W+ support. You can typically pull more than those wattage levels, but those are specified to standards as maximums.
There are four different VRM cooling styles, and the effectiveness of each will depend on how the motherboard is used and where it is used. Some vendors have a big heat sink with a lot of mass, and others have optimized surface area with thinner fins. What I call a "sink" is a lot of mass that could easily absorb a lot of heat (or big bursts), and what I call a "radiator" is designed to better dissipate the heat with much more surface area than mass.
The "sink" should excel on a test bench where there is little airflow while the "radiator" should do better in a case with excellent airflow. The vendor on the left uses a mixture of a sink and radiator and should do well in a case and on a test bench. The vendor on the right has taken a much different approach and really created a hybrid, with a solid sink located on the VRM itself, and a real fin-type radiator with an active cooling fan. The motherboard on the right will do well in a case and on a test bench, especially because it can provide its own airflow.
The vendor on the left has taken a different approach than the rest and created a standalone radiator type heat sink. However, they didn't stop there, and they include mounting hardware to mount a 120mm fan right over the VRM heat sink to properly cool the VRM. The vendor on the right has created a hybrid as well, but it's significantly more sink than radiator, and it has the most mass of any of the other heat sinks as far as I can tell.
Bottom line is that all vendors have done a decent job of avoiding the problems many early X299 motherboards had, the VRM shouldn't throttle the CPU on most X399 motherboards. The bottom line is that you either should have your system in a case with decent airflow, or you should provide active cooling on a test bench without airflow.
Most X399 motherboards also have clock generators that provide decent bus speed overclocking. While most people these days are used to multiplier overclocking, some people still want to experiment with bus speed control. If a motherboard has a clock generator, I will mention it in the review.
There are multiple types of clock generator implementations; you either get one clock generator or you get two clock generators and a quick switch that seems to be attached to one. I am not quite certain why some boards have two clock generators; perhaps one maintains the bus clock for system devices while the other can change the bus clock of the CPU alone.
One vendor has also implemented a switch to disable temperature protection for their VRM, and it's designed for extreme overclocking. The same vendor also has put 2oz of copper in their PCB, which is a good thing for overclocking. If a vendor has implemented 2oz of copper, you will see them advertise it, as it's not cheap.
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