We remarked upon the GTX 750 / 750 Ti reveal that passive cards were a distinct possibility, given the low TDP and ability of the cards to operate solely on motherboard PCI-e power. Hovering at a 55W TDP, nVidia’s GM107-powered GTX 750 doesn’t draw any power from the PSU and has a lower thermal footprint than any of its higher-powered brethren. With the right heatsink design, it’s always been an ideal candidate for a passively-cooled, silent, low-profile HTPC video card.
ZOTAC announced its “GTX 750 ZONE” passively-cooled solution just a few weeks ago. Standard GTX 750 specs apply, the one exception being that Zotac has nixed the active fan in favor of a larger aluminum and copper heatsink with no active components. Thermals are always a concern when operating a passively-cooled device, and with GPUs, thermals will directly impact the throttling and performance (FPS) output in games.
We benchmarked Zotac’s passive GTX 750 Zone video card for temperatures and framerate (FPS) in Metro, GRID, Battlefield 4, Titanfall, Watch_Dogs, and FurMark. These results can be extrapolated upon for a wider-spectrum understanding of the GPU’s worth for gaming.
Zotac GTX 750 ZONE Specs
|GPU||NVidia GTX 750 (GM107)|
1111MHz Boost CLK
|Memory Spec||1GB GDDR5
5000MHz Mem CLK
ZOTAC GTX 750 ZONE FPS Benchmark, Video Review, Gameplay
Looking at the GTX 750 ZONE
The video card is pretty standard-issue when it comes to accessories. You get the box, the card, a DVI/VGA adapter, and that’s about it. As with all GTX 750 devices, the ZONE is limited to 1GB of GDDR5 RAM, though this will be plenty of memory for HTPC users. The card will be more limited by the GPU’s computational performance than the memory at this point, anyway, and it’s really not meant for higher-end gaming than what a 1GB capacity can fuel.
Priced at $120, MSRP is just above what most GTX 750 active cards are selling for and just under the $140 GTX 750 Ti 2GB (active) card. Passive cards always bear with them some added cost over a reference design, but you’re paying for silence and slightly lower TDP.
How Does Passive GPU Cooling Work?
We haven’t discussed how a passive GPU cooler works before, though we’ve explained CPU heatsinks in complete detail. The methodology is identical.
For ease of understanding, here’s a photo of the 750 ZONE when disassembled:
A copper coldplate is mounted directly to the GPU, secured only by four screws on the rear side of the circuit board. The coldplate, as seen above, has thermal compound residue on it that matches up with the thermalpaste residue on the actual GPU (which is the small piece of silicon mounted to the board). This is the point of contact.
The copper coldplate conducts the heat away from the GPU with direct contact, using the thermalpaste as a means to aid in smoothing out the divets in the surface of the copper (TIM has a higher thermal conductivity than air, which is what would fill any imperfections if TIM were not present). Copper has a thermal conductivity (measured in Watts per Meter Kelvin) of nearly 400W/mK at 25C. Aluminum is approximately half of that, and for reference, thermal compound (TIM) tends to sit in the 4-8W/mK range.
Once the heat has been conducted to the coldplate by the copper’s natural thermal conductivity, the heatpipes begin their work. I previously made a graphic to describe how heatpipes work -- you can read that full article here (as already linked above).
What’s going on here is phase changing and basic physics. The heatpipes contain a small liquid (coolant) that would be exposed if cut open with a hacksaw. This liquid undergoes a phase change when heated by the heat conducted away from the GPU, turning it into a gas. Capillary action does its work as the gas moves to the opposite end of the copper heatpipe, condenses in the tip, and cycles back toward the GPU in liquid form to be reused. As the gas and liquid moves throughout the copper heatpipes (two, in the case of the 750 ZONE), the encasing aluminum fins from the larger heatsink help duct heat from the copper pipes (heat wants to spread outward and disperse). This cools down the pipes and liquid, allowing the recondensing into liquid.
It appears that the ZOTAC 750 ZONE has a third machined hole for another heatpipe, but there isn’t one there -- maybe it was deemed unnecessary.
The aluminum fins then must be cooled to ensure efficiency as the card continues to heat up. Normally, this is where the fan would come in -- the fan forces air through the heatsink to pull heat away from the fins as cooler air passes through. In the case of a passive solution, we’re relying primarily on static air in the environment or intake from case fans. Our test bench uses the Phantom 820, so the ZONE has several large (200mm+) fans pointed directly at it. The air from this intake is what cools the fins, which cool the heatpipes, which cool the coldplate, which cools the GPU.
This is almost a perfect system with no noteworthy loss of liquid, so it will effectively supersede the usable life of the GPU several times over.
Continue on to page 2 for the benchmark charts, FPS, and thermals!