Modeling Power Terminology

The power a circuit dissipates falls into two broad categories:

• Static power
• Dynamic power

Static Power

Static power is the power dissipated by a gate when it is not switching – that is, when it is inactive or static.

Static power is dissipated in several ways. The largest percentage of static power results from source-to-drain subthreshold leakage. This leakage is caused by reduced threshold voltage that prevent the gate from turning off completely. Static power also results when current leaks between the diffusion layers and substrate. For this reason, static power is often called leakage power.

Dynamic Power

Dynamic power is the power dissipated when a circuit is active. A circuit is active anytime the voltage on a net changes due to some stimulus applied to the circuit. Because voltage on a net can change without necessarily resulting in a logic transition, dynamic power can result even when a net does not change its logic state.

The dynamic power of a circuit is composed of

• Internal power
• Switching power

Internal Power

During switching, a circuit dissipates internal power by the charging or discharging of any existing capacitance internal to the cell. The definition of internal power includes power dissipated by a momentary short circuit between the P and N transistors of a gate, called short-circuit power.

Figure 1 Components of Power Dissipation

Figure 1 illustrates components of power dissipation and shows the cause of short-circuit power. In this figure, there is a slow rising signal at the gate input IN. As the signal makes a transition from low to high, the N-type transistor turns on and the P-type transistor turns off. However, during signal transition, both the P- and N-type transistors can be on simultaneously for a short time. During this time, current flows from VDD to GND, resulting in short-circuit power.

Short-circuit power varies according to the circuit. For circuits with fast transition times, the amount of short-circuit power can be small. For circuits with slow transition times, short-circuit power can account for up to 30 percent of the total power dissipated. Short-circuit power is also affected by the dimensions of the transistors and the load capacitance at the output of the gate.

In most simple library cells, internal power is due primarily to short-circuit power. For this reason, the terms internal power and short-circuit power are often considered synonymous.

Note:

A transition implies either a rising or a falling signal; therefore, if the power characterization involves running a full-cycle simulation, which includes both rising and falling signals, then you must average the energy dissipation measurement by dividing by 2.

Switching Power

The switching power, or capacitance power, of a driving cell is the power dissipated by the charging and discharging of the load capacitance at the output of the cell. The total load capacitance at the output of a driving cell is the sum of the net and gate capacitance on the driver.

Because such charging and discharging is the result of the logic transition at the output of the cell, switching power increases as logic transition increase. The switching power of a cell is the function of both the total load capacitance at the cell output and the rate of logic transitions.

Figure 1 shows how the capacitance (Cload) is charged and discharged as the N or P transistor turns on. Switching power accounts for 70 to 90 percent of the power dissipation of an active CMOS circuit.

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