Power Dissipation in CMOS Circuitry

What is CMOS, and why is it used?

CMOS - Complementary Metal-Oxide Semiconductor

It is a type of technology utilized in the production of

  • Computer processors,
  • Memory chips, and
  • Other electronic devices.

Overall power dissipation in a CMOS circuit may consist of two major factors.

1). Static Power Dissipation

2). Dynamic Power Dissipation

1). Static Power Dissipation

A quiescent mode is the amount of power consumed while no circuit activity occurs. If you remove the clocks and do not change the circuit inputs in presence of supply voltage, the circuit will still consume some power, which can be referred to as static power consumption.

It is primarily due to leakage currents that flow when the transistor is in an off-state. When the transistor’s junction diodes are reverse biased, a reverse bias leakage current occurs. When VGS Vth is present, sub-threshold leakage current flows across the channel from drain to source. The leakage power dissipation is inversely proportional to the threshold voltage.

2). Dynamic Power Dissipation

Dynamic power dissipation is the power consumed when the circuit is in operation, which indicates that we have supplied the supply voltage, clock, and modified the circuit’s inputs.

It is mainly caused by

  • Capacitance Current (Switching Power) and
  • Short Circuit Current (Short Circuit Power).

a) Switching Power

It refers to the elements that influence overall energy usage. It largely results from the charging & discharging of capacitive loads during transistor switching processes. When a CMOS transistor transitions from one state to another, there is a period of conduction during PMOS and NMOS, which results in a direct path between the power source and ground, resulting in substantial power dissipation. The amount of power dissipated is determined by several factors, including operating frequency, capacitive loads, & voltage swing across transistors. This can be reduced by employing techniques such as power gating, clock gating, and voltage scaling.

b) Short-Circuit Power Dissipation

When the inputs change, some of the transistors in the pull-up & pull-down networks will be switched ‘ON’ at the same time, forming a short circuit path from the power source to Ground.

Total power dissipation = P(static) + P(dynamic) + P(short)