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Dependence of Power Consumption on Overclocking

In addition to our power consumption tests discussed above we also decided to find out how greatly the power consumption of a typical graphics accelerator may be affected by overclocking. For this type of study we picked the most typical solutions of the today’s graphics market: ATI Radeon HD 5850 and Nvidia GeForce GTX 275. The latter is currently a little cheaper in retails, but is also a little slower while Radeon HD 5850 is a true sales hit in $320-$350 price range.

Since the top Radeon HD 5800 models are equipped with Volterra controllers, they allow software management of the GPU voltage levels that is why for our overclocking experiments we resorted to MSI Afterburner and AMD GPU Clock Tool.

The original vGPU setting for Radeon HD 5850 is 1.088 V and we decided to raise it only if the card loses stability at the given frequencies. To avoid possible overheating and failure for the card the Radeon HD 5850 cooler was working at the utmost of its capacity.

As FOR Nvidia GeForce GTX 275, things got a little bit more complicated here: the reference design of these graphics cards uses a VRM controller that doesn’t support any software management. Therefore, there is only one way to increase the GPU voltage besides hardware voltmodding: BIOS modification using NiBiTor or any other program with similar functionality and further use of this modified BIOS on the card. By default, GeForce GTX 275 graphics processor works at 1.05 V in 3D mode and 1.17 V in extreme mode. Using NiBiTor both settings can be increased to 1.18 V, but unfortunately, not any higher than that.

Since we are particularly interested to see how the cards will behave in games, we decided to use the same Crysis Warhead with DirectX10/Enthusiast settings on "frost" map in 1600x1200 resolution and enabled MSAA 4x. Here are the obtained results:


* - memory overclocked to 1250 (2500) MHz

First of all, we see that even if memory overclocking does affect power consumption, this influence is not dramatic: after raising the memory frequency on Radeon HD 5850 from 1000 (4000) MHz to the maximum possible frequency (without stability issues) of 1200 (4800) MHz, we managed to detect an about 10 W increase in power consumption. The same action performed on GeForce GTX 275, which memory was overclocked from the nominal 1134 (2268) MHz to 1250 (2500) MHz produced the same 10 W power consumption increase.

Further overclocking of Radeon HD 5850 core with 25 MHz increment continued successfully up until 825 MHz. This is when the card became unstable. In the interval from 725 to 850 MHz the power consumption increased by only about 20 W. by raising the core voltage to 1.14 V, we managed to hit 925 MHz core frequency. The card has immediately turned way more energy-hungry. However, the biggest challenge awaited us ahead: we managed to hit 950 MHz frequency only by pushing the voltage to 1.2 V, which bumped the power consumption from 165 to 200 W right away. We struggled to hit 1000 MHz barrier until we decided to take the risk and increase vGPU to a pretty dangerous level of 1.35 V. as a result, the height was taken, the card passed our stability tests successfully, but the peak power consumption readings in this case registered 276 W! I believe that with liquid-cooling the card can work like that for a mighty long time, but overheating is not the only threat during overclocking involving increased vGPU. When the chip voltage is increase by more than 30% over the nominal, electro-migration accelerated significantly and the chip may fail at any time. Moreover, the graphics adapter voltage regulator circuitry, which is also not designed to withstand loads like that for a long time gets overloaded as well.

As for GeForce GTX 275, things were a little easier on it, because it didn’t allow any extreme voltmodding without serious hardware modifications. As a result, the card worked stably at 734/1620 MHz without a modified BIOS and at 756/1674 MHz with one. It seems to be the best our GeForce GTX 275 can do: all further attempts to increase the GPU frequencies at least a little bit more didn’t succeed and every time we launched Crysis Warhead and started the test the system would reset everything and reload the driver. The power consumption difference between the minimum and maximum core frequencies was only 32.4 W, which is nothing compared with 148.8 W we have just seen by ATI Radeon HD 5850. However, there is also less risk involved with Nvidia graphics card overclocking, unless you resort to hardware modification of the GPU voltage regulator circuitry.

The results for each power line are pretty interesting:

The +3.3 V line that seems to be feeding the auxiliary circuitry of the graphics card hardly depends on the clock frequency. The internal +12 V line depends on them, but in a somewhat strange way: the current in it was increasing slowly as we continued overclocking, but as soon as we resorted to the first voltmodding measures, it dropped from the maximum registered level of 2.6 A down to 2.1 A and stayed there until the end of the experiment. The line marked “12V 6/8-pin” on the graph was connected to the top first power connector of Radeon HD 5850. During the entire overclocking session it behaved: the current was going up slowly from 2.3 A to 4.3 A and only when we increased vGPU to the extreme 1.35 V it jumped up to 6.5 A. The load on the lower power connector increases much more aggressively. Although the graph remains pretty gentle up to 1.14 V GPU voltage, we see a sharp surge at 1.2 V, and at 1.35 V the current in this line may easily hit 14 A or more, which is around 170 W! I have to repeat one more time: do not attempt to repeat these experiments at home, especially, if your favorite graphics card is the only one you have and there is nothing to replace it with once it fails.

As I have already mentioned, Nvidia GeForce GTRX 275 acts more calmly during overclocking and the graph below proves it clearly. The currents in the internal power lines remain almost unchanged, and grow gradually in the external ones, with the second power connector located closer to the PCB edge is loaded noticeably heavier. However, we didn’t manage to hit the same extreme numbers as we did with Radeon HD 5850 even here; the maximum current we recorded here was only 8.8 A, which means that the load never exceeded 106 W on this connector.

 
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