7.3 Power distribution network design

Power distribution networks (PDNs) are crucial for managing electromagnetic interference in electronic systems. They minimize noise, reduce signal integrity issues, and improve overall performance by effectively delivering power to components.

PDN design involves optimizing power sources, distribution networks, and decoupling capacitors. Key considerations include impedance profile targets, frequency domain analysis, and addressing power integrity challenges like voltage fluctuations and current transients.

Fundamentals of PDN design

• Power Distribution Network (PDN) design plays a crucial role in managing electromagnetic interference and ensuring compatibility in electronic systems
• Effective PDN design minimizes noise, reduces signal integrity issues, and improves overall system performance in the context of EMI/EMC

PDN components

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• Power sources supply electrical energy to the system (batteries, AC-DC converters, voltage regulators)
• Distribution network consists of copper planes, traces, and vias that carry power throughout the PCB
• Decoupling capacitors provide local energy storage and filter high-frequency noise
• Load circuits represent the various components that consume power in the system

Impedance profile targets

• Target impedance defines the maximum allowable PDN impedance across a frequency range
• Calculate target impedance using the formula $Z_{target} = \frac{\Delta V}{I_{max}}$, where ΔV is voltage ripple and I_max is maximum current draw
• Flat impedance profile aims to maintain consistent low impedance across a wide frequency range
• Impedance peaks indicate potential resonances that can amplify noise and cause EMI issues

Frequency domain considerations

• Analyze PDN performance across different frequency ranges (DC to GHz)
• Low-frequency response dominated by bulk capacitors and voltage regulators
• Mid-frequency range influenced by decoupling capacitors and board parasitics
• High-frequency behavior affected by package and die capacitance
• Consider skin effect and transmission line effects at higher frequencies

Power integrity challenges

• Power integrity focuses on delivering clean, stable power to all components in a system
• Addressing power integrity challenges reduces electromagnetic emissions and improves system reliability

Voltage fluctuations

• Voltage droops occur when sudden current demands exceed PDN capability
• Simultaneous switching noise (SSN) causes voltage fluctuations due to multiple outputs switching simultaneously
• Long-term voltage drift can result from temperature changes or component aging
• Voltage ripple from switching power supplies can propagate through the PDN

Current transients

• Fast current transitions in digital circuits create di/dt noise
• Load step responses test PDN's ability to handle sudden changes in current demand
• Inrush currents during power-up can cause voltage sags and potential component stress
• Oscillatory current transients may excite PDN resonances, leading to ringing and EMI

Noise coupling mechanisms

• Conductive coupling occurs through shared impedances in power and ground planes
• Capacitive coupling between adjacent PCB layers can introduce noise into sensitive circuits
• Inductive coupling from current loops in the PDN can induce voltages in nearby traces
• Ground bounce results from return current flowing through non-ideal ground planes

Decoupling capacitor selection

• Decoupling capacitors play a vital role in maintaining power integrity and reducing EMI
• Proper selection and placement of decoupling capacitors significantly impact PDN performance

Capacitor types and characteristics

• Ceramic capacitors offer low ESR and ESL, suitable for high-frequency decoupling (X7R, X5R dielectrics)
• Tantalum capacitors provide higher capacitance values for mid-frequency decoupling
• Aluminum electrolytic capacitors used for bulk decoupling at lower frequencies
• Consider capacitor parasitics (ESR, ESL) when selecting components for specific frequency ranges

Placement strategies

• Place small, high-frequency decoupling capacitors close to IC power pins
• Distribute decoupling capacitors across the board to provide localized charge storage
• Use via-in-pad designs to minimize inductance between capacitor and power/ground planes
• Implement power islands with dedicated decoupling for noise-sensitive circuits

Capacitor resonance effects

• Series resonance of capacitors occurs at frequency $f_{res} = \frac{1}{2\pi\sqrt{LC}}$
• Parallel resonance between capacitors and board inductance can create impedance peaks
• Spread capacitor values to create a wide, low-impedance region in the PDN
• Consider anti-resonance effects when combining multiple capacitors in parallel

PCB stackup optimization

• PCB stackup design significantly impacts PDN performance and EMI/EMC characteristics
• Optimizing stackup improves signal integrity, reduces crosstalk, and enhances power delivery

