New power-semiconductor technologies like SiC and GaN enable increased efficiency and higher switching frequencies, which allows smaller component sizes. But these gains come at the expense of greater radiated electromagnetic emissions, just as EMC regulations are getting tougher. How can engineers effectively minimize radiated EMI?
High-efficiency power conversion and pervasive wireless connectivity are two trends that can profoundly influence sustainability and living standards, from enhancing cost parity of renewable energy sources, to putting an affordable always-on communication device in every pocket, to powering and connecting the Internet of Things.
On the other hand, both present tougher challenges to ensure equipment satisfies electromagnetic compatibility (EMC) regulations. They are expected to function normally in the target environment, while also not interfering with other equipment in the vicinity. Moreover, EMC legislation in major markets worldwide is becoming increasingly stringent as high-speed switching and high-frequency RF devices crowd the electromagnetic environment.
Looking ahead, innovations like connected cars are expected to up the ante even further, adding a safety-critical dimension to the EMC issues surrounding daily-use consumer-class electrical equipment.
The Wide Bandgap Effect
In the power-conversion field, wide bandgap semiconductor technologies such as silicon carbide (SiC) and gallium nitride (GaN) are now becoming commercialized to improve performance over conventional silicon parts: conduction losses are lower, die size and therefore cost can be reduced, breakdown voltage is greater, temperature capability is increased, and faster switching allows smaller smoothing and decoupling components.
However, although increased switching frequencies allow greater power density and lower energy losses, the picosecond switching edges result in harmonics deep into radio-frequency territory. Slew rates can be much higher than with conventional silicon devices: the gate voltage must swing between typically +15V and -3V to ensure reliable switching of SiC devices, for instance, compared to 0-10V for a standard MOSFET, and dV/dt across the transistor can be high, too, if a higher DC-link voltage is used for greater efficiency. For a switching frequency of about 1MHz, the amplitude of associated harmonics can be troublesome even up to several hundred MHz. These must be dealt with to ensure EMC compliance.
At the same time, EMC regulations are becoming stricter as evolving applications and usage trends mean more and more equipment must coexist within close proximity. Increasingly, these are wireless devices such as mobiles, tablets, and IoT infrastructure connected via cellular, WLAN, PAN, LPWAN or other networks in various frequency bands: sub-GHz RF, GSM/CDMA, 2.4GHz or 5GHz Wi-Fi, or Bluetooth® 5 at 2.4GHz.
The latest EU EMC directive, 2014/30/EU, provides a good example. Revised technical limits require lower conducted and radiated emissions, and greater immunity, to demonstrate compliance, and the EU’s New Legislative Framework places greater emphasis on market surveillance to identify and remove non-compliant products from sale.
The EMC directive 2014/30/EU references a variety of technical specifications, including new documents such as EN 50121-4 for railway signalling equipment and 50121-5 for power equipment, EN 55014 for household electrical products and appliances, and EN 55022 and 55032 for IT equipment and multimedia equipment. Meeting these technical specifications is one aspect of demonstrating compliance; the other being maintaining satisfactory documentation.
In North America, the Federal Communications Commission (FCC) has set out EMC requirements in its Part 15 legislation. For light industrial and industrial applications respectively, the international IEC 61000-6-3 and IEC 61000-6-4 EMC standards are used.
Dealing with Power Supply Noise
So, EMC compliance is becoming more important, but harder to achieve, at the same time power-systems design is pushing switching frequencies higher resulting in noise signals in or near the ISM radio-frequency bands.
Historically, the typical noise spectrum of switching power converters containing conventional silicon IGBTs or MOSFETs has spanned the frequency range from about 10kHz to 50MHz. Much of this is within the range for conducted emissions (9kHz to 30MHz) defined by CISPR/CENELEC and FCC noise standards.
