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Microsoft Takes Heat from Xbox 360

Microsoft Takes Heat from Xbox 360

A week ago, Microsoft adjusted its warranty policy on the Xbox 360 to cover general hardware failure signaled by the E74 error message.

The Xbox 360 has been a huge success since its launch in 2005, notching sales of more than 28 million worldwide, but there have been thermal management problems with the high-tech gaming device. In the course of developing new technology to dissipate heat from microprocessors, Microsoft was awarded two U.S. patents.

The power in the Xbox 360 was more than doubled from the original Xbox, going from under 100 to more than 200W, and several other important features were added, including an online capability. The box is smaller, and had to be cost competitive. The Xbox mechanical engineering team took steps to deal with thermal management, but overheating and reliability problems dogged some of the models.

The original engineering of the Xbox 360 was coordinated by Jeff Reents, the lead mechanical engineer. In an interview called "Xbox 360: The Guts," posted on the xbox.com Web site, Reents provides insight into how thermal issues were attacked.

"We originally wanted to put in a single 80-mm fan. But we couldn't fit it in the final form factor chassis," said Reents. "We couldn't just go with one 60-mm fan because it would not provide the required cooling - as the diameter of the fan increases the air flow performance per revolution increases exponentially. So we ended up putting in two 60-mm fans, and we came up with a water-cooled heat sink, as well, for the CPU."

The heat sink includes a copper base, aluminum-stamped fins and a copper heat pipe, which contains water. A vacuum is pulled on the pipe prior to sealing. Water boils faster under a vacuum. The steam rises and moves the heat higher into the fins where the air flow from the fans can extract the heat more efficiently. The steam condenses and flows back to the copper base of the heat sink, which is attached to the processor.

Copper and other conductive materials have replaced aluminum as the material of choice for heat sinks.

The required cooling capacity of the Xbox 360 is determined by system electronics. Fans cool the unit, but can be noisy. "We use thermal sensing diodes in the main ICs (GPU and CPU) and thermal algorithm firmware that reads the diode temperatures and adjust the fan RPMs by altering the fan voltage to a predetermined minimum requirement that minimizes the fan speed (minimizes the acoustics) while providing the required cooling capacity for the system," says Reents.

There have been significant revisions to the Xbox 360 since its release. They include the mundane, such as surrounding the CPU and GPU with epoxy to prevent board movement during heat expansion. More importantly, a second heat sink was added.

Two Patents issued

Microsoft and Reents have been issued two patents on new thermal management concepts for electronics devices since engineering work began on the Xbox 360.

A patent was awarded last December for an integrated heat sink that provides efficient transfer of heat from a non-planar surface. In an example, one of the heat sinks is compressively coupled to an integrated circuit via an integral spring assembly. The spring assembly pushes one of the heat sinks against one of the integrated circuits while allowing the other heat sink to remain positioned against the other integrated circuit. The integrated heat sink compensates for variations in circuit height, which occur due to manufacturing tolerance issues.

In 2006, Microsoft was awarded a patent for Reents' invention of a system in which a heat sink is mounted to a processor using a spring fastener. It's an interesting way to boost heat release from the processor.

A plastic clip had typically been used to provide a tight connection of the heat sink to the processor. As a result, a poor conducting material (plastic) is placed over the hottest section of the heat sink. Use of a metal fastener would prevent the use of fins across a portion of the surface of the heat sink.

Reents' design goal was to develop a heat sink mounting system that could avoid placing excessive forces on the PCB, while still allowing the heat sink to tightly press against the CPU.

In Reents' invention, the spring element includes a thrust plate made of an insulating material located between the spring element and the PCB and presses on the bottom of the PCB to create a force that keeps the CPU and the heat sink together.

It's not known to what extent the invention was used in the Xbox 360 because Microsoft turned down an interview request from Design News. "It is our policy not to discuss specific details of internal components or manufacturing processes," said Jeremy Bartram, a company spokesman.

The Xbox 360 is available in three models: the "Arcade," the "Premium" and the "Elite" console, each with its own accessories. The central processing unit (CPU) is a 3.2-GHz PowerPC Tri-Core Xenon.

The majority of the heat problem in electronics, such as the Xbox 360, is caused by the electrical leakage of the transistors that make up a CPU. Efforts to reduce the heat output of CPUs have been mostly unsuccessful because the number of transistors on a CPU increased from about 6,000 transistors on an Intel.RTM.8080 in 1974 to more than 50 million resistors on CPUs in recent years.

The maximum die temperature for a silicon-based CPU is around 90C. Temperatures that high, however, will cause a short operating life.

Reents was one of the original dozen Microsoft employees recruited by Todd Holmdahl  to work on the first Xbox. Holmdahl is corporate vice president of the Gaming and Xbox Product Group at Microsoft Corp., and is responsible for the design, engineering, testing and manufacturing of all Xbox consoles worldwide.

Other Design News stories on the Xbox 360:

"Learning from Failure" by Dean Takahashi

"Software-Defined Instrumentation" by Kevin Bisking

A water-cooled heat sink cools the GPU on the Xbox 360. Photo Xbox.com
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