Printed circuit boards (PCBs) are becoming more and more complex. Having multiple clocks, multiple power supplies, and many data channels on a single PCB has become more of a norm. As the complexity of PCBs increase as do their demands on their power delivery networks (PDN). These complex systems require that the voltage on the PDN is stable. The margin for voltage ripple has become smaller and smaller as designers need to ensure the reference voltage is stable to have confidence in their device performance.
When investigating voltage ripple we need to make sure we are discussing all that entails power integrity within a device. That means the many inductive loops, capacitive planes, and at times the complex resistance paths need to be considered. That is why it is imperative that designers use a simulation tool to simulate their PDN and perform power integrity (PI) analysis. Ansys provides their users a streamlined workflow through Ansys SIwave to perform PI analysis with a step-by-step workflow wizard shown below.
Fig 1. Ansys SIwave workflow wizard
Ansys SIwave allows users to import many different board designs including ODB++, IPC-2581, and DXF. This allows users of popular PCB designer tools such as Altium, Cadence, or Mentor Graphics to quickly bring in their PCB into Ansys tools. Once imported, pad stacks and stackups can be verified and components can be mapped through the inclusion of S-parameter or spice models or through the internal component library to Ansys SIwave. After these verifications and component mappings, users can then begin to set up their first simulation.
The DCIR simulation allows users to assign voltage sources and current sinks to visualize the voltage, current, and power on their traces, layers, and vias. As well as providing power mapping and verification of proper voltage and current supply to critical components. Figure two depicts an example of a voltage result from Ansys SIwave. In addition to a single-phase VRM, multi-phase VRM systems can be simulated within the DCIR analysis. One of the advantages of performing the DCIR analysis in Ansys SIwave is that the user can use the resulting data as a source for a CFD thermal simulation within Ansys Icepak. So the user can take plane, trace, and via level information for their thermal simulations.
Figure 2. Voltage results from DCIR Simulation
Within the PI analysis, users will extract the SYZ parameters for their PCBs. Within the tool, there is a smooth workflow for assigning ports to VRMs, ICs, and decoupling cap positions to get a full system extraction of the SYZ-parameters.
Users can then take the extractions into the PI Advisor Wizard to optimize the decoupling capacitors on their board. Required impedance characteristics can be set over frequency, as well as the number of capacitors to optimize. Figure three depicts the PI Advisor Wizard.
Figure 3. PI Advisor in Ansys SIwave
Once simulated, the PI advisor will give several schemes that outline the capacitors and their values as well as an estimated price. In addition, the tool will show the new impedance for the user-chosen scheme. Figure four demonstrates the optimized results for one of those schemes.
With the new PDN being optimized and simulated, the z-parameter results can be taken into the Ansys circuit tool for a transient analysis. This allows users to fully include spice and component models for their PDN as well as the newly extracted impedance. The transient simulation can be performed, and the resulting voltage ripple can be examined to verify that it meets the component manufacturer’s spec.
Ansys SIwave and Ansys Circuit can be used to perform frequency and transient domain simulation on PDNs. Using the PI wizard, users can easily optimize their decoupling capacitors and their placements. As well as perform a DCIR analysis to ensure proper voltage and current are getting to their ICs. In addition, the DCIR results can be used as sources for a CFD thermal simulation on the user’s PCB. With Ansys tools, users can simulate complex PDNs and optimize their designs to ensure they are meeting the intricate and at times difficult requirements of their ICs.
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