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Method for controlling CPU fan speed using CPU power consumption as the feedback parameter

IP.com Disclosure Number: IPCOM000019984D
Publication Date: 2003-Oct-15
Document File: 7 page(s) / 126K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for controlling central processing unit (CPU) fan speed using CPU power consumption as the feedback parameter. Benefits include improved functionality, improved performance, and improved reliability

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Method for controlling CPU fan speed using CPU power consumption as the feedback parameter

Disclosed is a method for controlling central processing unit (CPU) fan speed using CPU power consumption as the feedback parameter. Benefits include improved functionality, improved performance, and improved reliability

Background

         A high-speed CPU must dissipate up to 60W of heat during worst-case operations. A passive heatsink cannot provide sufficient dissipation. As a result, the use of a fan is required. One typical approach is to use an always-on fan that constantly operates at a full speed and provides for the worst-case condition. However, the CPU functions most of its time well under worst-case conditions.

Temperature feedback approach

         The typical fan-speed control uses temperature as a feedback element. However, a time lag can occur from the moment the CPU increases power consumption to the moment its temperature increases. The time lag is dependent on the effective thermal mass. Additionally, the CPU must be operating at an elevated temperature for the fan to spin faster.

         An experiment can be performed to determine the temperature rise of a CPU when changing state from being idle to being fully stressed. The CPU die temperature is read out using a monitor application. Results indicate that many seconds elapse before the CPU approaches its steady state operating temperature (see Figure 1).

General description

         The disclosed method uses CPU power consumption as a feedback element for controlling CPU fan speed. The disclosed method results in reduced audible noise, reduced power consumption, and increased useful life.

Advantages

         Some implementations of the disclosed structure and method provide one or more of the following advantages:

•         Improved functionality due to using CPU power consumption to control the CPU fan speed

•         Improved performance due to enabling speed changes almost instantaneously with changes in the CPU heat dissipation

•         Improved performance due to ensuring that the CPU temperature is approximately constant throughout its operating power range

•         Improved reliability due to increasing the mean time between failure of the CPU due to reduced heat stress

Detailed description

         The disclosed method varies the CPU fan speed based on the actual CPU power consumption. The VCC of a CPU is roughly constant during its operations (disregarding the Vreg load line). The parameter that varies directly with power consumption is the current that is being drawn.

         The components required are minimal, one dual-op-amp and one positive-negative-positive (PNP) transistor. Because of its low cost, this solution can be easily incorporated into most motherboards. An alternative is to include this solution into motherboard management ASICs, easing motherboard-level integration.

         The electrical current flowing into the CPU must be measured (see Figure 2). A power plane has very small electrical resistance. However, due to the large currents involved (~...