EV inverters, also known as traction inverters, convert the dc electricity from the battery into the three-phase variable frequency ac that’s needed to drive the motor at a given speed (Figure 1). EVs use asynchronous (induction) motors or sometimes synchronous motors. Both are types of ac motors, with the speed of the motor determined by the ac frequency.
The inverter can, therefore, control the speed and torque of the motor independently. A traction inverter is a type of motor drive or motor controller, also called a variable frequency drive (VFD).
The development of inverters is driven by the need to improve vehicle efficiency and performance, reduce cost, and create more compact drivetrains. Improved efficiency extends vehicle range while lowering running costs.
Figure 1. Inverters play a critical role in electric vehicles, converting the battery’s dc into ac power, which is used to operate the electric motor. (Image: MES, Inc.)
Traction inverter fundamentals
The basic principle of all rotary electric motors is that there are fixed magnets called stators and rotating magnets called rotors. When the stators and rotators have their opposite poles aligned, they repel each other, causing rotation. However, if all magnets maintain the same polarity, the motor could only rotate through a maximum of 180°.
To reverse the polarity of either the rotor or the stator, all electric motors in practical use are fundamentally ac motors. What’s referred to as dc motors use mechanical switching within the motor to alternate the current flowing through the magnets. So, in a sense, the brush and commutator arrangement within a dc motor can be considered an extremely simple inverter that produces a square wave input.
The highly efficient motors used in EVs and most large industrial machinery are three-phase ac motors. These require an input of three separate sinusoidal currents, with a 120° phase shift between them. The inverters that create this supply are more sophisticated than a simple brush/commutator arrangement that produces a single square wave ac in a dc motor. However, they still work by using switches to turn the current on and off and to reverse its direction. Switching on and off quickly enough allows power to be regulated without any noticeable vibration in the motor.
Semiconductor switches in inverters switch on and off thousands of times per second to produce smooth sinusoidal ac waveforms. This approach of varying the duration of off-and-on periods is known as pulse width modulation (PWM). Switching is an energy-efficient way of regulating power without the inherent resistive losses of linear regulators. Though as the switch is transitioning between the off and on states, there’s a brief period where resistance does occur, leading to some power loss and some heating.
Figure 2. Aside from converter dc to ac power for the motor, traction inverters also play important roles in boosting voltage, protecting switches, and enabling regenerative braking. (Image: Allied Market Research)
Faster switching is better in terms of efficiency and from a practical perspective — with resistive heating requiring thicker conductors and thermal management. Inverters with faster switches are typically compact, lighter, and more efficient.
Practical ac motor drives that use PWM require two things:
- Semiconductor switches that can turn the circuit off and on at the required frequency (orders of magnitude higher than the ac frequency
- Microprocessors that can calculate the switching pattern required to produce a sinusoidal waveform at the required frequency
EV inverter switches must handle currents of hundreds of amps per phase and voltages of up to 800 Vdc without overheating. Since semiconductor-based inverters were established in the ’80s, MOSFETs have been used below about 150 V and IGBTs for higher voltages.
Wide band-gap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) have recently allowed MOSFETs to be used in high-voltage applications. MOSFETs switch more quickly, so SiC-based inverters offer efficiency, mass, and size advantages. All switches used are three-terminal devices, with two terminals forming the power connections on either side of the switch, and the third acting as the control input. While the control input would be called the gate in a transistor, it is known as the gate in a MOSFET or IGBT.
Inverter switches generate heat through the switching loss mentioned above and resistive losses in conducting elements. Cooling these increasingly compact and high-powered systems is challenging because materials that are good conductors of heat also tend to be good conductors of electricity, and electrical isolation is required.
As a result, ceramics are widely used since they’re good electrical insulators with reasonably high thermal conductivity. Boards of copper sheet bonded to a ceramic substrate can be etched to form printed circuit boards with good thermal dissipation characteristics. But the rapid thermal cycling caused by the switching losses is a change in terms of fatigue.
Power density
A significant trend is increasing power density, with inverters achieving the same power output with reduced mass and volume. EVs are moving from inverters with silicon-based IGBTs to those based on silicon carbide (SiC). SiC can handle higher voltages, temperatures, and frequencies, leading to smaller and lighter inverters. There has been a progression from MOSFETs to Insulated Gate Bipolar Transistors (IGBTs), which are now widely used in production EVs and are starting to be replaced by SiC.
The ability of SiC inverters to operate at higher frequencies enables them to control higher speed motors, offering potential cost savings. Motors running at higher speeds can be smaller and lighter, leading to a reduction in overall system size and cost.
