|
Demo:
CMOS
Power
Consumption
|
CMOS logic has (almost) no static power dissipation. If the gate voltage
is either '1' or '0', there is no conducting path from VDD to
GND, i.e. there is no static current path through the inverter.
A short-circuit condition arises only when the gate voltage is
switched, typically during a short interval in time. Typical switching
times are 1 ns or less. During this interval, a short-circuit power
dissipation occurs.
However, there is a significant dynamic power dissipation in CMOS gates.
The applet below illustrates this effect for the CMOS inverter.
The gate of a MOS transistor forms a small capacitor, Cgs. Typical values for the gate capacitance is Cgs = 5 fF.
If the inverter input is connected to VDD at time t1,
this capacitor is charged:
Qgs = Cgs * VDD
If the input is connected to GND at time t2, the capacitor is
discharged. The net effect of this is a current of
I = dQ/dt = (Cgs * VDD)/(t2-t1)
The later is called dynamic power consumption. Though the contribution seems
small, the total current drawn by the entire CMOS chip can be quite large:
- A modern microprocessor may contain about millions of transistors.
Typically, about one percent of all gates switch during one cycle.
- Operating frequencies are up to 200 MHz (cycle time 5 ns)
at an operating voltage of e.g. VDD = 3.3V.
- On VLSI chips, the wires connecting the gates have a substantial
capacitance Cw (e.g. 100 - 500 fF)
compared to the transistor gate
capacitances, Cgs.
The wire capacitances are part of the load, i.e.
charged and discharged:
Ctotal=Cgs+Cw
The applet illustrates the current dissipation in the CMOS inverter.
If the input voltage stays at '1' or '0', either the N-type or the
P-type transistor in nonconducting, and no current flows through
the inverter.
When the input is switched, the gates of the transistors are
charged/discharged. The applet draws a moving electron to illustrate this.
During the switching, the input voltage passes the region close to
VDD/2, i.e. a short-circuit current flows through the inverter.
This current again is shown by a moving electron.
Top of demo
Modified by Flemming Stassen on 26 August 1999 stassen@imm.dtu.dk