CPU core voltage

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The CPU core voltage (VCORE) is the power supply voltage supplied to the CPU (which is a digital circuit), GPU, or other device containing a processing core. The amount of power a CPU uses, and thus the amount of heat it dissipates, is the product of this voltage and the current it draws. In modern CPUs, which are made using CMOS, the current is almost proportional to the clock speed, the CPU drawing almost no current between clock cycles. (See, however, subthreshold leakage.)

In order to help conserve power and manage heat, many laptop and desktop processors have a power management feature that allows software (usually the operating system) to adjust the clock speed and core voltage dynamically.

The trend is towards lower core voltages, which conserve power. This presents the CMOS designer with a challenge, because in CMOS the voltages go only to ground and the supply voltage, the source, gate, and drain terminals of the FETs have only the supply voltage or zero voltage across them.

The MOSFET formula: \,I_D = k((V_{GS}-V_{tn})V_{DS}-(V_{DS}/2)^2) says that the current I_D supplied by the FET is proportional to the gate-source voltage reduced by a threshold voltage V_{tn} which is dependent on the geometrical shape of the FET's channel and gate and their physical properties, especially capacitance. In order to reduce V_{tn} (necessary both in order to reduce the supply voltage and to increase current) one must increase capacitance. But, the load being driven is in fact another FET gate. The current needed to drive it is proportional to capacitance, which thus requires the designer to keep it low.

The trend towards lower supply voltage therefore works against the goal of high clock speed. Only improvements in photolithography and reduction in threshold voltage allow both to improve at once. On another note, the formula shown above is for long channel MOSFETs. With the area of the MOSFETs halving every 18-24 months (Moore's law) the distance between the two terminals of the MOSFET switch called the channel length is becoming smaller and smaller. This changes the nature of the relationship between terminal voltages and current.

When a processor is overclocked its clock speed is increased at the cost of system stability. To withstand higher clocks, the core voltage often needs to be increased at the cost of power consumption and heat dissipation. This is known as overvolting. Overvolting generally involves running a processor out of its specifications, which may damage it or shorten CPU life.

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