In this piece, we’ll investigate what SMPS EMI is and where it comes from so that we can better understand how to build against it.
Sources of EMI in SMPS
If you want to fix an EMI issue, you need to know three things: where the interference is coming from, how it will be transmitted to other circuits (the victims), and what kind of circuits will be impacted. During product development, it is nearly difficult to predict how EMI will affect end users; as a result, most EMI control efforts are directed toward reducing the number of emission sources and the number of coupling paths.
Inherent nature and switching characteristics of SMPS power supplies can be traced back as the primary cause of electromagnetic interference. Turning on and off at high frequencies, the MOSFET switching components in SMPS generate a false sine wave (square wave) during AC-DC or DC-DC conversion, which can be characterized as the sum of many sine waves with harmonically related frequencies using a Fourier series. Switching generates a complete Fourier spectrum of harmonics, which are then transmitted as EMI from the power source to the rest of the device’s circuits and any nearby electronic devices that are vulnerable to these frequencies.
EMI in SMPS is caused in part by the rapid changes in current (dI/dt) and voltage (dV/dt) that occur during switching.(which are, well, also related to switching). According to Maxwell’s equation, this alternating current and voltage will generate an alternating electromagnetic field, and while the magnitude of the field decreases with distance, it interacts with conducting parts(like copper traces on the PCB) that act as antennas, resulting in additional noise on the lines, or electromagnetic interference (EMI).
Now, EMI at the source is not so dangerous (at times) until it is coupled into nearby circuits or devices (victims). As a result, EMI can usually be reduced by eliminating or minimizing the potential coupling paths. Conduction (via unintended or repurposed paths or “sneak circuits”), induction (via inductive or capacitive elements like transformers), and radiation are the three main types of EMI coupling described in the “Introduction to EMI” article. (over-the-air).
When designers are aware of the impact of these coupling paths on EMI in switch-mode power supplies, they can craft systems to minimize the coupling path’s effect and limit the spread of interference.
Different Types of EMI Coupling Mechanisms
We’ll go over all the different coupling methods in the context of SMPS and identify the design considerations that led to their development.
Radiated EMI in SMPS:
Source and receiver (victim) become radio antennas during radiated coupling. The source emits an electromagnetic pulse that travels through the air to reach its target. Switched currents with high di/dt, which are exacerbated by the presence of loops with rapid current rise times due to sloppy layout and wiring that causes leakage inductance, are a common cause of radiated EMI propagation in SMPS.Consider the circuit below;
In addition to the expected voltage, a noisy voltage (Vnoise) is produced by the rapid current shift in the circuit. (Vmeas). Since the coupling process is analogous to that of transformers, the Vnoise can be calculated using the formula;
Where M/K is the coupling factor that, like in a transformer, varies with the separation, area, and orientation of the magnetic loops in issue, as well as the magnetic absorption between them. Thus, there tends to be a higher degree of radiated EMI in design/PCB layouts with poor loop orientation consideration and a big current loop area.
Conducted EMI in SMPS:
Conduction Coupling happens when EMI emissions are transmitted from a source to a receiver via a conductor (such as a wire, cable, enclosure, or copper trace on a printed circuit board). This type of H-field-heavy EMI coupling is typically found in the power supply cables.
Conduction Both Common Mode and Differential Mode coupling are possible in SMPS, with the former producing interference that is in phase on the +ve and GND line. (the interference appears out of phase on two conductors).
Common mode conducted emissions are typically brought on by board architecture, switching voltage waveform across the switch, and parasitic capacitances like the heatsink and transformer.
Differential mode conducted emissions, on the other hand, are brought about by the switching process itself, which generates switching bursts and input current pulses, both of which contribute to the presence of differential noise.
Inductive EMI in SMPS:
Inductive coupling occurs when there is an electrical (due to a capacitively coupled) or magnetic (due to an inductively coupled) EMI induction between the source and the victim. Electrical coupling or Capacitive coupling occurs when a varying electric field exists between two adjacent conductors, inducing a change in voltage across the gap between them, while a Magnetic coupling or Inductive coupling occurs when a varying magnetic field exists between two parallel conductors, inducing a change in voltage along the receiving conductor.
In conclusion, while high frequency switching action and the resulting fast di/dt or dv/dt transients are the primary source of EMI in SMPS, poor component selection, poor design layout, and the existence of stray inductance/capacitance in current paths are enablers that facilitate the propagation/spreading of the generated EMI to potential victims on the same board(or external systems).
Design Techniques to Reduce EMI in SMPS
It may be helpful to review EMI/EMC standards and regulations prior to reading this part as a refresher on the design goals. The FCC EMI Control regulations and the CISPR 22 (Third Edition of the International Special Committee on Radio Interference (CISPR), Pub. 22) are two of the most widely accepted standards, and are now recognized for certification in most regions, despite differences in the standards between countries/regions. The article on the EMI standard that we discussed previously provided a summary of the various nuances of these two standards.
Maintaining emission levels below the values specified in the standards is necessary for passing EMC certification processes or for ensuring your devices function well when in close proximity to other devices.
We will do our best to discuss the various methods used in SMPS design to reduce electromagnetic interference (EMI).
