"Range anxiety," caused by limited driving range, is a major obstacle for many consumers adopting electric vehicles. Increasing battery density and improving the efficiency of energy conversion processes are key to extending vehicle range and alleviating this anxiety. A crucial area where energy efficiency is paramount is the main drive inverter, which converts the DC battery voltage into the required AC drive to power the motor.
In this technical article, we discuss how VE-Trac™ IGBTs and silicon carbide (SiC) modules enable higher battery density and provide a more efficient conversion process to extend the range of electric vehicles, thereby helping to overcome consumer concerns.
The main drive inverter is the core of an electric vehicle, connecting the battery and the main drive motor. They convert the DC battery voltage to the AC drive required by the motor, typically ranging from 80 kW to over 150 kW in power. Battery voltage, depending on the size of the battery pack, is usually in the 400 V DC range, but 800 V DC is becoming increasingly common to significantly reduce current and thus lower losses.
While the cost of lithium-ion (Li-Ion) batteries has decreased by 40% over the past three years, or 90% over the past decade, it remains the highest cost item in electric vehicles. This price reduction trajectory is expected to continue until around 2025, at which point prices will stabilize. Given this cost, the immediate priority is to utilize every joule of stored energy as efficiently as possible to reduce the cost and size of battery packs.
This electric drive provides extremely high torque and acceleration. The responsiveness of the inverter and electric motor combination directly affects the vehicle's "perception," and thus the consumer's driving experience and satisfaction.
Function of switching devices
The main drive inverter typically contains three half-bridge elements, each consisting of a pair of MOSFETs or IGBTs, referred to as the upper and lower bridge switches. There is one half-bridge for each motor phase, for a total of three, with each switching device controlled by a gate driver.
Figure 1: Overview of the main drive inverter
The primary function of the switch is to turn the DC voltage and current from the high-voltage battery on and off, providing AC drive for the vehicle's motor. This is a demanding application because it operates under high voltage, high current, and high operating temperature conditions, while an 800V battery can provide over 200 kilowatts of power.
The main drive inverter based on a 400 V battery system requires power semiconductor devices with a VDS rating between 650 V and 750 V, while the 800 V scheme increases the VDS rating requirement to 1200 V. In a typical application, these power devices must also handle peak AC currents exceeding 600 A for up to 30 seconds (s) and maximum AC currents of 1600 A for approximately 1 millisecond (ms).
In addition, the switching transistors and the gate drivers used in the device must be able to handle these large loads while keeping the main drive inverter highly efficient.
IGBTs have long been the preferred devices for main drive inverter applications because they can handle high voltage, fast switching, deliver high energy efficiency, and meet the automotive industry's challenging cost targets.
Switching and power density
Modern cars are extremely cramped—at least the space containing technology is. This illustrates that power density is a crucial parameter, especially the power density of the powertrain. Physical size (and weight) must be minimized, as any amount of weight will reduce the vehicle's range.
Besides the physical dimensions of the components, energy efficiency is also a major driving factor in the design. Higher energy efficiency means less heat is generated, and the inverter structure can be more compact.
Switching (whether IGBT or MOSFET) has the most significant impact on heat loss. Lower on-resistance (RDS(ON)) reduces static losses, while improvements in gate charge (Qg) reduce dynamic or switching losses, enabling faster system switching speeds. Faster switching speeds allow for significant reduction in the size of passive components such as magnets, thereby increasing power density.
The maximum operating temperature of a switch also affects power density, because if a device can operate at a higher temperature, less cooling is required, which in turn reduces the size and weight of the design.
Modular solutions increase power density
In many main drive inverter designs, key components are often packaged separately. While this is a very efficient approach, it does not necessarily provide the most compact or highest power density design.
Another approach is to use pre-configured modules to construct the half-bridge required for the main drive inverter. On Semiconductor's VE-Trac Power Integrated Module (PIM) is one such solution, specifically designed for automotive functional electronics applications, including inverters.
The VE-Trac Dual power module integrates a pair of 1200 V ultra-field-stop (UFS) IGBTs in a half-bridge architecture. These devices utilize proven and reliable trench UFS IGBT technology, providing high current density, reliable short-circuit protection, and the higher blocking voltage required for 800 V battery applications. The intelligent IGBT integrates current and temperature sensors, offering unique advantages and faster response times for overcurrent (OCP) and overtemperature protection functions, resulting in a more stable and reliable solution.
These chips are packaged and mounted on an Al2O3 copper-clad substrate (DBC substrate) with 4.2 kV (basic) insulation capability, featuring copper on both sides for cooling. The unwired module has twice the expected lifespan of a similarly packaged module with wire bonding. Co-packaging this IGBT with a diode reduces power loss and enables soft switching, thereby improving overall energy efficiency.
The VE-Trac Dual module packages the bare chip in a small size, making it easier to integrate into compact designs. High efficiency, low losses, and dual-sided water cooling ensure easy thermal management, while continuous operation at 175°C allows for higher peak power delivery to the traction motor.
Each phase of the main drive inverter typically requires a VE-Trac Dual module, whose mechanical design is inherently suitable for multiphase applications, providing simple scalability, including paralleling the modules to provide more power in each single phase.
While the IGBT-based VE-Trac module is sufficient for most automotive applications, an enhanced version based on SiC MOSFETs can be used for the most demanding applications. This product utilizes the latest wide bandgap (WBG) technology, further reducing the size of the main drive inverter design and improving energy efficiency.
Summarize
Enabling electric vehicles to travel further between charges is one of our major technological challenges. Driven by government mandates and environmental aspirations, these vehicles will see rapid adoption in the coming years.
Electric vehicles will be more attractive and adopted more quickly if consumers' "range anxiety" is alleviated. The best way to achieve this is to improve energy efficiency, which not only extends driving range but also increases power density and improves reliability.
Semiconductor switches are key to achieving high energy efficiency. While discrete devices offer excellent performance, the best solution is PIMs (Power Injection Moldings) designed specifically for automotive applications, such as ON Semiconductor's VE-Trac module. These IGBT-based designs provide the required high energy efficiency, high performance, and scalability in a compact form factor, simplifying thermal design.
