Central processing unit power consumption

Description

Central processing unit power dissipation or CPU power dissipation is the process in which central processing units (CPUs) consume electrical energy, and dissipate this energy both by the action of the switching devices contained in the CPU (such as transistors or vacuum tubes) and by the energy lost in the form of heat due to the impedance of the electronic circuits.

Designing CPUs that perform tasks efficiently without overheating is a major consideration of nearly all CPU manufacturers to date. Some CPU implementations use very little power; for example, the CPUs in mobile phones often use just a few watts of electricity, while some microcontrollers used in embedded systems may consume only a few milliwatts or even as little as a few microwatts. In comparison, CPUs in general-purpose personal computers, such as desktops and laptops, dissipate significantly more power because of their higher complexity and speed. These microelectronic CPUs may consume power in the order of a few watts to hundreds of watts. Historically, early CPUs implemented with vacuum tubes consumed power on the order of many kilowatts.

CPUs for desktop computers typically use a significant portion of the power consumed by the computer. Other major uses include fast video cards, which contain graphics processing units, and power supplies. In laptops, the LCD’s backlight also uses a significant portion of overall power. While energy-saving features have been instituted in personal computers for when they are idle, the overall consumption of today’s high-performance CPUs is considerable. This is in strong contrast with the much lower energy consumption of CPUs designed for low-power devices. One such CPU, the Intel XScale, can run at 600 MHz consuming under 1 W of power, whereas Intel x86 PC processors in the same performance bracket consume a few times more energy.

There are several factors contributing to the CPU power consumption; they include dynamic power consumption, short-circuit power consumption, and power loss due to transistor leakage currents.

Power consumption due to leakage power emanates at a micro-level in transistors. Small amounts of currents are always flowing between the differently doped parts of the transistor. The magnitude of these currents depend on the state of the transistor, its dimensions, physical properties and sometimes temperature. The total amount of leakage currents tends to inflate for increasing temperature and decreasing transistor sizes.

Both dynamic and short-circuit power consumption are dependent on the clock frequency, while the leakage current is dependent on the CPU supply voltage. It has been shown that the energy consumption of a program shows convex energy behavior, meaning that there exists an optimal CPU frequency at which energy consumption is minimal.