EMI Filter for Simulating Noise Suppression with Altium Designer 20 and Altium 365

Getting your new product designed and evaluated will involve multiple simulation runs, especially if you need to design an extremely stable power regulation strategy. For power supplies, one essential evaluation step is an EMI filter simulation. This can involve simulating Gaussian random noise directly in the frequency domain, or it can involve analyzing the frequency response of a circuit to identify where conducted/radiated noise can produce a strong response in the circuit.

In a real EMI filter simulation for power supplies, you need to examine how parasitics and coupling between reactive components produce a complex transfer function with multiple resonances. Because we want to ensure the EMI filter only passes the 60 Hz AC line voltage and DC, we need to examine how a real EMI filter needs to be modified to produce low-pass behavior.

Designs for these systems can get complex and often require multiple collaborators work on the project together. When you need to share your project with collaborators, you can use the sharing and managed content features in Altium 365® to securely share data with your team. Your schematics, simulation data, component models, and any other required data in your project can be easily shared after you’ve qualified your design in Altium Designer. Here’s what your simulation workflow can look like when you use Altium Designer with the Altium 365 platform.

EMI Filter Simulations From Your Schematics

Creating an EMI filter simulation is simple when you use a schematic editor with an integrated SPICE-based simulator. The unified environment in Altium Designer lets you immediately generate analog simulation results in the time or frequency domain. If you’re still in the design qualification phase, simply create your schematic with the Miscellaneous Devices Library, which can be accessed from the Components panel. You can also create custom simulation models and import them as custom components for use in an EMI filter simulation.

The image below shows my schematic for an EMI filter simulation. An input filter is placed after the voltage source, which is then connected to a ferrite bead (L1) between a filter capacitor (C1) and an RC shunt network (C2). Two output capacitors (C3 and C4) are included for additional AC filtration. Here, I’ve included some typical ESL values in capacitors C1-C4, as well as parasitic capacitance and parallel resistance for the ferrite bead model. Note that the ESR values for the capacitors have been ignored as they are typically in the milliOhm range, so they won’t have any effect in this simulation.

The output from my EMI filter is connected to a 1 MOhm load. Here, we are examining how a 60 Hz AC signal from the grid and any high frequency superimposed noise (whether radiated, ripple, or conducted) will be affected by this EMI filter. This can be done in two ways:

1. Generate random noise in the time domain and perform transient analysis to examine how this noise propagates to the output. 
2. Examine the circuit’s response in the frequency domain and use the EMI filter’s transfer function to identify resonances due to coupled parasitics in the circuit.

Here, I’ll take the latter approach as the circuit shown above is purely linear, thus transfer functions are well-defined for any AC signal.

Step 1: Identifying Resonances

The image below shows results from an initial frequency sweep from DC to 1 MHz; I’ve set the lower limit on the x-axis to 1 kHz for clarity. This coarse simulation shows the output signal seen with a 50 mV input AC sine wave, which was used to identify any resonances in this EMI filter. Since we want to filter out broadband noise, such as might be generated from a downstream switching regulator or an upstream rectifier, we want to identify any of these resonances and damp them to the greatest extent possible.

In the above results, I’ve identified the components responsible for producing these large resonances. These resonances tell you which noise components (specific frequencies) can generate a large voltage spike at the output. The resonance at ~22 kHz is quite dramatic and has a gain of ~10. In other words, narrowband noise that overlaps with this particular resonance as small as 1 uV can produce a 50 mV spike at the output when measured in the same bandwidth. This would easily be enough to blow past the allowed ripple on the output. In addition, any switching noise or higher order harmonics generated by the rectifier could excite this resonance, which would produce strong noise in an EMI measurement with a near-field probe.

The C1 resonance is large enough that it can be damped by adding an RC shunt network in parallel with C1 (before L1), or by adding some resistance in series with C1. The L1 resonance and the C3 + C4 resonance can be damped in the same way: simply add a resistor in series with these two networks.

Step 2: Damping the C1 Resonance

The C1 resonance in the above plot can be damped by adding a small resistor in series. Frequency sweep results for the case with a 1 kOhm series resistor in the C1 leg of the filter are shown below. We can clearly see that this additional series resistor damps the C1 resonance to the point it is no longer visible in the filter’s transfer function. However, we’ve now created a new problem in that the ~550 kHz resonance has a huge gain of ~10. This occurs due to coupling among reactive components and their parasitics, which is typical behavior in nontrivial circuits with multiple reactive components. A great example can be seen in this article.

Step 3: Damping the 20 kHz and 550 kHz Resonances

To damp out the remaining resonances, I’m going to try adding a series resistor between L1 and C3. As we’ll see, it only takes a very small resistor to provide the required damping. Ideally, you should use the smallest possible resistor as you do not want to drop any power, yet you still want to damp out these resonances. The modified schematic with a 10 Ohm series resistor (RD) is shown below.

Here, I’m going to use a parametric sweep to vary the value of RD and determine the best value to use for damping the 20 kHz and 550 kHz resonances. As shown below, it takes a very small RD value to significantly dampen both resonances. I’ve swept the value of RD from 1 to 6 Ohms; it only takes a couple Ohms of series resistance to totally dampen the 20 kHz resonance; the 550 kHz resonance also experiences significant damping with this additional resistor.

Here, I would settle for a ~2 Ohm resistor as I would rather not sacrifice power transfer when damping these resonances. The ultimate determinant will be EMI tests, which would then need to be compared against CISPR or FCC standards to determine EMI/EMC compliance.

Step 4: Sharing on Altium 365

Now that you’ve finished your EMI filter simulation, you can push it to your Altium 365 workspace and share it with your collaborators. Everyone on your team will be able to access your schematics with your EMI filter model, and they can run their own simulations in Altium Designer. Your team won’t have to send emails back-and-forth to share design data when you use Altium 365 for sharing and collaboration.

Summary

In summary, we’ve used Altium Designer to create and run an EMI filter simulation, and the results are shared on Altium 365. Once the project is formally released, your collaborators will be able to download the SDF file with the simulation results and run their own analyses.

It may not be obvious from looking at the above frequency sweep results, but there may also be stop-bands in your own EMI filter. This can be more easily seen in a pole-zero plot for the damped circuit. This type of analysis looks at the transfer function for the EMI filter and calculates the critical points in the transfer function.

Read this article to learn more about pole-zero analysis and transient analysis.

The steps shown above were for an EMI filter simulation, but you could use the same process to design any other type of filter or circuit when you use the integrated simulation features in Altium Designer®. Once you’re ready to create your design in a PCB layout, you can instantly capture your schematic as a new layout and share your project data at any time through the Altium 365 platform.

Altium Designer on Altium 365 delivers an unprecedented amount of integration to the electronics industry until now relegated to the world of software development, allowing designers to work from home and reach unprecedented levels of efficiency.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.