Switching Power Supply Noise, Radiated EMI, and EMCBy ZM Peterson • Jan 24, 2022
Power supply designs are rapidly evolving, with newer semiconductors being implemented across modern power electronics and RF applications. Outside of municipal power distribution, DC-DC converters of various topologies are the most common type of power converter, both as standalone highly-integrated components and systems built from discrete components.
Among the various options for purchasing or building a low-noise AC-DC or DC-DC converter circuit, topology and physical layout are important determinants of conducted and radiated EMI. High-power, low-noise devices normally use a switching power converter circuit with AC input (e.g., flyback or resonant LLC topologies), or with a high voltage DC input (flyback or buck/boost). Application areas where EMI in these power systems has suddenly become a prominent concern include EVs, alternative energy, industrial systems, and aerospace.
In this article, we’ll look at some of the factors that contribute to switching power supply noise, and particularly noise generated or received in the regulator section in a switching power supply. As the target areas listed above are grid-connected, noise-sensitive, or operate at high powers, we take a somewhat different view of EMI/EMC in these systems compared to low-power, low-profile electronics.
Sources of Switching Power Supply Noise
In a switching power supply, DC power is regulated at the required value by modulating the current across a reactive element (in this case, an inductor) at high frequency. Switching rates can be sub-100 ns in modern power electronics at MHz frequencies. Examples include RF power supplies, where multiple nodes are switched in a phased manner. These devices can potentially produce large amounts of radiation from the switching nodes, as well as strong coupling around a PCB layout.
In the process of generating and coupling radiation around a physical layout, noise is generated or transferred around a system through several possible mechanisms:
- Conduction: The output waveform from a switching converter will be a superposition of the desired DC output and high frequency ripple from the switching element in the circuit.
- Radiation: The switching signal can radiate strongly from the switching noise. Noise can radiate into the far field, or it can couple within the PCB layout. Radiation is generation by high dV/vt and high dI/dt action at switching nodes.
- Common-mode noise: Noise coupled by an external source around a closed loop in the converter can propagate common-mode noise to the output. This needs to be filtered from the output, and within the system has to be suppressed to remove this noise.
- Conducted switching noise: The noise that appears as ripple on the output voltage waveform does not originate via coupling, but it still needs to be filtered or reduced in certain systems. For example, multiphase converters are preferred when ultra-low noise power is needed.
- Parasitics and transients: Transient ringing may occur due to parasitics in the PCB layout or in components, such as ESL in capacitors. Usually, some very small resistance (just a few Ohms) wil be sufficient to dampen this noise.
The challenge in diagnosing the source of switching power supply noise starts from looking at the noise spectrum as gathered in an EMC test. Another strategy is to compare the noise spectrum with simulations, which can be used to diagnose some possible paths that would generate emissions.
Switching power supply noise is typically broadband and will emit very strongly at a range of frequencies. This can make it difficult to diagnose specific noise sources in a PCB layout.
Given all these possible noise sources, where do they tend to occur and what are some simple strategies for reducing noise? Let’s briefly look at each of these factors.
Emissions From a PCB Layout
Switching power supply noise can be emitted via conduction or radiation. Conducted emissions can be detected with a voltage or current measurement, and each can be classified as either common-mode or differential-mode conduction. To make matters more complicated, the connecting wires' finite impedance allows voltage conduction to trigger current conduction and vice versa. Similarly, differential-mode conduction can trigger common-mode conduction and vice versa.
Radiation always occurs in switching power supplies, and omitting some simple layout choices can make noise much worse. There are some simple design guidelines that will help reduce differential-mode emissions and common-mode emissions, as well as improve the overall level of safety in the device:
- Maintain clear return paths near the switching power supply circuit, as shown below. The idea here is to take advantage of the shielding provided by nearby ground and the reduced emissions from lower loop inductance.
- Connect grounds across transformers with a large safety capacitor. This will provide a clear path back to the chassis or earth while still maintaining isolation between PGND and SGND sides of the power supply.
- Eliminate ground offsets between the system ground and chassis if possible with a low-impedance connection between these regions. This is typically done through mounting holes in most designs. RF power supplies will use a molded enclosure that contacts exposed copper guard trace arrays in the PCB layout.
This layout is created to have small inductance about the loop spanning from the central regulator chip, D2, L2, and C22.
The first point is easily accomplished with a ground plane in the adjacent layer. In the case where high power is needed, switching speeds are very fast, or the board is only 2 layers with a thick dielectric, copper pour on the surface layer will be needed.
Line Input and I/O Filtering
Line filtering serves two purposes when used to address switching power supply noise: remove conducted noise on the input power line (such as from the grid), or remove noise conducted to the output, particularly over a cable. Once noise reaches a cable and can act as a propagating signal, the cable itself becomes a significant source of radiation, which must be addressed. The simplest and most effective way to do this is with filtering.
Noise that reaches a filter circuit could be either common-mode or differential-mode; the former is more important in terms of switching power supply noise, especially when a surging MOSFET couples noise around the chassis and through any output cable. Traditional LC circuits are typically used in the input and output of the power supply as these are the main points where conducted EMI measurements will be gathered, thus conducted EMC testing will be most important here.
The schematic below shows conceptually where filtering elements would be placed on the input and output in a flyback converter. The input and output both use low pass filtration with capacitors, as well as coupled inductors (choke coils) to eliminate conducted noise. Together, these components target both common-mode and differential-mode noise. Note, however, that the parasitic elements (Cp1, Cp2A, and Cp2B) can still couple noise around the board, as well as produce power losses if switching speeds are fast enough.
Switching noise can also be targeted with filtering, although more elegant solutions involve strategies like multiphasing, Parasitics in real components can also induce oscillations in filter circuits, but simple solutions where damping is added (lower L or higher R) will help address this additional problem from switching noise. When these other methods fail, more inductance will also help reduce switching power supply noise from the driver section.
Newer switched-mode power supply designs include wide bandgap semiconductors like SiC and GaN. These devices can be switched at higher switching speeds and run at higher frequencies (in RF systems), and together with the layout of a printed circuit board (PCB) these devices can become a significant source of electromagnetic interference (EMI). Working with advanced devices in high speed PCB design, RF PCB layout, and many other products can be challenging, and EMI/EMC have to be considered early in the design process to ensure a smooth transition to market.
Whether you’re designing an ultra-rugged aerospace system or feature-rich embedded computing products, you’ll need to confront switching power supply noise early in the design process to ensure successful deployment of your product. NWES is an experienced design firm that develops IoT platforms, RF systems, data center products, aerospace systems, and much more. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.