1. Go Linear
Using a linear Power Supply can alleviate a great deal of EMI strain, provided that your application can tolerate its increased size and lower efficiency. They are easier and cheaper to create because they do not produce noticeable EMI. Although LDO linear regulators’ efficacy isn’t quite up to that of SMPS, it’s still respectable.
2. Use Power Modules
Sometimes, even when using best techniques, EMI performance is still subpar. If you’re struggling to tune and achieve desirable EMI results, one solution that often works is to swap to Power modules.
While power modules certainly have their flaws, one area in which they excel is in protecting you from common EMI pitfalls like poor architecture and parasitic inductance/capacitance. Some of the best power modules available today take into consideration the necessity of overcoming EMI and are made to facilitate the rapid and simple development of power supplies that exhibit good EMI performance. There is a broad selection of SMPS Modules available from manufacturers like Murata, Recom, Mornsun, etc. that eliminate the need for us to worry about EMI and EMC interference.
For instance, most of the module’s components, including inductors, are typically connected internally within the package; this results in a very tiny loop area within the module, reducing radiated EMI. To avoid radiated EMI from the coil, some modules completely enclose the inductors and the switch node in a shield.
3. Shielding
Shielding the SMPS with metal is a brute-force method for reducing EMI. To accomplish this, noise-making components are housed inside the power supply in a grounded, conductive (metal) enclosure; in-line filters serve as the only point of contact with exterior circuits.
However, shielding increases the project’s material and PCB size costs, so it may not be the best choice for projects trying to keep costs down.
4. Layout Optimization
One of the most important factors in EMI’s ability to spread across a circuit is its design structure. Therefore, Layout Optimization is one of the overarching, generic methods for lowering EMI in SMPS. Depending on the context, it could refer to eliminating parasitic components, isolating noisier nodes from noise-sensitive nodes, decreasing current loop areas, etc.
Here are a few suggestions for improving SMPS designs’ layouts:
Protect Noise-sensitive nodes from Noisy nodes
You can stop electromagnetic coupling by putting them as far apart as feasible from one another. In the chart below, we see several instances of both noise-sensitive and noisy nodes.
Keep traces for Noise-Sensitive Nodes Short
Copper traces on PCB act as antennas for Radiated EMI; therefore, keeping the traces directly connected to Noise-Sensitive nodes as short as possible by moving the components to which they are to be connected, as close as possible, is one of the best ways to prevent the traces from acquiring Radiated EMI. For instance, radiated EMI can be picked up by a long trace from a resistor divider network that goes into a feedback (FB) pin. When noise is fed into the Feedback pin, it multiplies at the system’s output and degrades its efficiency.
Reduce Critical(antenna) Loop Area
It’s ideal to place the traces/wires that transport the switching waveform in close proximity to one another.
Since the level of radiated EMI is proportional to the product of the current (I) and the area of the loop (A) through which it flows, we can decrease the radiated EMI by decreasing the area of the current/voltage. For power lines, this is best accomplished by routing the powerline and return route along adjacent layers of the printed circuit board (PCB).
Minimize Stray Inductance
Increasing the size of the tracks(powerline) on the PCB and routing it parallel to its return route can reduce the inductance of the tracks, which in turn reduces the impedance of a wire loop, which is a contributor to radiated EMI as its proportional to its area.
Grounding
When placed directly below the EMI Source, an uninterrupted ground plane on the PCB’s outer surfaces provides the shortest return route for EMI, greatly reducing radiated EMI. However, if other traces are allowed to cross through the ground level, it could cause issues. Since the return current will have to find a longer route to go around the cut in order to return to the current source, this could increase EMI levels and the effective loop area.
Filters
EMI Power sources cannot function without filters, most importantly for reducing conducted EMI. They typically sit at the power source’s intake and/or output. They work to filter out mains-related noise at the input and block it from spreading to the remainder of the circuit at the output.
When designing EMI filters to reduce conducted EMI, it is essential to distinguish between common-mode and differential-mode conducted emission, as the filter parameters for each will be distinct.
To effectively suppress differential mode current at the lower fundamental switching frequency and also at higher harmonic frequencies, input filters for differential mode conducted EMI filtering are typically constructed from a combination of electrolytic and ceramic capacitors. When additional attenuation is needed, a single-stage L-C low pass filter is created by connecting an inductor in line with the input.
Bypass capacitors connected between the input and output power lines and ground provide efficient filtering for common mode conducted EMI. Coupled choke inductors can be connected in series with the power lines to provide additional reduction if necessary.
When choosing components for a filter, it is best practice to think about the worst-case situations. Differential-mode EMI is worst when the input voltage is low and the load current is high, while common-mode EMI is worst when the input voltage is high.
Conclusion
Taking into account all of the above when building switching power supplies can be difficult at first, and is in fact one of the reasons why EMI mitigation is called a “dark art,” but after some practice, it will become second nature.
Electromagnetic compatibility and the general ability of each device to function properly under normal operating conditions without negatively impacting the operation of other devices in close proximity is even more important than before thanks to the Internet of Things and various advancements in technology. There can be no interference with other devices if one device is not vulnerable to electromagnetic interference (EMI) from adjacent intentional or unintentional sources, and vice versa.
Considering EMC early in the SMPS design process is essential for saving money. In most instances, particularly for embedded SMPS, the power supply will be certified together with the device as one unit, so it’s essential to think about how connecting the power supply to the main device impacts the EMI dynamics in both devices.