I recently heard an amusing quote from a colleague: “When it comes to EMC simulation, no one believes the results—except the person who did the simulation. When it comes to EMC testing, everyone believes the results—except the person who ran the test.” Anyone who’s worked in the field of EMC will probably smile knowingly at that.
Thanks to advancements in simulation tools, it’s now possible to build fairly accurate models using 3D solvers to simulate conducted emissions, radiated emissions, surface currents, and more. But such simulation tools come at a price—not only in terms of expensive software licenses (which often puts them out of reach for small to medium-sized companies), but also the steep learning curve. Whoever runs these simulations needs to thoroughly understand the product and its circuitry—including parasitics, physical layout, and component behavior—and must also be skilled in building the simulation model itself. A good model can take weeks, even months, to develop. And after all that effort, how do you validate it? You still need test results to back it up.
As a practical engineer, I often lean more toward hands-on diagnostics. To paraphrase a well-worn saying: “My best simulation tool is my soldering iron.” (Though in the EMC world, maybe we should say: “My best simulation tools are my near-field probes and current clamps.”)
Still, is there a middle ground? Can simple, quick, and even free simulation tools help practical engineers in troubleshooting EMI?
After years of solving EMI issues in the field, I think the answer is yes—especially in specific scenarios. And I’m not talking about the heavy‑duty full-wave simulators, but rather SPICE-based tools that are free to use and quick to learn.
Last year, Dr. Eric Bogatin presented a session on free power and signal integrity simulation tools at the EMC+SIPI symposium.1 Coming from an electronics design background myself, I’ve always supported the use of free SPICE tools. My personal favorite is SIMetrix, though LTspice is just as capable.
So how do I use them in EMI work? Here are the three most common scenarios:
- Filter design;
- Testing assumptions and educating clients; and
- Validating test results and informing next-gen designs.
Following are a few case studies illustrating each.
Filter Design with Simple SPICE Tools
When building accurate filter simulations, the devil is in the details. Parasitics, coupling, and damping all play critical roles. As discussed in one of my papers,2 it’s not easy to get right. But when you’re simply trying to determine how much impedance you need, a basic simulation might be enough.
In one example, I needed to estimate the required common-mode impedance/inductance for a small buck converter. I built a simple model in a few minutes: 25 ohms on the LISN side (for an automotive test setup, 25 ohm represents the common mode impedance3), with an estimated 150 pF parasitic capacitance between the converter and the test ground plane (you can simply use the math C=ε0εRA/h, where A is the area of the PCB and h is the height of insulation support, in this case, 50mm). The model showed that with a 250 µH common-mode choke, noise attenuation would begin around 1 MHz.
Would I trust the model at higher frequencies? Absolutely not—it doesn’t account for turn-to-turn winding capacitance or complex parasitics. But for the 1–100 MHz range, it gave me reasonable confidence in choosing a choke.
Figure 2 shows the actual measurement results. Noise reduction began around 1.5 MHz and had an effect up to about 100 MHz.
Simulating EMC Test Setup Errors
Here’s a rare but valuable case: sometimes, EMC test failures are due not to the product, but to incorrect test setups.
During one EFT/burst test, a product failed unexpectedly—even though it used a correctly installed, high-quality input filter from a reputable brand. Upon investigation, I discovered a long ground lead had been connected to the wall in the shielded room (rather than a short ground lead bonded to the test ground plane).
To illustrate this to the team, I built a quick SPICE model. As shown in Figure 3, the simulation clearly demonstrated that a long ground lead (500nH inductance) could result in significant current spikes—validating our conclusion that the setup, not the product, was at fault. This was later confirmed during the test.
Validating Test Results and Supporting the Next Design
In this case, a DC-DC converter using TO-247 packaged MOSFETs failed conducted emissions testing. The failure occurred in the 20 MHz range.
Oscilloscope measurements revealed overshoot and ringing during hard switching events. The resonant frequency of this ringing aligned perfectly with the EMC failure frequency.
We built a simple SPICE model of the switch node, incorporating parasitic inductance (primarily from the MOSFET leads). The simulation confirmed that the long legs of the TO-247 package were a major source of ringing.
For the next-generation product, we strongly recommended switching to surface-mounted FETs to reduce inductive parasitics and improve EMC performance.
Conclusion
While full-blown EMC simulation tools are valuable, they’re often impractical for fast troubleshooting or small teams. In contrast, quick, free SPICE-based simulations can be an efficient way to explore design ideas, test assumptions, and educate customers—without major investment.
They won’t replace the lab bench, and they won’t predict every nuance of your emissions profile. But when used wisely, they can be powerful tools in the hands of a practical engineer.
Sometimes, your best simulation tool really is your soldering iron and current probe—but your SPICE model might just be your second-best.
ENDNOTES
- Eric Bogatin, “Five or More Favourite Free SIPI Tools,” Signal Integrity Journal online, January 9, 2025.
- Min Zhang, “A Simple and Effective SPICE-Based Simulation Model To Assist Your Filter Design,” Signal Integrity Journal online, October 26, 2021.
- Timothy Hegarty, “The Engineer’s Guide To EMI in DC-DC Converters (Part 2): Noise Propagation and Filtering,” How2Power Today, January 2018.