Layer allocation

• Dedicate separate layers for power and ground planes to minimize impedance
• Interleave signal layers between power/ground planes for improved return path
• Use multiple ground planes to reduce overall ground impedance and improve shielding
• Consider split planes for multiple voltage domains while maintaining continuous return paths

Power plane design

• Implement solid power planes to minimize DC resistance and provide low AC impedance
• Use stitching vias to connect power planes across different layers
• Avoid slots or cuts in power planes that can create discontinuities and increase inductance
• Implement guard traces or moats around high-speed signals to isolate noise

Return path considerations

• Ensure continuous return paths for all high-speed signals to minimize EMI
• Place ground vias near signal vias to provide low-impedance return paths
• Use ground floods on signal layers to provide additional return path options
• Analyze and optimize return current paths for critical signals using simulation tools

PDN modeling techniques

• PDN modeling enables designers to predict and optimize power delivery performance
• Accurate modeling techniques help identify potential issues before PCB fabrication

Lumped element models

• Represent PDN components as discrete R, L, C elements
• Simplify complex structures into equivalent circuits for quick analysis
• Capture basic PDN behavior at lower frequencies
• Limitations in modeling high-frequency effects and distributed nature of real PDNs

Distributed models

• Treat PCB planes and traces as transmission lines with distributed parameters
• More accurately represent high-frequency behavior of PDN structures
• Include skin effect and dielectric losses for improved accuracy
• Use tools like SPICE or specialized PDN analysis software for simulation

3D electromagnetic simulations

• Provide most accurate representation of PDN behavior across all frequencies
• Account for complex geometries, material properties, and electromagnetic coupling
• Use finite element method (FEM) or method of moments (MoM) for full-wave analysis
• Require significant computational resources but offer highest fidelity results

Impedance analysis methods

• Impedance analysis helps characterize PDN performance and identify potential issues
• Various techniques allow designers to measure and optimize PDN impedance profiles

Target impedance calculation

• Determine maximum allowable PDN impedance based on voltage ripple and current requirements
• Consider different frequency ranges and adjust target impedance accordingly
• Account for voltage regulator bandwidth and decoupling capacitor effectiveness
• Use target impedance as a benchmark for evaluating PDN design performance

Self vs transfer impedance

• Self-impedance measures impedance at a single point in the PDN
• Transfer impedance characterizes impedance between two points in the network
• Analyze both self and transfer impedance to ensure proper power delivery to all components
• Use network analyzer or VNA to measure S-parameters for impedance calculations

Impedance measurement techniques

• Time-domain reflectometry (TDR) provides impedance profile along transmission lines
• Vector network analyzer (VNA) measures S-parameters for accurate impedance characterization
• Two-port shunt-through method offers improved accuracy for low-impedance measurements
• In-circuit impedance probes allow measurement of powered boards in real operating conditions

Power supply noise mitigation

• Effective noise mitigation in power supplies reduces EMI and improves overall system performance
• Various techniques and component selections help minimize power supply noise

Voltage regulator selection

• Linear regulators offer low noise but lower efficiency (suitable for noise-sensitive analog circuits)
• Switching regulators provide higher efficiency but introduce switching noise
• Consider load current, input voltage range, and noise requirements when selecting regulators
• Implement post-regulation techniques for noise-sensitive loads

Low-dropout regulators vs switching

• Low-dropout (LDO) regulators provide clean output with minimal voltage drop
• Switching regulators offer higher efficiency and wider input voltage range
• Use LDOs for noise-sensitive analog circuits and low-power applications
• Implement switching regulators with proper filtering for higher power applications

Filtering techniques

• LC filters reduce high-frequency noise from switching regulators
• Ferrite beads provide high impedance to high-frequency noise while maintaining low DC resistance
• Pi-filters combine multiple stages for enhanced noise attenuation
• Active filtering techniques using op-amps for improved low-frequency noise rejection

PDN resonance management

• PDN resonances can amplify noise and create EMI issues if not properly managed
• Understanding and controlling resonance effects is crucial for optimal PDN performance

