Static and Dynamic Power Dissipation

In today's world, we need sleeker devices with more capabilities and longer battery life. This can be achieved by packing more components on smaller chips, thus moving to low geometry chip design. However, power dissipation occurs in all the circuits that are currently used, which increases the overall power consumption, making it less suitable for mobile applications which need longer battery life.

The rising demand for portable devices such as mobile phones and even wearable electronic devices for need longer battery life, low power consumption and lesser device weight. Considering this, there seems a need to develop a solution that can make use of low voltage and low power design techniques. The power consumption is also considered as an important criterion in VLSI design along with timing and area. In order to create an ideal solution for this problem, Low Power Design has to be considered as a crucial factor.

Power dissipation has become one of the major problem as it results in heating up of the device which will affects the operation of a chip. There are many kinds of external heat sinks and software based methods are provided with the system, but we have scope to save the power during operation of the chip.

Types of Power Dissipation

The power dissipation is classified in two categories

  1. Static power dissipation
  2. Dynamic power dissipation
Total power dissipation is the sum of the dynamic and static power (leakage power). Dynamic power is the sum of two factors: switching power plus short circuit power. Dynamic power is dissipated only when switching but static power (leakage power) due to leakage current is continuous. 

Total Power = Pswitching + Pshort-circuit + Pleakage

 Static power dissipation

The power dissipation occurs in the form of leakage current when the system is not powered or is in standby mode. In other words, power will be dissipated irrespective of frequency and switching of the system. Leakage power due to leakage current is continuous become more dominant at lower technology nodes. In geometries smaller than 90nm, leakage power has become the dominant consumer of power whereas for larger geometries, switching is the larger contributor.

In circuits, there are several sources of leakage current which cause static power dissipation.

  1. Sub-threshold current
  2. Gate oxide leakage
  3. Diode reverse bias current
  4. Gate induced leakage

Static power (leakage power) is a function of the supply voltage, the switching threshold voltage and the transistor size. 

PLeakage = f (Vdd, Vth, W/L)

Where Vdd = the supply voltage, Vth = the threshold voltage, W = the transistor width and L = the transistor length.

Dynamic power dissipation

There are two reasons for dynamic power dissipation; Switching of the device and Short circuit path from supply to ground. This occurs during operation of the device.

Switching power is dissipated when charging or discharging internal and net capacitances.

Each time the capacitor Cgets charged through the PMOS transistor, its voltage rises from 0 to Vdd and a certain amount of energy is drawn from the power supply. Part of this energy is dissipated in the PMOS device, while the remainder is stored on the load capacitor. During the high-to-low transition, this capacitor discharged, and the stored energy is dissipated in the NMOS transistor. This way power is dissipated through charging and discharging. This is known as Switching power dissipation. 

Pswitching = a.f.Ceff.Vdd2

Where a = switching activity factor, f = switching frequency, Ceff = the effective capacitance and Vdd = the supply voltage.

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. When the input transition time is very high, there will be certain duration of time “t”, for which both the devices (PMOS and NMOS) are turned ON. Now, there is a short circuit path from Vdd to ground.

Short Circuit Path from Vdd to ground

Pshort-circuit = Isc.Vdd.t

Where Isc = the short  circuit current, Vdd = the supply voltage and t = short circuit time.

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