SIwave is a comprehensive SI/PI/EMI analysis platform from Synopsys/Ansys that provides a wide range of solvers to help designers model, verify, optimize, and debug PCB designs. With its intelligent automation and guided wizards, users can set up simulations quickly and with minimal risk of configuration errors. SIwave imports designs from multiple ECAD formats, including EDB (Altium), IPC-2581, ODB++, and .brd files.
In this section, we will highlight one of its most useful capabilities: the Compute AC Currents solver.
DCIR analysis allows designers to visualize and track DC current flow, helping them identify high-impedance regions—especially across vias and narrow traces. The AC current solver extends this concept to AC current distribution, which is equally important for both power integrity and signal integrity. For example, examining AC current flow at a resonant frequency provides valuable insight into how energy is being distributed or trapped within the structure.
To run AC current analysis, the designer must place voltage or current sources. This can be done directly using the toolbar icons or through the DCIR wizard.
We will prepare the model as though we are running a standard DCIR analysis. After configuring, validating, and optionally simulating to view the DC current, we can inspect the DC current distribution on the VCC plane.
Configure, validate, and simulate if you want to see the DC current.
And this is the DC current in the VCC plane:
Next, we switch to the Simulation panel and run the AC Current solver, then plot the resulting AC current distribution.
Plot the currents:
Effect of Resonance on AC Current
If the PCB structure exhibits a resonance, the AC current distribution at that frequency changes dramatically.
Without decoupling capacitors:
The resonance causes very high circulating currents within the plane.
With capacitors:
The addition of decoupling capacitors significantly alters the current profile and suppresses the resonance.
The difference between the two cases is substantial. Without capacitors, large amounts of current are driven into the structure under the same voltage-source and load conditions.
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At 190 MHz, the power dissipated in the no-cap scenario is approximately 2 mW.
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With capacitors, it drops to around 48 µW—roughly 1/40 of the no-cap case.
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At 500 kHz, the impedance is only 0.004 Ω, resulting in large current levels; however, the total radiated power remains low, at about 29 µW.
