Low Power Design | Physical Design | Dynamic: Switching & Short Circuit Power | Part 3

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Low Power Design | Physical Design | Dynamic: Switching & Short Circuit Power | Part 3


Dynamic Power:

Dynamic power is the energy consumed by a circuit when it's actively switching, processing information, and performing tasks. This type of power consumption is a result of the dynamic currents that flow through the circuit, including capacitance currents and short-circuit currents.

Switching Power Dissipation

Switching power dissipation occurs when the circuit's capacitances are charged and discharged, resulting in a flow of current. This power dissipation can be calculated using the equation Pswitch = α * (Vdd)^2 * CL * f, where α is the switching activity, Vdd is the supply voltage, CL is the total load capacitance, and f is the frequency of operation. The switching activity, α, is a measure of how often the circuit's outputs change state, and it's a critical parameter in determining the circuit's power consumption.

Pswitching = α * (Vdd)^2 * CL * f, where:

  • α represents the switching activity, which is the probability of a signal transitioning from 0 to 1 or vice versa
  • Vdd is the supply voltage
  • CL is the total load capacitance
  • f is the frequency of operation

Question: what are the available options for a Physical Design Engineer to reduce switching power?

As a physical design engineer, reducing switching power is crucial to minimize power consumption and heat generation in digital circuits. Since the switching power equation is Pswitching = α * (Vdd)^2 * CL * f, and α, Vdd, and f are fixed parameters, the only variable that can be controlled is CL, the total load capacitance.

To reduce CL, physical design engineers can employ several techniques:

  1. Optimize gate sizing: By optimizing the size of gates in the circuit, the load capacitance can be reduced. This can be done by using smaller gates or by using gates with lower input capacitance.
  2. Use low-capacitance interconnects: The interconnects between gates can contribute significantly to the total load capacitance. Using low-capacitance interconnects, such as those with smaller widths or shorter lengths, can help reduce CL.
  3. Minimize clock load: The clock signal is a major contributor to the total load capacitance. By minimizing the clock load, the overall CL can be reduced. This can be done by using clock gating or by optimizing the clock tree.
  4. Optimize placement and routing: The placement and routing of gates and interconnects can also impact the total load capacitance. By optimizing the placement and routing, the CL can be reduced.
  5. Use low-power design techniques: Low-power design techniques such as power gating can also help reduce the switching power.

Short-Circuit Power Dissipation:

Short-circuit power dissipation occurs when the circuit's transistors are momentarily turned on simultaneously, creating a short-circuit path between the power supply and ground. This can happen when the input signals change slowly, causing the transistors to overlap in their on-states. During this time, a significant amount of current can flow, resulting in power dissipation. This type of power dissipation is particularly important to minimize in low-power design, as it can significantly impact the circuit's overall energy efficiency.



In practice, the finite rise and fall times of input signals result in a direct current path between the power supply and ground, leading to short-circuit power dissipation. During switching, both NMOS and PMOS transistors in the circuit conduct simultaneously for a short period, creating a short-circuit current. This occurs when the input voltage exceeds the threshold voltage of the NMOS transistor, causing it to turn on, and similarly, when the input voltage reaches Vdd-|Vt,p|, the PMOS transistor remains on. As a result, both transistors are on for a brief period, allowing short-circuit current to flow.

The short-circuit power is directly proportional to the rise time, fall time, and k. To reduce the short-circuit current component, it is essential to minimize the input transition times. However, this must be balanced with propagation delay requirements.

Short-circuit currents become significant when the rise/fall time at the input of a gate is much larger than the output rise/fall time. This is because the short-circuit path remains active for a longer period. To minimize the total average short-circuit current, it is desirable to have equal input and output edge times. In this case, the power consumed by short-circuit currents is typically less than 10% of the total dynamic power.

How to Completely Eliminate Short Circuit Power:

An important consideration is that if the supply voltage is lowered to be below the sum of the thresholds of the transistors, Vdd < VTn + |VTp|, the short-circuit currents can be eliminated. This is because both devices will not be on at the same time for any value of input voltage, eliminating the short-circuit path.


#scripting #coding #Tcl #pd #physicaldesign #physicalverification #pv #vlsi #design #india #semiconductor #freshers #professional #hardware #floorplanning #motivation #leadership #magnet #bound #placement #synopsys #cadence #interview #vlsi #question #power #irdrop #powerswitches #rushcurrent #lowpower #staticpower #dynamicpower #wifi #bluetooth #leakage #current






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