Illustration showing an exploded 3D view of an IGBT with gel encapsulant.
PV inverters convert the direct current (DC) produced by solar panels into the alternating current (AC) used by homes and businesses. They are also used with battery energy storage systems in solar, wind and other renewable energy resources.
To convert high-voltage DC into grid-available AC, solar inverters use insulated gate bipolar transistors (IGBTs) as fast electronic switches. Seventh-generation IGBTs (IGBT7s) are designed to manage higher currents than first-generation devices, but higher currents generate more heat. Unless this high heat is dissipated, there is a risk of IGBT failure due to thermal overload.
Thermal management for IGBT7 modules
Thermal management solutions for IGBT modules include heat sinks, cold plates, heat pipes, turbulators, direct liquid cooling and vapor cooling loops. Advances in IGBT technology have also improved thermal management. The next-generation IGBT7 modules are designed to manage typical operating junction temperatures up to 150°C and a maximum transient overload temperature of 175°C for short durations.
Furthermore, IGBT7 technology supports higher power densities for improved power efficiency and the handling of greater DC input loads, which are typically higher than AC output ratings by a ratio of 1.1 to 1.3. Electrical loads vary by PV installation, but integral overload protection automatically disconnects circuits if there is a risk of overheating.
Silicone gels are currently used for IGBT7 thermal management in PV inverters. Gels are a special class of encapsulants that become extremely soft after curing. They are usually applied in thick layers but flow easily into tight spaces before curing. Cured gels provide the dimensional stability of a solid elastomer and maintain their shape, size and function over time and under changing conditions. Yet cured gels also retain some qualities of a liquid, such as the ability to dissipate pressure from an impact. They can also self-heal by flowing back together if separated.
Gels that use advanced silicone chemistries provide excellent heat resistance up to 180°C for continuous exposure. This exceeds IGBT7 modules’ maximum temperatures and provides greater high-temperature resistance than standard encapsulants. Advanced silicone gels also exhibit excellent thermal stability, the ability of a material to retain its physical properties when subjected to heat. Importantly, these gels maintain their flexibility and strength through repeated heating and cooling cycles, a common occurrence since solar power production and ambient temperatures increase during the day before power production stops and ambient temperatures fall at night. In power electronics like IGBT7 modules, thermal cycling is also important because load cycling and switching losses can cause temperatures to fluctuate significantly.
Moisture resistance and high-voltage protection
Advanced silicone gels resist moisture and contaminants that can cause short circuits or corrosion in electronics. When poured into an electronic enclosure, these gels encapsulate electronics and fill voids between IGBTs. Photovoltaic inverters are usually housed in metal or plastic cabinets, but poorly sealed doors, vents or cable entry points can allow the ingress of unwanted substances. Because PV enclosure cabinets are often located outdoors, the ingress of moisture and dust are concerns. These contaminants can also enter an enclosure during routine checks or maintenance activities.
To help prevent contamination, PV enclosures are sealed using mechanical gaskets, but these seals are not enough. Ingress Protection (IP) standards describe the degree of protection that a sealed enclosure provides. Higher IP ratings denote greater levels of protection against water and dust, but there is still a risk of contamination. That is because enclosures with higher IP ratings are so well-sealed that when air inside the enclosure draws in moisture-rich air from outdoors, it is more difficult for water vapor to escape. Condensation can also occur because of temperature fluctuations inside the enclosure caused by power electronics or exposure to direct sunlight.
As an extra layer of moisture protection, Silicone gels can be used to encapsulate sensitive electronics and protect them against moisture which can cause significant issues. In PV inverters, moisture can corrode metal parts and weaken solder joints. It can also cause short circuits because moisture that condenses on internal components creates an electrically conductive pathway. In addition, moisture can increase an insulating material’s dielectric constant, which can slow signal propagation speeds.
Importantly, advanced silicone gels also provide electrical insulation against high voltages. With their excellent dielectric strength, these encapsulants readily prevent the flow of electricity. Individually, solar panels produce DC in low voltages. In large scale solar arrays, however, panels are wired in a series to add their individual voltages together. Connecting multiple panels in a long string, such as in larger commercial and utility-scale systems, can result in voltages up to 1500 volts of direct current (VDC). A central PV inverter needs to withstand these voltages and the resulting heat that is generated.
Reducing thermal and mechanical stresses
Among their advantages, advanced silicone gels protect circuits and interconnects from thermal and mechanical stresses because these advanced materials are soft and stress-relieving. Thermal stresses build up in a material when there are changes in temperature and the material’s movement is constricted. Compressive stress occurs if the material cannot expand when temperatures increase, and tensile stress occurs if the material cannot contract when temperatures decrease. When silicones are subjected to temperature changes, they expand or contract more than other elastomers. Silicones also exhibit less thermal stress for the same amount of temperature change.
In PV inverters, mechanical stresses are caused by vibrations. For example, converting DC to AC power may cause IGBTs to vibrate because of the changing magnetic fields produced by the alternating current. This can result in physical damage such as bond wire failure and solder joint fatigue. Because IGBT7 technology is designed to manage higher switching frequencies, the resulting vibrations can be more extreme. A combination of thermal and mechanical stresses is typical, and higher operating temperatures risk softening an encapsulant and reducing its vibration damping capabilities. With their ability to withstand temperatures up to 180°C, however, advanced silicone gels are well-suited to meet these challenges.
Seventh-generation IGBTs are more rugged than their predecessors, but vibrations or mechanical shock can still cause physical damage to power electronics. Along with their softness, the self-healing properties of advanced silicone gels support vibration management. By repairing small cracks without intervention, self-healing gels maintain their ability to absorb and dissipate energy from vibrations over time, preventing degradation that would otherwise reduce the gel’s effectiveness. Advanced silicone gels have a longer functional life than other types of encapsulants, and they can help extend the useful life of IGBT modules while reducing electronic waste.
Primer-less adhesion and flexible processing
Unlike standard encapsulants, advanced silicone gels do not require a primer to adhere to the IGBT7 module. By eliminating the need for surface preparation, these self-priming products save time and labor during assembly. With their medium viscosity, they spread easily and remain in place when dispensed. In addition, advanced silicone gels provide greater processing flexibility. These two-part products can cure at room temperature with no need for ovens, but heat-accelerated curing can be used for faster processing. For manufacturers, the benefits include shorter cycle times and lower energy costs.
Material selection and expertise
As the solar industry increasingly adopts PV inverters with higher power densities, power efficiencies will improve and electrical loads will increase. Seventh-generation IGBTs are enabling these advances, but their elevated performance presents challenges that traditional encapsulants cannot adequately meet. Advanced silicone gels can successfully address these challenges with their higher-temperature resistance, thermal stability, self-healing properties and flexible processing, making them well suited for high-density applications.
While there are various thermal management materials available, selecting the best solution requires specialized knowledge. By working with an experienced partner, PV inverter manufacturers can receive help with material selection, use local sourcing, and achieve more efficient and reliable product designs.
Cody A. Schoener, PhD, is marketing manager for Dow Performance Silicones at The Dow Chemical Company. In this role, he is serving the consumer electronic, industrial electronic and pressure sensitive industry. His responsibilities include short and long-term strategy development, innovation portfolio management, and leading a global team specifically for industrial electronic strategy and global accounts. Prior to his role in marketing, he spent eight years as a technical and developmental scientist for The Dow Chemical Company servicing polyurethane and cellulosic chemistries. He received his BS and MS degrees in Biomedical Engineering from Texas A&M University, College Station, Texas, in 2007 and 2009, respectively. He earned his PhD in chemical engineering from University of Texas at Austin in 2012.
