CMOS Power Dissipation Explained

July 27, 2023

Cadence PCB Solutions

Key Takeaways

CMOS power dissipation is the amount of power dissipated by complementary metal-oxide semiconductor (CMOS) integrated circuits (ICs) during operation.
Minimizing CMOS power dissipation is crucial for achieving power efficiency, ensuring thermal management, maintaining signal integrity, meeting design constraints, addressing environmental concerns, and reducing operational costs.
PCB design affects CMOS power dissipation by optimizing power delivery, managing thermal issues, improving signal integrity, and ensuring stable power distribution.

CMOS power dissipation

CMOS power dissipation refers to the amount of power dissipated by complementary metal-oxide semiconductor (CMOS) integrated circuits (ICs) during operation. CMOS is widely used in modern electronic devices, such as microprocessors, memory chips, and other digital logic circuits, due to its low power consumption, high noise immunity, and compatibility with scaling down transistor size.

Unplanned or uncontrolled power dissipation can have several negative impacts on electronic devices and systems: heat generation, reduced efficiency, performance degradation, reliability concerns, and potential safety risks.

CMOS power dissipation is the amount of power dissipated by complementary metal-oxide semiconductor (CMOS) integrated circuits (ICs) during operation.

Sources of CMOS Power Dissipation

There are several sources of power dissipation in CMOS ICs:

Dynamic Power Dissipation: This is the power dissipated when the transistors in CMOS ICs switch states from 0 to 1 or vice versa. Dynamic power dissipation occurs during the charging and discharging of the parasitic capacitances associated with the transistors.
Short-Circuit Power Dissipation: During the transition period when both the P-channel metal-oxide semiconductor and N-channel metal-oxide semiconductor transistors are partially on, there is a brief moment where a direct path exists between the power supply and the ground, resulting in a short-circuit current flow that causes power dissipation in CMOS ICs.
Leakage Power Dissipation: Even when a transistor is supposed to be in an off state, a small amount of current still flows through it due to insufficient insulation, known as leakage current. As the number of transistors on a chip increases, the total leakage current increases, leading to leakage power dissipation.
Static Power Dissipation: Static power dissipation occurs when transistors are static, meaning they are not switching. This includes both leakage power dissipation and the power consumed by the direct current flowing through the circuit.
Power Supply Noise: Variations and fluctuations in the power supply voltage can cause noise in the CMOS circuits. The power supply noise can lead to additional power dissipation as the transistors respond to the changes in voltage.
Capacitive Coupling: The capacitance between adjacent circuit elements can cause power dissipation when the voltage on one element changes, leading to coupling and charging/discharging of the capacitance.

Power dissipation directly affects the energy efficiency of CMOS ICs and generates heat within the IC, which has a negative impact on the circuit’s performance. Therefore, it is advisable to minimize CMOS power dissipation where possible.

How to Minimize CMOS Power Dissipation

Minimizing CMOS power dissipation is crucial for achieving power efficiency, ensuring thermal management, maintaining signal integrity, meeting design constraints, addressing environmental concerns, and reducing operational costs.

To minimize power distribution, various techniques are employed at different levels of design, including circuit-level, architectural-level, and system-level techniques. See the chart for a list of commonly used methods.

Methods to Minimize CMOS Power Dissipation
Power Gating	Power gating involves selectively turning off power to specific circuit blocks or modules when they are not in use to reduce power consumption
Clock Gating	Clock gating is a technique where the clock signal to specific circuit elements or modules is selectively enabled or disabled based on their activity to reduce unnecessary switching activity.
Voltage Scaling	Voltage scaling involves adjusting the supply voltage to different circuit blocks or modules based on performance and power requirements to reduce power consumption.
Multi-VDD Design	In multi-VDD designs, different voltage levels are used for different circuit blocks based on their power and performance requirements to control power consumption.
Power-Aware Circuit Design	Power-aware circuit design includes optimizing transistor sizing, reducing switching capacitances, utilizing low-leakage transistors, and implementing sleep transistors.
Data Compression	Data compression techniques can reduce power consumption during data transfer and storage.
Energy-Efficient Coding and Algorithms	Energy-efficient coding techniques and algorithms can minimize power consumption by optimizing the computation and data flow, reducing unnecessary operations, and maximizing hardware resource use.
System-Level Power Management	Power management techniques at the system level include dynamically adjusting power modes, task scheduling, and workload balancing to optimize power consumption.

These methods are not mutually exclusive, and a combination of multiple techniques is often employed to achieve the desired power reduction while meeting performance and functionality requirements.

How Does PCB Design Affect CMOS Power Dissipation

PCB design can have a significant impact on CMOS power dissipation. PCB designers should consider power distribution, signal integrity, thermal management, and grounding to minimize power dissipation for a more efficient and reliable IC operation. PCB design is crucial in managing CMOS power dissipation for the following reasons:

Power Delivery Network Design: The PCB layout includes power and ground planes and power distribution traces. Proper placement of decoupling capacitors, power integrity analysis, and careful routing of power and ground connections help ensure a low-impedance and noise-free power supply to the CMOS circuits, reducing power dissipation.
Thermal Management: Power dissipation in CMOS ICs leads to heat generation. Efficient thermal management prevents excessive temperature rise, affecting circuit performance and reliability. The PCB design should incorporate heat sinks, thermal vias, and proper component placement to facilitate effective heat dissipation and cooling.
Signal Integrity and Routing: Proper signal integrity practices in PCB design can minimize unnecessary switching transitions and reduce dynamic power dissipation. Techniques such as controlled impedance matching, minimizing signal reflections, and reducing crosstalk help optimize the design and lower power consumption.
Power Plane Partitioning: Dividing the power planes into appropriate regions or using multiple power planes can help isolate noise and minimize power fluctuations. This ensures stable power distribution and reduces power dissipation due to voltage drops or noisy power signals.

Overall, PCB design considerations impact CMOS power dissipation by optimizing power delivery, managing thermal issues, improving signal integrity, and ensuring stable power distribution. These factors contribute to CMOS circuits' efficient and reliable operation while minimizing power consumption and dissipation.

PCB Design Solutions from Cadence

PCB designers can use Cadence suite of PCB Design and Analysis Software tools to help minimize CMOS power dissipation. When you are ready for prototyping, OrCAD PCB Designer's speed and comprehensive functionality accelerate board layout and reduce time-to-market.

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