Integrated Simulations for Switched-Mode Power Converter EMI/EMC

I'm going to discuss switching power supply noise. I'll talk about that and some of the workflows we have with the Ansys tools. I'll first discuss the overall capabilities of some of the tools, the electromagnetic field solvers we use, as well as the circuit simulation that will be coupled to those.

I will then show an example workflow for a specific design example demonstration. I'll cover conductive emissions as well as radiated EMI. The first field solver you may have heard of in the Ansys stable is SI Wave. SI Wave has been around for many years as a specialized solver for PCB.

It's a specialized solution for the stack-up. It imports the stack-up directly and can read the stack-up from an ODB++ database.

If you have a BMI, you can couple that with the signals that are flowing on the board and look at near fields as you would with a probe at different locations around the board to understand what's going on and diagnose issues, as well as look at emissions at locations away from the board to make sure you don't have compliance issues later.

It also has some really nice utilities built into SI Wave that can look at issues just from the actual import.

These are looking at signal integrity, cross-talk issues, high coupling that is undesired between the different traces, or maybe high-speed lines that are designed for controlled impedance and are not meeting the target value.

One of these is specifically for EMI design rules and violations of those rules. Before doing a simulation, you can read the entire board in and it will scan the board very quickly to look for issues related to 25 plus different rules that are related to signal integrity, power integrity, and EMI.

One of the examples I'll show here is related to a net signal flowing and it's over a ground plane. Most of the time you do not want to split that ground plane.

If you do split the ground plane, you can put local capacitors there that will enable the AC current to travel right underneath the signal path, and that avoids any EMI issues like radiation from a slot antenna.

Using the scanner, you can quickly identify if you have those types of issues on the board, even before you do any in-depth simulations.

There is an Explore utility that will let you look at the placement of those capacitors, the values of those capacitors, and see what effects it would have on the radiated emissions. Once you are ready to do simulation with SI Wave, there is a lot of EMC and EMI related capabilities.

Typically, one workflow would be taking the layout and assigning ports to the problem, so you have the different nets that come in, and then you're letting SI Wave characterize the parasitic effects of those nets and the ground plane. This is called parasitic extraction.

Once you've extracted the response, the frequency response of the board, then you can couple that into a circuit schematic. And that circuit schematic can apply an IBIS model or a driver and receivers, whatever signals are flowing on those lines.

And then we can put that back into the SI wave domain, in the frequency domain, and look at near fields and far field emissions from the design. Other things that are relevant to radiated emissions could be unintended emissions from a plane, like a power plane or ground plane.

It does a frequency dependent scan that looks for modes that may exist at different parts of the board. You can reshape the board or look at coupling capacitors to avoid that type of radiated emissions issue. And it also does susceptibility analysis where you can hit it with a plane wave.

Say you're expecting antennas in your system to be located next to the board. You can do a frequency sweep and see what signals are coupled onto which nets. So that lets you maybe look at design decisions as well to see if you can harden your board.

The other electromagnetic solver is called HFSS, which stands for High Frequency Structure Simulator. It's a full wave three-dimensional electromagnetic field solver. It's been around for many years and it's based on the finite element method.

It's considered one of the industry standard tools for extracting S-parameters for different designs. You may see it used for intentional radiation such as antennas or antenna arrays, looking at microwave transitions, RF designs, and the human body.

For EMI simulations, it can do very accurate simulations. This is one example of a radiated emissions measurement that was made at one meter away from a device that was carrying current through different types of PCB traces.

And then you can model the antenna that's receiving that and get correlation between the amplitudes of the signals and the frequencies of the signals that are on the board and emanating away from the board. There is a lot of capability in HFSS related to EMI, EMC.

I can't cover all that today, but I can give you a quick overview for immunity type of applications. It may be looking at bulk current injections or ESD. Those are looking at coupled currents and coupled voltages.

There's tools that are built into HFSS that are basically templates, 3D components that already exist and are relevant to these standards. The last solver I'll talk about relative to EMI is a circuit tool. This circuit tool has been around for a long time.

It's a full-featured circuit tool with time domain and frequency domain, linear and nonlinear analysis. Solvers are very powerful and they can be dynamically linked to the solutions that I just referred to from either SI Wave or HFSS.

It has a toolbox full of the components you would need to look at the output for convective emissions, and then you can link it back to those field solvers to look at the radiant emissions results.

This is an example with an HFSS model of a car, an electric drive, a powertrain for an electric vehicle. We've got the model of the HFSS simulation up there on the right with the motor and the traction inverter and the batteries and the cables.

And then we can solve that in the frequency domain and put that into our schematic. And then we can build up the detailed models. So it would be piece-wise models perhaps for the semiconductor devices and the controller electronics that would go into the traction inverter.

And then the battery is supplying the motor. So we run that as a simulation. And this is a high voltage system. These types of systems are high voltage so we can look at our three-phase currents that are delivered to the motor.

But we can also look at the output of the conducted emissions and this can be a concern for automotive electronics if you have other things that's connected to the power distribution network. And these types of switching systems, you can definitely see there's high and low-frequency emissions.

Once we have the conducted emissions in the circuit, we can then push those excitations back to HFSS, look at these different frequencies, and see the magnetic field distribution inside of the vehicle. Here's another example of a conductive emissions for a flexible switching DC to DC converter.

This can either take an input voltage and step it up or step it down. It's called a CPIC converter and it's taking inductive and capacitive energy and storing that and then delivering that to a load.

If we were to measure the conducted emissions for that type of device, we would need the setup you're seeing down there in the middle. Picture with the line impedance stabilization network.

That provides a very stable interface between the device you're testing and the supply, the actual supply for the energy. And it allows you to get a measurement port output to see what is coupled back onto that as a voltage.

To reproduce that in the Ansys tools we would simply take the layout for the PCB and then do that extraction that I was talking about, do the frequency domain extraction, we model that geometry, some of the cabling that's attached, we can do that in the cable tools, and then we would combine that in the schematic with the active components.

Thank you for watching.