In this three-part series, Exro's Chief Technology Officer, Eric Hustedt, helps us explore what a traction inverter is, how inverters work, EV traction inverter development, and the latest advancements in technology for traction inverter design. This third part of the article focuses on the recent a Contact online >>
In this three-part series, Exro''s Chief Technology Officer, Eric Hustedt, helps us explore what a traction inverter is, how inverters work, EV traction inverter development, and the latest advancements in technology for traction inverter design. This third part of the article focuses on the recent advancements related to electric vehicle inverter design. Specifically, we explore switches in traction inverters, semiconductor advancements, and cooling methods, and other developments that have contributed to the evolution of traction inverter design.
Welcome to part three of our series on inverter technology. In part one, we provided an introduction to inverters and how they work, and in part two, we explored the early advancements in inverter technology and the differences between AC and DC motors. Now, in part three, we will dive deeper into the latest advancements in inverter technology and take a closer look at critical components such as switches, semiconductor advancements, cooling methods, and interconnects.
Since its invention, the fundamental concept behind a three-phase inverter has not changed; however, there have been major advancements in the devices, fabrication techniques, and components used. These advancements have enabled the production of smaller, more affordable, and more powerful inverters. In the following section, we will delve into each of these crucial developments in greater detail.
The switches in an inverter play a crucial role in regulating the flow of electrical energy and converting DC to AC power. They are responsible for switching the current ON and OFF at a rapid rate to create the desired AC waveform. The type, construction, and cooling of the switching elements are arguably the most significant elements of an inverter design.
The switches used in modern inverters must be able to handle high currents sometimes exceeding 500 amps per phase or more. They must also rapidly switch this current on and off with voltages ranging from 400V to 800V DC. This is no small task and requires switches that can handle this level of power without generating excessive heat or voltage.
To put the power requirements of an inverter into perspective, it is helpful to compare them to an average household outlet. Household outlets typically operate at only 15 amps and 120V, or 10 amps and 240V, depending on the region. In contrast, inverters used in electric vehicles and other high-power applications must often handle an order or two magnitudes larger currents and voltages. This highlights the complexity and sophistication required in designing the switches for these applications.
Since the 1980s, MOSFETs have been the preferred device for lower voltage inverters, while IGBTs have been the go-to choice for higher voltages of around 150V or higher. IGBTs remained the top choice in the high voltage market until the mid-to late-2010s when wide band gap semiconductors like silicon carbide (SiC) MOSFETs became commercially viable.
Wide band gap semiconductors are materials that require more energy to be applied to them to transform them from insulators to conductors compared to conventional semiconductors such as silicon. This reduces sensitivity to external energy, allowing them to operate at higher voltages, frequencies, and temperatures.
As of 2023, two wide-bandgap semiconductors are commercially available for power devices: silicon carbide (SiC) and gallium nitride (GaN). At present, SiC is leading in terms of cost per ON-resistance and are available with higher voltage capability; as such, it has become the dominant choice for inverter power semiconductors.
Power semiconductor switches, regardless of type, generate heat when operating, and how well they can be cooled determines how much silicon area is required for a given application. Silicon area is directly proportional to the cost of the switch, therefore improving the cooling methods has a direct cost benefit.
The heat generated in the switches is a result of two main factors: conduction loss and the already discussed switching loss. Conduction loss is the heat generated by the movement of electric current. An example of this can be seen when a coiled extension cord is connected to a high-powered device, such as a space heater, causing the cord to become warm. With the exception of superconductors, conduction loss occurs in all materials through which current flows, including power semiconductors, bus bars, and power delivery cables.
As power semiconductors become more compact and high-powered, the challenge lies in effectively removing heat from these smaller devices. Additionally, the cooling solution must provide electrical isolation, as the chips within them operate at hundreds of volts and are "live", while the cooling system is typically made of metal and connected to the chassis/ground.
Various ceramics can be utilized, each providing different levels of performance and cost. Alumina (aluminum oxide) is frequently used and is the most economical. Other ceramic materials such as Aluminium Nitride (AlN) and Silicon Nitride (SiN) or Zirconium-doped Alumina, offer improved thermal performance, enhanced mechanical strength, or both, with a corresponding increase in cost.
An alternative method of substrate construction is called Active Metal Brazing (AMB), which bonds the metal to the ceramic using a high-temperature soldering process instead of an oxidation process. In this process, copper is soldered to the ceramic to create the substrate.
Once an appropriate substrate has been selected, the next step is to attach the semiconductor to the substrate and the substrate to the cooling system. The process of connecting the semiconductor to the substrate, also known as "die attach," was traditionally done using solder. Although solder has a reasonable thermal conductivity, it is not very mechanically durable and repeated mechanical stress must be limited to prevent fatigue failure.
