14 September 2007

Dynamic (switching) power

As we seen in earlier blog the average power consumed
by the CMOS circuit can be devided into three
different components[1]. They are:

1)Dynamic (switching)power consumption

2)Short circuit power consumption

3)Static (Leakage) power consumption

Dynamic (switching) power dissipation

As the name indicates it occurs when signals which
goes through the CMOS circuits change their logic
state. At this moment energy is drawn from the power
supply to charge up the output node
capacitance.Charging up of the output capacitnce
causes transition from 0V to Vdd.Considering an
inverter exaple power drawn from the power supply is
dissipated as heat in pMOS transitor.
On the other hand charge down process causes NMOS
transistor to dissipate heat.

Output capacitance of the CMOS logic gate consists of
below components:

1)Output node capacitance of the logic gate: This is
due to the drain diffusion region.

2)Total interconnect capacitance: This has higher
effect as technology node shrinks.

3)Input node capacitance of the driven gate: This is
due to the gate oxide capacitance.

To find the avearage power energy required to charge
up the output node to Vdd and charge down the total
output load capacitance to ground level is integrated.
Applied input periodic waveform having its period T is
assumed to be having zero rise and fall time. Note
that average power is independent of transistor size
and characteristics.

Internal power

This is the power consumed by the cell when an input
changes, but output does not chnage[2]. In
logic gates not every change of the current running
through an input cell necessarily leads to a change in
the state of the output net. Also internal node
voltage swing can be only Vi which can be smaller than
the full voltage swing of Vdd leading to the partial
voltage swing.

How to reduce dynamic power?

1)reduce power supply voltage Vdd
2)reduce voltage swing in all nodes
3)reduce the switching probabilty (transition factor)
4)reduce load capacitance


[1] Sung Mo Kang and Yusuf Leblebici, CMOS digital
integrated circuits-analysis and design, Tata McGraw
hill, third edition, 2003
[2]Astro User Guide, Version X-2005.09, September 2005


  1. clock gating technique also can be used to reduce dynamic power

  2. clock gating technique also can be used for reducing dynamic power.


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