Continued development of SiC inverters is expected to reduce system weight and size, increase efficiency, and reduce cost. Larger SiC fabrications with higher throughput will achieve lower costs, while increasing wafer sizes to 200 mm and beyond will also reduce costs. While these changes are expected within the decade, SiC development is challenging, with new products taking several years to reach production.
Increasing the integration of the inverter with other electric drive unit components, such as the gearbox and motor, will increase power density further and reduce material use and manufacturing costs.
Efficiency
All of the power that reaches the motor must flow through the inverter, meaning that the inverter’s efficiency has a direct impact on the vehicle’s range and efficiency. Greater efficiency clearly means a vehicle will travel further with the same battery. It also means that a smaller battery may be feasible, with shorter charging times, lower cost, and reduced weight. Reduced weight further increases range and vehicle performance.
EVs are moving from inverters with silicon-based semiconductors to those based on silicon carbide (SiC). This is primarily due to an increase in efficiency from 97 to 98% for silicon to 99% for SiC. With efficiency already so high, there must be more scope for further improvement. For example, there might be potential to increase the overall vehicle efficiency by more deeply integrating the inverter with the motor to control individual motor coils precisely. This could improve the motor’s efficiency, torque, and power output.
For instance, technology company Exro has developed an inverter that switches the motor between different coil configutations. (Figure 3). Their CTO, Eric Hustedt, explained this: “Fundamentally it’s a drive that that switches the machine between two different coil configurations… for example, at low speed where you want a lot of torque, we would switch into series mode where we’re producing a lot of current density in the machine, which is generating a large amount of low end torque. Then, as the machine speed increases, we switch into parallel, which allows the machine to continue operating at high speed without sacrificing performance.”
Figure 3. Inverters developments are advancing and providing additional benefits. For example, this EV traction inverter offers coil switching technology that boosts torque at low speeds and improve power and efficiency at high speeds. Essentially, it enables two operating modes in a single motor. (Image: EXRO)
Higher voltages
There’s a drive toward higher dc-link voltage in EV, driven primarily by increased charging speed. For instance, 800 V is replacing 400 V as the standard. Charging anxiety is becoming more important for consumers than range anxiety. This is partly about being able to quickly and easily find an available charger when needed, and partly about the waiting time to complete charging on a long journey. Reducing charging times is essential for mass EV adoption.
Higher voltages mean multi-level structures could effectively replace two-level inverters. This may lead to higher efficiency, higher power density, better waveform quality, and inherent fault-tolerance. While GaN inverters over some advantages over SiC, GaN is not really suitable for voltages over about 650 V.
With EVs moving to 800 V to enable rapid charging, using a GaN inverter would require additional conversion, which would add cost and complexity.
Thermal management and material selection
The semiconductor switches used in EV inverters must be mounted so that heat can be dissipated, electrical isolation is maintained, and the thermal cycling caused by the high-frequency intermittent heating of switching losses does not cause a mechanical fatigue failure. This presents challenges for the laminated substrate and the attachment of the semiconductor to the substrate. A substrate cooling system is also required. The manufacturing cost is often considerably higher than that of mounting and cooling the semiconductor switches.
The solder is usually unsuitable for die attachment due to insufficient fatigue resistance. Sintered attachments are more durable using a paste of silver and copper power with an organic filler. These sintered attachments not only have far higher fatigue resistance but also around three times the thermal conductivity and a much higher melting temperature, allowing higher temperature operation and, therefore, even more efficient heat dissipation. Despite these advantages, sintering is a costly process that significantly increases the expense of an inverter.
A common substrate used in inverters is direct bond copper (DBC), which consists of a thin layer of copper bonded to a ceramic substrate using a high-temperature oxidation process. A common and low-cost ceramic is alumina (aluminium oxide). More expensive ceramics — such as aluminium nitride (AlN), silicon nitride (SiN), and zirconium-doped alumina — can improve heat dissipation and fatigue resistance.
Substrate cooling systems consist of a heat sink soldered, clamped, or glued to the underside of the substrate, with a fluid cooling system. Sintering is increasingly being used in high-performance inverters due to the same advantages that make it suited to the die attach. The heat sink or cold plate is often pre-attached to the substrate to ensure good thermal contact.
Conclusion
With inverters already achieving up to 99% efficiencies, there is only minor scope for improvement in this area. Inverters are also relatively light and compact — say, five liters and 10 kg for an EV weighing over 2,000 kg — meaning that further reductions will have a negligible effect on overall vehicle performance.
However, changes that facilitate improvements in other areas may have more impact. For example, SiC inverters enable higher dc voltages than Gan, meaning faster changing times and smaller conductors. Faster switching also enables higher-speed motors with significant cost and mass reduction potential.
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