Patrik Kalbermatten, Senior Manager - Distribution Promotion, Product Management MSABG - Magnetic, Sensor and Actuator at KEMET
Electrification is a major factor in moves towards smart industry and smart driving. The trend is bringing increasing numbers of high-powered electrical systems into close proximity with each other, such as arrays of industrial robots inside factories. In automotive markets, the latest vehicles are adding greater numbers of powerful electrical subsystems (figure 1) including electric power-steering, water pumps, oil pumps, and the air-conditioning compressor (eCompressor) that also cools the battery pack in hybrid and electric vehicles (H/EVs). Figure 1 also shows the H/EV main traction inverter and motor, on-board charger (OBC) and battery system, which operate at the battery voltage. This can be as high as 800V in vehicles designed for fast charging and high performance.
Changes in Power Systems
Electromagnetic compatibility (EMC) between the electrical/electronic systems and subsystems is essential to ensure all can function safely and correctly. The IEC 61000 series of standards are widely applied in the industrial space, to ensure minimum electromagnetic immunity and limit electromagnetic emissions. On the other hand, the automotive standards landscape is changing quickly with the rapid pace of vehicle electrification.
As far as the equipment designs are concerned, there are various techniques for suppressing electromagnetic interference (EMI) generated within the equipment, from paying careful attention to board layout and signal characteristics to inserting filters at strategic locations such as at power inputs.
In addition, some topical technical trends are changing the requirements for power-line filter design. One is that designers are adopting new wide-bandgap power semiconductors like silicon-carbide (SiC) MOSFETs in switched-mode power supplies, inverters, and DC/DC converters, to leverage their greater energy efficiency and reliability.
The efficiency advantage of SiC MOSFETs is partly due to superior switching performance, completing turn-on and turn-off transitions much more quickly than ordinary silicon alternatives can manage. Power supply designers have the option to select a high system switching frequency, which permits smaller passive components like capacitors to achieve a stable output voltage with fast dynamic response.
Regardless of the switching frequency selected, the fast-switching transitions result in a greater overshoot with longer ringing that causes large electromagnetic interference signals across a wide frequency range. This EMI can be reduced in several ways such as inserting a snubber to minimize the magnitude of the overshoot and reduce the ringing duration. Adding resistance in series with the MOSFET reduces EMI by lowering the current flow through the device. However, this reduces the net gain in switching efficiency. Also, optimizing the circuit board layout and package sizes to minimize the power loop can help reduce EMI, but has only a limited effect. And, of course, adding a powerline filter to attenuate the energy coupled onto the power unit’s external connections is always considered best practice.
In addition, SiC-based power circuits permit operation at higher temperatures, which allows designers to simplify thermal management and specify smaller, lower-cost heatsinks. Conventional high-permeability magnetic materials used in components like inductive EMI chokes are effective up to about 120°C. SiC devices can operate above this temperature, creating a requirement for magnetic materials that can withstand the more hostile conditions.
Moreover, general demands for increased power delivery in both the industrial and automotive spaces are driving designs towards higher operating voltages to offset any increase in current that leads to higher resistive losses and extra weight. This trend is illustrated starkly by the general increase in typical Li-ion battery voltage of EVs, which is being raised to enable charging at high power and thus minimize the time needed to fully recharge the vehicle’s battery. Suitable filter components are needed to handle these increased voltages.
New Materials Science
To simplify the design of powerline filters that meet these demands, KEMET is using magnetic materials that enable chokes to deliver greatly improved characteristics in filter circuits. These materials permit a more favorable combination of core size and magnetic permeability than conventional ferrite core materials.
KEMET offers three different core material options within their standard product line of automotive chokes. The SCR-XV and SCT-XV series provide high-performing ferrite material cores for high permeability and high-temperature needs. The newly launched SCF-XV adds a nanocrystalline option to this product line to take advantage of the advanced material properties.
The frequency band in which noise attenuation is most effective depends on the material properties and the magnetic permeability in particular. With ferrite materials, the effective frequency range varies depending on core shape, size and number of windings. Figure 2 compares the effective frequency ranges of MnZn and NiZn ferrites with the typical performance of nanocrystalline core material, which enables chokes to cover a broader frequency range.
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Figure 2. Comparing nanocrystalline and ferrite-core materials
The increased permeability of the nanocrystalline material allows more freedom to optimize the wire thickness and number of turns in the winding to achieve the desired component parameters for a given core size. In particular, fewer winding turns are needed than with conventional materials to build up a given inductance on the same core size. The choke with fewer turns has a lower ohmic resistance as well as less parasitic capacitance, leading to superior high-frequency filtering performance and wider frequency range.
In addition, the nanocrystalline materials have very high Curie temperatures and thus maintain their magnetic properties in challenging application environments thereby ensuring consistent performance and safe operation.
Nanocrystal materials do not only have advantages, one disadvantage to point out is the sensitivity to unbalanced and common mode currents which can effectively reduce the inductance in the application. The existence of those currents is linked to the switching devices, their capacity to chassis and also the load itself, like a BLDC motor and its winding capacity. It is always a good possibility to consider also the SCR-XV and SCT-XV versions as well (respectively using proprietary ferrite materials S15H and 7HT), as those have less sensitivity against those parasitic currents. 7HT is a premium high B ferrite material that offers higher electrical robustness and still a >150°C Curie temperature. S15H is a high µ material and offers high permeability when the component temperature keeps below 120°C. The graph from Figure 3 shows the inductance in relation to the common mode current present.
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Figure 3. SCF-XV, SCR-XV and SCT-XV common mode current response
High-Voltage Common-Mode Filers
KEMET SCF-XV AEC-Q200 qualified common-mode chokes (figure 4) are built with nanocrystalline core material and have the industry’s highest rated voltage, at 1,000V AC/DC. This permits use with high-voltage EV batteries including 800V systems. The core properties permit a reduced footprint, with outer core diameters of 19mm, 25mm, or 29mm, vertically or horizontally oriented. The complete series covers a range of current ratings from 5A to 35A, with DC resistance (DCR) values from 0.65mΩ to 40.3mΩ, and the inductors can operate at temperatures up to 150°C.
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Figure 4. Toroidal common-mode choke with vertical coil orientation
The wide range of properties allows engineers to meet the needs of most applications by selecting devices from the standard product offering. In contrast, when using conventional chokes, the frequency range and size limitations often call for designing a custom part. When designing with standard components, samples and production quantities can be obtained immediately and engineering costs are avoided.
In addition to their small size, high-voltage capability, and wide operating temperature range, these chokes also permit creative mechanical design to withstand harsh conditions such as high levels of vibration in automotive applications as well as excellent protection against ingress of moisture and other contaminants.
The requirement for EMI filtering is deeply ingrained in industrial and automotive applications. The latest nanocrystalline core materials enable common-mode chokes to have a wider effective frequency range, smaller footprint, higher temperature capability, and lower DC resistance than chokes built with conventional ferrite cores. These characteristics ease the design of filters suitable for use with SiC wide-bandgap power semiconductors and high-voltage bus systems.