Conducted noise can exist as differential-mode noise, also known as normal-mode, or common-mode noise, and is coupled between a source and power or signal lines. Differential noise is the result of expected operation of the equipment, and follows the signal or power lines, whereas common-mode noise is coupled between signal or power lines and unintended conduction paths such as chassis parts or an earth ground.
Conducted noise is usually dealt with by inserting a powerline or signal filter containing capacitor/s and/or inductor/s. Typically, the capacitor faces high-impedance circuitry – which may be the source or load – while an inductor is needed to interface to a low-impedance circuit. If the source and load are both high-impedance, a pure capacitive filter can be used, or a pi-filter for steeper frequency response.
Standards bodies globally have established specifications for passive filters, such as the European EN 60939 specification based on IEC 60939, and UL 1283 or MIL-F-15733 that apply in USA. KEMET’s filters comply with the applicable standards and are available in various configurations including single- or three-phase, chassis-mount, board-mount or feed-through filters spanning current ratings from below one Amp to 2500A. There are also special filters for applications such as medical equipment, or lighting equipment that must meet the EN 55015 emission standard to be marketed in the EU.
Attenuating High-Frequency Noise
Interference signals at frequencies above 30MHz is classified as radiated emission, according to North American and European standards. Major sources of radiation include cables and poorly designed PCB tracks. Applying best design practice, including keeping these cables and tracks as short as possible, and positioning any tracks carrying signal pairs closely together on the board, is always recommended. However, it is not always possible to design-out EMC challenges in this way, and extra measure to attenuate high-frequency noise signals are required.
Fundamentally, the strategy for dealing with radiated noise is to convert the high-frequency noise energy into heat by imposing magnetic losses. A cable, for example, is passed through a ferrite core to attenuate high-frequency radiated EMI. The magnetically permeable core interacts with the magnetic field generated by common-mode noise currents due to the self-inductance of the cable, presenting a high impedance at high frequencies. Passing the cable through the core multiple times increases the noise attenuation at any given frequency. Differential currents and low-frequency signal currents generate minimal magnetic flux, and so experience little attenuation.
Flexible Shielding Solution
Other high-frequency noise radiators, such as PCB tracks, must be addressed differently, usually with some form of shielding. A grounded metallic shield can be effective, but adds cost and a small enclosure may not provide enough space for the shield and its mechanical fixings and ground connections. If a noise issue is discovered late in the project, there may not be time to design such a component.
Flexible shielding materials made of high-permeability magnetic materials (figure 1) can offer a convenient and cost-effective solution. This is a widely recognized approach, and, indeed, methods for measuring their electromagnetic characteristics are standardised under IEC 62333. This standard aims to ensure that sheet manufacturers demonstrate the performance of their products clearly, and end users can achieve comparable results in practice.
Figure 1. the composition of suppression sheets combines energy-absorption properties with flexibility.
KEMET’s Flex Suppressor is IEC 62333 compliant and provides effective attenuation of frequencies beyond 1GHz. The material can be trimmed to a suitable size and shape to shield specific circuit features, such as a power-switching stage, to absorb radiation or protect against external interference. It can be fixed to the inside of a casing, close to the circuity in question, or in other locations such as between closely stacked boards to prevent crosstalk. The material can also be wrapped around cables, to perform in a similar way to a ferrite choke.
Other proven applications include ESD protection, wireless-charging and RFID-range enhancement, and countering receiver de-sense in multi-radio devices like laptops and mobiles by preventing reflected interference. Flex Suppressor is available in several permeability grades, giving designers effective choices for a wide range of noise frequencies. These include standard grade with relative permeability of 60, and extra-high permeability materials with 130. There is also an ultra-low permeability of 20 variant that provides extremely high noise attenuation in Wi-Fi frequency ranges and higher.
High-frequency noise sources and tougher legislation challenge power designers seeking to use wide-bandgap semiconductors in their latest designs. Ferrite cores and high-permeability suppression materials are evolving to combat radiated noise at frequencies up to 1GHz and beyond.