How to deal with EMI and EMC in high frequency?
From increasing the cost parity of renewable energy, to enabling each of us to have an affordable, always-on communication device, to powering and connecting the Internet of Things, efficient power conversion and widespread wireless connectivity will These are two trends that profoundly affect sustainability and living standards.
On the other hand, in order to ensure that the equipment meets electromagnetic compatibility (EMC) regulations, both have more severe challenges. They need to operate normally in the target environment without interfering with other devices nearby. In addition, as high-speed switches and high-frequency RF equipment crowd the electromagnetic environment, EMC regulations in major global markets are becoming stricter.
Looking ahead, innovative technologies such as Connected Vehicles are expected to intensify competition, adding a safety-critical aspect to the EMC issues surrounding everyday consumer-grade electrical equipment.
Wide band gap effect
In the field of power conversion, wide-bandgap semiconductor technologies such as silicon carbide (SiC) and gallium nitride (GaN) are being commercialized to improve the performance of traditional silicon devices: lower conduction losses, lower chip size, and thus lower costs, Higher breakdown voltage, increased temperature performance, and faster switching speeds can use smaller smoothing and decoupling components.
However, although increasing the switching frequency can achieve greater power density and lower energy loss, picosecond-level switching edges will cause harmonics to penetrate into the RF field. The slew rate of new power devices will be much higher than traditional silicon devices: for example, compared to the standard MOSFET 0-10V gate voltage, to ensure reliable switching of SiC devices, the gate voltage must be between + 15V and -3V In addition, if higher DC bus voltage is used to improve efficiency, the dV / dt across the transistor will also be high. For a switching frequency of about 1 MHz, the amplitude of the relevant harmonics can be troublesome even for frequencies up to several hundred MHz. To ensure compliance with EMC standards, these issues must be addressed.
At the same time, with the continuous development of applications and usage trends, more and more devices inevitably coexist in adjacent areas, and EMC regulations are becoming stricter. These wireless devices will be more and more, including mobile devices, tablets and IoT infrastructure, they are through cellular, WLAN, PAN, LPWAN or sub-GHz RF, GSM / CDMA, 2.4GHz or 5GHz Wi-Fi or 2.4 Various other frequency bands such as Bluetooth® 5 of GHz realize network connection.
The latest EU EMC Directive 2014/30 / EU provides a good example. The revised technical limits require reduced conduction and radiated emissions and improved immunity to prove compliance. The European Union ’s new legislative framework places greater emphasis on market surveillance in order to detect and exclude the sale of non-compliant products.
Various technical specifications are cited in the EMC Directive 2014/30 / EU, including EN 50121-4 for railway signal equipment, 50121-5 for power equipment, EN 55014 for household electrical products and equipment, and EN 55022 for IT equipment and multimedia equipment And new files such as 55032. Meeting these technical specifications is one aspect that demonstrates compliance, and the other is maintaining satisfactory documentation.
In North America, the United States Federal Communications Commission (FCC) has specified EMC requirements in its Part 15 legislation. For light industry and industrial applications, the international IEC 61000-6-3 and IEC 61000-6-4 EMC standards are used, respectively.
Respond to power supply noise
Therefore, as the power system design pushes the switching frequency higher and the noise signal enters the ISM radio frequency band or nearby, EMC compliance becomes more and more important but more difficult to achieve.
Historically, the typical noise spectrum of switching power converters containing traditional silicon IGBTs or MOSFETs has covered a frequency range of approximately 10 kHz to 50 MHz. Most of them are within the conducted emission range (9kHz to 30MHz) specified by the CISPR / CENELEC and FCC noise standards.
Conducted noise can exist in the form of differential mode noise (also known as normal mode) or common mode noise, and is coupled between the power supply and the power line or signal line. Differential mode noise is generated due to the expected operation of the device and flows along the signal line or power line, while common mode noise is coupled between the signal line or power line and unintended conduction paths (such as chassis components or ground).
Conducted noise is usually handled by inserting power lines or signal filters containing capacitors and / or inductors. Generally, capacitors face high-impedance circuits-which may be power supplies or loads-and inductors are used to connect low-impedance circuits. If the power supply and load are high impedance, you can use a pure capacitor filter, or use a π-type filter to achieve a steeper frequency response.
Global standards bodies have developed passive filter specifications, such as the European EN 60939 specification based on IEC 60939, and UL 1283 or MIL-F-15733 for the United States. KEMET ’s filters comply with applicable standards and can be provided in a variety of configurations, including single-phase or three-phase, chassis mounting, circuit board mounting, or feedthrough filters, with current ratings from less than 1A to 2500A. There are also special filters for medical equipment or lighting equipment that must comply with the EN 55015 emission standard and can be sold on the EU market.
Attenuating high frequency noise
North American and European standards classify interference signals with frequencies above 30 MHz as radiated emissions. The main radiation sources include cables and poorly designed PCB traces. Engineers should always use best design practices, including shortening these cables and traces as much as possible, and arranging closely any traces that carry signal pairs on the circuit board. However, this method does not always solve EMC challenges, and we need to take additional measures to attenuate high-frequency noise signals.
Fundamentally, the strategy for dealing with radiated noise is to convert high-frequency noise energy into heat by applying magnetic losses. For example, passing a cable through a ferrite core can attenuate high-frequency radiated EMI. Due to the self-inductance of the cable, the magnetic conductor core interacts with the magnetic field generated by the common-mode noise current and exhibits high impedance at high frequencies. Passing the cable through the core multiple times can increase the noise attenuation at any given frequency. The differential mode current and the low-frequency signal current generate the smallest magnetic flux, so the attenuation is small.
Flexible shielding solution
PCB traces and other high-frequency noise radiation sources must be resolved in different ways—usually using some form of shielding. Grounded metal shielding is very effective, but it will increase the cost and small shell, and may not provide enough space for the shield and its mechanical fixing and grounding connection. If noise problems are discovered late in the project, there may not be time to design such components.
Flexible shielding materials made of high permeability magnetic materials (Figure 1) can provide convenient and economical solutions. This method is widely recognized. In fact, the method used to measure its electromagnetic characteristics has been standardized in IEC 62333. The standard aims to ensure that sheet manufacturers clearly demonstrate the performance of their products, while enabling end users to obtain comparable results in practice.
Figure 1: The composition of the suppression sheet combines energy absorption characteristics and flexibility.
Other mature applications include ESD protection, wireless charging and RFID range enhancement, and in multi-radio devices such as notebook computers and mobile devices, the reduction in receiver sensitivity is offset by preventing reflected interference. Flex Suppressor has several permeability levels, providing designers with an effective choice of various noise frequencies. They include standard grades with a relative permeability of 60 and ultra-high permeability materials with a value of 130. There is also an ultra-low permeability version with a value of 20, which provides extremely high noise attenuation in the Wi-Fi frequency range.
To sum up
High-frequency noise sources and stricter regulations pose challenges to power supply designers who seek to use wide-bandgap semiconductors in their latest designs. Ferrite cores and high permeability suppression materials are constantly being developed in order to resist radiated noise at frequencies up to 1GHz or higher.
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