Cavity resonance effects

• Parallel plate resonances occur between power and ground planes
• Calculate resonant frequencies using the formula $f_{res} = \frac{c}{2\sqrt{\epsilon_r}}\sqrt{(\frac{m}{a})^2 + (\frac{n}{b})^2}$, where a and b are plane dimensions
• Higher-order modes create multiple resonance peaks at different frequencies
• Resonances can lead to increased impedance and noise amplification at specific frequencies

Damping strategies

• Add lossy materials between power and ground planes to absorb resonant energy
• Use embedded capacitance materials to increase capacitance and lower resonant frequencies
• Implement resistive termination at board edges to reduce reflections
• Strategically place decoupling capacitors to provide damping at resonant frequencies

Anti-resonance considerations

• Anti-resonance occurs when parallel capacitors create high-impedance peaks
• Spread capacitor values to avoid overlapping self-resonant frequencies
• Use series resistors with capacitors to reduce Q-factor and flatten impedance profile
• Analyze and optimize capacitor placement to minimize anti-resonance effects

EMI reduction in PDNs

• PDN design plays a crucial role in overall EMI performance of electronic systems
• Implementing proper EMI reduction techniques in PDNs helps meet regulatory requirements

Common-mode vs differential-mode noise

• Common-mode noise flows in the same direction on multiple conductors
• Differential-mode noise flows in opposite directions on paired conductors
• Identify and address both noise types for comprehensive EMI reduction
• Use common-mode chokes and differential filters to target specific noise modes

Shielding techniques

• Implement ground planes and power planes as shields for high-frequency noise
• Use guard traces and ground fills to contain noise from sensitive or noisy circuits
• Consider board-level shielding with metal enclosures or conductive coatings
• Implement cable shielding and proper termination to reduce radiated emissions

Ground bounce mitigation

• Minimize inductance in ground return paths to reduce ground bounce
• Use multiple ground vias for IC packages to lower overall ground impedance
• Implement ground planes with minimal splits or discontinuities
• Consider controlled ESR in decoupling capacitors to dampen ground bounce oscillations

PDN design verification

• Verification ensures that the PDN design meets performance requirements and EMI/EMC standards
• Various analysis techniques help identify potential issues before production

Time-domain analysis

• Simulate PDN response to step load changes and measure voltage droop
• Analyze power supply noise using time-domain reflectometry (TDR) measurements
• Evaluate PDN performance under various switching scenarios and load conditions
• Use oscilloscopes with high bandwidth and low noise floor for accurate measurements

S-parameter measurements

• Characterize PDN impedance and transfer functions using vector network analyzers
• Convert S-parameters to Z-parameters for impedance analysis
• Measure both self and transfer impedance to evaluate PDN performance
• Use calibration techniques to ensure accurate low-impedance measurements

Eye diagram assessment

• Evaluate impact of PDN noise on signal integrity using eye diagram measurements
• Analyze eye height, width, and jitter to assess overall system performance
• Consider both vertical (voltage) and horizontal (timing) effects of PDN noise
• Use high-speed oscilloscopes or bit error rate testers (BERTs) for accurate eye measurements

Advanced PDN concepts

• Advanced PDN design techniques address challenges in modern high-speed and complex systems
• Consideration of these concepts is crucial for maintaining power integrity in demanding applications

Power integrity for high-speed designs

• Implement power aware signal integrity analysis for accurate simulation results
• Consider via transitions and plane changes in high-speed signal paths
• Use power-aware SI/PI co-simulation to capture interactions between signals and power
• Implement power modulation techniques to reduce EMI in high-speed interfaces

Mixed-signal PDN considerations

• Separate analog and digital power domains to minimize noise coupling
• Implement star-ground topologies to reduce ground noise in sensitive analog circuits
• Use ferrite beads or LC filters to isolate noisy digital supplies from analog circuits
• Consider ADC/DAC performance impact from PDN noise in mixed-signal systems

PDN for multi-board systems

• Analyze system-level PDN performance across multiple boards and interconnects
• Consider cable inductance and connector parasitics in inter-board power delivery
• Implement distributed filtering and decoupling strategies across the system
• Use simulation tools to optimize PDN performance at the system level