Power electronics issues Increases in both power density and voltage in power electronics, along with trends in packaging, also present challenges to potting and encapsulation. Power electronic systems include unidirectional and bi-directional inverters, DC-DC converters, BMS and battery chargers (both on-board and external), and they are also used in advanced driver-assistance systems. Epic Resins’ Staller notes that EV manufacturers are increasingly combining two or more of these systems, often housing multiple components in one enclosure to reduce the total length of cabling and coolant lines in the vehicle. “Individually or combined, these power electronics process an extraordinary amount of electrical energy while orchestrating precise control over a multitude of components,” he says. “Managing this energy requires a delicate balance of properties to ensure safety, efficiency and longevity.” Staller adds that choosing the appropriate advanced polymer that can simultaneously dissipate heat, mitigate thermal cycling and insulate high voltages while inhibiting flame propagation is critical. With high-voltage inverters, for example, the challenges are to prevent insulation breakdown and short circuits, says Axalta’s Koschmieder. Whether they are based on insulated gate bipolar transistors (IGBTs) or silicon-carbide metal-oxide semiconductor field-effect transistors (MOSFETs), the solid-state switches inside inverters produce stepped sine waves. “The voltage increase when the semiconductor switch turns on affects inductive components, creating a voltage spike of opposite polarity for a short time. That will lead to a voltage peak twice as high as that of the intermediate circuit in the inverter. With these very high voltages, there can be discharges into the surrounding materials, especially if there is any air remaining inside the enclosure, and these discharges can cause very quick destruction of the insulation,” he explains. This, he adds, will reduce the lifetime of the motor drastically. The need to prevent such damage is driving demand for materials with high dielectric strength and good penetrating properties. Henkel stresses that the increasing density of electrical components, and the need for rapid charging and discharging of batteries via the power electronics are, as elsewhere in the powertrain, driving demand for higher thermal conductivity. “This inherently boosts the loading of thermal filler required, and significantly impacts viscosity and flowability. The impact on viscosity is manageable for encapsulating systems, but for potting applications the flow properties throughout the shelf life must be carefully managed to ensure complete filling of the component and integration with customer production lines,” White explains. Novel motor topologies and locations also affect requirements for protective materials, and they are making custom solutions vital to success and longevity, says Staller at Epic Resins. There are multiple topologies where motor encapsulation is essential, he adds, citing axial flux motors as an example, particularly in-wheel applications. “This location exposes the motor to greater environmental stresses, including water and vibration. Completely encapsulating the conductors gives them excellent protection.” In radial flux machines, particularly in interior permanent magnet (IPM) synchronous reluctance motor (syn-RM) applications, the magnets must be secured to the motor. “These compounds fasten the magnets in the correct position, creating a precise magnetic-flux location for optimised electric and magnetic field coupling, yielding maximum torque and efficiency,” Staller explains. Alongside thermal conductivity, thermal stability is a vital property when choosing a motor encapsulation compound, because it maintains the polymer’s properties over extended periods of thermal exposure, and it is important to assess this. “Monitoring weight change and hardness change of a polymer at elevated temperatures gives a great comparison to determine the stability of a motor compound,” he says. Repair and recycling The need for repair and recycling is a significant challenge for potting and encapsulation materials, and Henkel reports significant interest in effective strategies for controlling the release force without affecting in-service performance. “Silicone-based materials are inherently well suited for repair due to their lower strength, as they can be readily removed from components manually and subsequently washed off with appropriate cleaners, allowing a component to be reused or recycled without contamination,” according to Brown at Henkel. “Henkel has an advanced research team, focused on de-bonding technologies. The team is continuing to test and develop solutions that support in-plant repair, vehicle service and end-of-life recycling.” Insight | Potting & encapsulation March/April 2024 | E-Mobility Engineering On-board charger, with potting and encapsulation applied to electronic and electromagnetic components (Image courtesy of Henkel) 42
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