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PCB Design and Cooling System

As we wrote in our review of the Nvidia GeForce 7950 GT, Nvidia’s partners are not limited to utilizing the reference PCB design with that graphics card. Gigabyte took the opportunity and developed a product that has little in common with Nvidia’s reference card:

It’s clear that the PCB design of the Gigabyte GeForce 7950 GT was developed from scratch. There are a lot of differences, from the color of the lacquer that covers the PCB to the memory chips being turned around by 90 degrees. There is a standard 4-pin Molex plug instead of a 6-pin power connector typical of PCI Express graphics cards. That’s why you won’t find a power adapter among the card’s accessories.

The power circuit is fully redesigned: as opposed to the reference card, the power transistors are located on both sides of the PCB while the control elements were moved to the face side. These are NexSem NX2415 and NX2305 PWM-controllers that are responsible for powering up the GPU and memory, respectively. We haven’t yet seen such controllers in our labs. What’s interesting, the NX2415 typical connection schematic uses not only a +12V power line, but also a +5V one, so the appropriate pin of the Molex connector is really made use of. The +5V voltage powers up the controller itself and doesn’t seem to be used anywhere else, so the load on that line is very low. This power circuit design is most unordinary for graphics cards with a PCI Express interface which normally use +12V and +3.3V lines only.

The card carries eight chips of GDDR3 memory from Qimonda (ex-Infineon) that are marked as HYB18H512321AF-14. Each chip has a capacity of 512Mb (16Mx32), an access time of 1.4 nanoseconds and a voltage of 2.0V.

The chips are rated to work at 700 (1400) MHz and this is indeed the frequency they are clocked at on the Gigabyte GeForce 7950 GT, in full compliance with Nvidia’s specifications. The GPU frequency is exactly as specified by Nvidia, i.e. 570MHz for the vertex processors and 550MHz for the rest of the GPU subunits. So although the Gigabyte GeForce 7950 GT has a unique PCB design, its technical characteristics match precisely those of the reference card from Nvidia.

Among everything else we can note such things as the neat plastic plugs on all the external connectors as well as on the SLI connector. Note also the emblem telling us that the Gigabyte GeForce 7950 GT is manufactured using lead-less technologies.

Although the GPU and memory of the Gigabyte GeForce 7950 GT work at the frequencies of the reference card, this device is equipped with an advanced cooler, the VF700-AlCu model from Zalman. This is not a flagship product in Zalman’s line-up, yet it boasts a total ribbing area of about 500 sq. cm and a high-quality 80mm fan on two frictionless bearings. The heatsink is shaped like Zalman’s CNPS7x00 series coolers for the CPU, but is modified a little to adapt to the GPU geometry. The ribs are profiled in such a way that the bottom part of the heatsink is much smaller than its top part and the latter goes beyond the dimensions of the PCB. The bottom part won’t press against the mainboard, though.

The cooler is somewhat simplified in comparison with the retail version. First, the fan is adapted for a two-wire connection. Second, the memory chips lack heatsinks which are included with Zalman’s VF700-AlCu. And third, the cooler is fastened to the card without using a plate that would prevent the PCB from bending. The lack of memory heatsinks is not a problem because the cooler is designed in such a way as to cool the memory chips with the airflow from the fan. Moreover, modern GDDR3 memory working at frequencies about 700 (1400) MHz generates little heat and is quite capable of doing without any cooling at all. Having such an advanced cooler the Gigabyte GeForce 7950 GT should do well at overclocking, but the overall success will also depend on other factors like the frequency potential of the particular sample of the G71 chip and the overall design of the PCB.

 
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