Components of power consumption
The power consumed in a device is composed of two types – dynamic, sometimes called switching power, and static, sometimes called leakage power. In geometries smaller than 90nm, leakage power has become the dominant consumer of power whereas for larger geometries, switching is the larger contributor. Power reduction strategies can be used to minimize both types of power.
Power Dissipation in CMOS
Total power is a function of switching activity, capacitance, voltage, and the transistor structure itself.
Total power is the sum of the dynamic and leakage power
Total Power = Pswitching
Dynamic power is the sum of two factors: switching power plus short-circuit power.
Switching power is dissipated when charging or discharging internal and net capacitances. Short-circuit power is the power dissipated by an instantaneous short-circuit connection between the supply voltage and the ground at the time the gate switches state.
Where a = switching activity, f = switching frequency, Ceff
= the effective capacitance and Vdd
= the supply voltage.
= the short-circuit current during switching, Vdd
= the supply voltage and f = switching frequency.
Dynamic power can be lowered by reducing switching activity and clock frequency, which affects performance; and also by reducing capacitance and supply voltage. Dynamic power can also be reduced by cell selection—faster slew cells consume less dynamic power.
Leakage power is a function of the supply voltage Vdd
, the switching threshold voltage Vth
, and the transistor size.
= f (Vdd
= the supply voltage, Vth
= the threshold voltage, W = the transistor width and L = the transistor length.
Of the following leakage components, sub-threshold leakage is dominant.
• I1: Diode reverse bias current
• I2: Sub-threshold current
• I3: Gate-induced drain leakage
• I4: Gate oxide leakage
While dynamic power is dissipated only when switching, leakage power due to leakage current is continuous.
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