System Analysis Knowledge Bytes - Importance of VRM Modeling in PDN Simulation

18 Jun 2024 • 5 minute read

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Voltage Regulator Modules (VRMs) play a critical role in Power Integrity (PI) analysis due to their significant impact on the stability, reliability, and performance of electronic systems. Switching power noise can cause severe Power Integrity (PI) and Signal Integrity (SI) issues on a PCB, making the modeling of DC-DC buck converters, also known as VRMs, important for analyzing noise coupling from the switching power noise induced by VRMs to nearby signal and power nets.

Key reasons why VRMs are important in PI analysis include:

Voltage regulation
Noise suppression
Transient response
Current delivery capability
Impedance matching
Thermal considerations

PI analysis helps you validate the VRM's performance and optimize its design to meet the requirements of the electronic system. The VRM model serves as a fundamental component for simulating and understanding the behavior of the power-delivery system within electronic devices.

Voltage Regulator Model Equivalent Circuit

The purpose of a VRM is to convert one DC voltage to another, for example 5V to 1.8V. This is done by a feedback mechanism. The VRM senses the voltage near the load and adjusts the output current to regulate the voltage at the load.

Power distribution networks (PDNs) aim to supply a stable output voltage within a tight tolerance range during large current step changes and high slew rates. Given the maximum operating current and voltage ripple tolerance, the target impedance of the PDN can be calculated. The target impedance must be maintained across all possible operating current transient frequencies.

The VRM dominates the PDN impedance in the KHz bandwidth, before being dominated by bulk capacitors. It is desirable to find a linear model to represent the VRM in simulations, as Simulation program with integrated circuit emphasis (SPICE) analysis of such models runs very fast.

Here is a simplified block diagram for a buck switching regulator, commonly found in VRMs. The buck regulator is non-linear because switches open and close as a function of time.

Here is the equivalent four-element linear circuit model of a VRM, which consists of an ideal voltage source and four passive elements. Many components are common between non-linear and linear models.

Pre-Defined VRM Models in OptimizePI

The VRM consists of a DC-DC converter and a feedback-control circuit that supplies the reference voltage required by the system for the output stage. The VRM is basically a nonlinear system. However, when the non-linear model is implemented in the power transfer model, the simulation run time becomes long, and it becomes difficult to set the parameters that determine the characteristics of each element in the VRM.

Sigrity OptimizePI offers three predefined VRM model types with adjustable parameters. This method is both easy and fast, and you can choose any of the three VRM model types for your PDN simulation.

Resistor Model

The resistor model for VRMs is a simplified representation used to approximate the electrical behavior of VRMs within the PDN. While it is not as detailed as more complex models, such as RLC (Resistor-Inductor-Capacitor) models, the resistor model is useful for understanding basic VRM behavior and its impact on the PDN.

Resistor-Inductor in Series

In the VRM RL series model, users can approximate the impedance behavior of the VRM within the PDN. This simplification allows for analysis of the VRM's impact on voltage ripple, transient response, and overall stability of the power-delivery system.

More-Elaborate Model

A commonly used equivalent circuit model for VRMs in PDN analysis is the RLC model, which represents the VRM’s electrical characteristics in terms of R, L, and C. In OptimizePI, a four-element linear VRM model can be defined as More-Elaborate Model for PDN simulation.

In PDN analysis, understanding and optimizing these parameters is crucial for ensuring stable power delivery, minimizing voltage fluctuations, and meeting the performance requirements of the electronic system. Users often use simulation tools like OptimizePI to assess the impact of these parameters on the PDN's behavior and optimize the VRM design accordingly.

You can perform PDN simulations on a typical six-layer board with different VRM model types (Resistor, Resistor-inductor in series, and More elaborate model) in OptimizePI, and you will get the following PDN Impedance results for all three cases with different decapacitors mounted on a PCB layout.

Conclusion

You can model VRM circuits in Sigrity Optimize to optimize the design. It is evident from the above PDN impedance results that each R, L, and C parasitics in each VRM circuit model significantly impacts the PDN impedance results, ranging from very low frequency to high frequency in each case.

For a more in-depth exploration, refer to the following RAK: VRM Modeling and PDN Simulation with Different VRM Models in Sigrity OptimizePI by Cadence AE, Marthanapalli Shiva Shankar.

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Stay with us as we continue to explore what’s new in the world of Cadence Sigrity and Systems Analysis. For information about the most recent enhancements, check the Sigrity and Systems Analysis 2024.0 What's New. Happy reading!