Mechanical fatigue is a phenomenon that arises when a material is subjected to repeated loads, resulting in the gradual development and spread of cracks over time. In this case, a single application of the load will not cause immediate failure, but repeated application and removal of the load will eventually lead to failure. For instance, bending a wire once will not break it, but if this wire is repeatedly bent and straightened, it will eventually fail due to fatigue at the bend point.
The fatigue life characteristics of different materials vary, this characteristic is generally presented in a chart indicating the peak load and the number of cycles before failure occurs. Understanding the behavior of solder materials under cyclic strain is complex, as factors such as relaxation time significantly impact reliability.
In general, it is accepted that solder is not a material suitable for joints subjected to large mechanical loads, leading to extensive research on alternative methods for die attach. One of the more robust alternatives is sintering, which involves a complex process of forming a solid mass from a powder mixture of metals using pressure and temperature, but without melting. Forming a hard snowball from loose snow by pressing it together in your hands is a form of sintering. Sintering is also commonly used in the manufacture of ceramics, powder metal parts, and some plastics.
The sintering method for die attachment typically uses a paste made from a mixture of silver and copper powder, along with an organic filler material, which evaporates during the sintering process. The die is placed on the mixture and subjected to pressure and heating in a controlled environment to produce the final joint.
The sintered joint that results has greatly improved thermal conductivity, often three times greater than that of solder, and a significantly higher melting point, typically over 960°C, compared to the melting point of solder which is around 220°C. The most notable advantage of the sintered joint is its capability to withstand cyclic mechanical stress, making it ideal for use in power modules where the die-to-substrate joint is subjected to cyclic thermal stress driven by the different thermal expansion of the materials.
The sintering process for die attachment is significantly more expensive than soldering due to the cost of the materials involved and the additional process steps. The most sophisticated cooling technologies, including ceramics, sintering, and related procedures, can cost more than the semiconductor being cooled. Currently, there is ongoing research to find ways to reduce the cost of sintering, such as developing sinter pastes with reduced or no silver content. For instance, pure copper sintering is being explored as a cost-effective alternative.
Having connected the semiconductor chips, or die, to an electrically insulating substrate, the next step is to remove the heat from the system by connecting the substrate to a fluid cooler. A common method of doing this is by soldering the substrate to a heat exchanger, as both sides of the substrate are equipped with a metal layer. Other options for attachment include mechanical clamping and the use of thermal paste or thermal adhesives.
Sintering is also gaining traction for high-performance power modules for attaching the substrate to the cold plate, however since the involved areas are much larger than the semiconductor die, the cost is significant. Often the cold plate is pre-attached to the substrate by the power module manufacturer to ensure optimal thermal performance.
This construction method, sintering the die to a DBC or AMB substrate and then to a pin fin cooler, is providing the best thermal performance and reliability from the semiconductor junction to the cooling fluid. Nonetheless, there are other system-level factors to consider, and various approaches being taken by other organizations, but they are outside the scope of this article.
Semiconductor switches, including MOSFET (Si or SiC), transistors, and IGBTs, are all three-terminal devices. Two of the terminals are the power connections, one for each side of the switch, and the third is the control input. The control input is connected to a small signal, usually, a voltage or current, which is used to turn the switch ''ON'' or ''OFF''. In MOSFETs and IGBTs, this control input is known as the gate, while in bipolar transistors, it is called the base.
The bottom of the die is either soldered or sintered to the copper layer of the substrate, which then also acts as the Drain connection or the collector in the case of an IGBT. The remaining two connections, the gate, and source (or gate and emitter for IGBT), are established on the top of the die.
The most commonly used method for creating these top-side connections is through the use of a wire bonder. This machine uses very fine, pure aluminum wires to "stitch" together the tops of the die and other traces on the substrate or bus bars to form all the electrical connections.
The wires used for these connections are incredibly thin, with a typical diameter of 0.125mm for the gate connection and 0.5mm for the power or source connections. For larger bonds, such as those found in power devices, pure aluminum is typically used, while for smaller devices like microprocessors, even thinner wires made from gold are employed due to their superior electrical conductivity and ease of bonding.
Although aluminum wire bonding is still the preferred method for the majority of semiconductors, the latest power semiconductor die has surpassed its capabilities in power module applications. Alternative methods of creating top-side connections that can carry larger currents and offer better reliability have been the focus of ongoing research. While there are many ideas on how to electrically connect devices, the real challenge lies in developing the material science and manufacturing processes that make these ideas feasible.
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