Power Dissipation

Sources of power dissipation

The power consumed in a VLSI circuit can be broadly classified into two types – Static power dissipation and Dynamic power dissipation.

  1.  Static Power

Static power is the power consumed when there is no circuit activity or you can say, when the circuit is in quiescent mode. In the presence of a supply voltage, even if we withdraw the clocks and don’t change the inputs to the circuit, the circuit will still consume some power, called the static power consumption. 

It is mainly due to the leakage currents that flows, when the transistor is in off-state. There are many types of leakage currents, however in the diagram below I have shown only two common leakage currents.

Reverse bias leakage current flows when the junction diodes within the transistors are reverse biased. Similarly sub-threshold leakage current flows from drain to source through the channel, when V_GS ≲ V_th [V_th is the threshold voltage of the transistor]. Typically the leakage power dissipation in a transistor is inversely proportional to its threshold voltage.

  2.  Dynamic Power

Dynamic power is the power consumed when the circuit is in operation, which means we have applied supply voltage, applied clock and changing the inputs.

It is mainly due to the dynamic currents, such as capacitance currents (switching power) and short-circuit currents (short-circuit power) as described below –

    2.1.  Switching power dissipation

This is due to the charging and discharging of total load, which includes the output capacitors and other parasitic capacitors. At a very high level, we can say the switching power dissipation, P_switch = α.(V_dd)².C_L.f , where –

       α = switching activity
       V_dd = supply voltage
       C_L = total load capacitance
       f = frequency of operation

All the parameters mentioned above is pretty much understandable except switching activity. So what exactly is switching activity? Well, the output of a circuit will not change in each and every cycle rather it will change depending upon the functionality of the circuit. So we can calculate probability of transition to 0 and 1, and find out a parameter known as the switching activity.

Note: If the signal is a clock then α = 1 and if the signal switches once per clock cycle then α = 0.5.

    2.2.  Short-circuit power dissipation

As the input changes slowly, there will be certain duration of time for which some of the transistor(s) in the pull-up network and pull-down network are turned ‘ON’ simultaneously, forming a short-circuit path from VDD to GND. Consider a simple example of an inverter, as shown in the figure below between time t₁ & t₂ and between time t₃ & t₄, both the transistors Q1 and Q2 are turned ‘ON’ due to slew in input signal.