Development overview and technical direction of SiC motor controller (inverter)

Tips: The information is collected from the Internet and is for learning purposes only.

1 Overview

From 2022 to 2023, the third generation semiconductor silicon carbide will be pushed into a new craze. We all know that as long as it is the semiconductor industry, it will definitely attract world attention. Today we take the automotive industry as an example to understand the advantages and technical routes of the silicon carbide platform.

2. Electric vehicle power system design trends

high pressure, high speed

The motor controller is responsible for converting battery energy into the power needed to control torque and speed, and therefore is the largest factor affecting the range, performance and driving experience of electric vehicles. Torque is proportional to motor size, while power provides both torque and speed. While keeping the power constant, if you want to reduce the motor size and torque, you need to increase the speed. This is a challenge because component size typically increases as power levels and torque increase, especially when there are design inefficiencies such as losses due to mechanical or electrical non-idealities. Therefore, it becomes important to downsize not only the motor, but also the electrical system of the traction inverter itself.

To extend range, reduce motor size and weight without reducing power levels, traction motors need to be able to rotate at higher speeds (>30,000rpm). This requires fast sensing and processing capabilities, as well as efficient DC to AC voltage conversion. To achieve these goals, traction inverter design trends include using advanced control algorithms, adopting SiC MOSFETs as switching transistors in the power stage, using 800V high-voltage batteries, and integrating multiple subsystems to achieve high power density.

Current rapid detection

The control loop is the path through which the sensed current flows from each phase of the traction inverter back to the isolated precision amplifier and through the microcontroller (MCU) for processing. This path ultimately returns the signal to the control output of the traction inverter. Fast and accurate feedback is achieved by optimizing the motor control loop so the motor can respond quickly to changes in speed or torque. The highlighted portion in Figure 1 shows the motor control loop.

The supply and control currents in traction inverters are usually isolated by isolating semiconductor components. Three isolated amplifiers or modulators measure the motor current through shunt resistors and then feed the signals into the MCU's field-oriented control (FOC) algorithm. Increasing motor speed requires a higher bandwidth current sensing feedback loop, which means that in-phase current must generate a modified inverter output as quickly as possible. Delay in the current sensing feedback loop is a primary consideration, especially as power transistor switching frequencies (insulated gate bipolar transistors [IGBT]/SiC MOSFETs (Figure)) increase to tens of kilohertz and control signals must Vary the pulse width on a cycle-by-cycle basis to achieve higher rotational speeds. Noise generated by large currents can also affect loop reliability.

In order to improve the feedback speed and system reliability, Ti has proposed a series of solutions for reference: "Traction Inverter – The driving force of automobile electrification"

3. Gate driver and drive power supply configuration

Control signals generated by the MCU and current sensing loop feed into the power stage, which is the link between the battery and the motor. The power stage consists of a high-voltage DC bus that is decoupled by a large capacitor bank connected to the three phases of power transistors such as IGBTs or SiC MOSFETs. The power stage should have minimal power losses in converting DC voltage to AC and should be small in size to use the battery efficiently, thereby extending the car's range. However, this is a challenge because the higher the voltage and power, the
larger the component size naturally becomes. Fortunately, with continuous breakthroughs in related technologies, it is possible to provide higher power levels with the same component size.

Two factors influence the size of a traction inverter: the type of high-voltage transistor, and the voltage level of the battery. SiC MOSFETs have lower switching losses and smaller die size than IGBTs with the same voltage rating, so some engineers use SiC MOSFETs in traction inverter designs. When SiC transistors are properly controlled, they have lower losses and higher reliability under all operating conditions of the inverter (such as temperature, speed and torque), thus enabling longer driving ranges.

Although SiC MOSFETs are more efficient, like any other transistor, they incur some power losses when switching, and these power losses affect the efficiency of the traction inverter. During switching transients, voltage and current edges overlap and cause power loss, as shown in the figure. The high gate driver output current enables fast charging and discharging of the SiC FET gate, resulting in lower power losses. However, switching behavior changes over temperature, current, and voltage, so switching as fast as possible is not
ideal. Rapid voltage transitions on SiC FETs (voltage transients (dv/dt) called drain-to-source voltage (VDS)) can produce voltage overshoots and electromagnetic interference (EMI) in the form of conducted ground currents. The motor itself is subject to high dv/dt due to possible short circuits in the capacitance between the windings. Gate driver circuits control power losses and switching transients.

Controlling the output source and sink current of the gate driver by using gate resistors helps optimize the trade-off between dv/dt and power loss. Figure shows a gate driver implementation with adjustable output drive strength to optimize for SiC MOSFET slew rate changes over temperature and current ranges.

Adjustability benefits traction inverter performance because it enables lower EMI and lower losses, which in turn increases efficiency to help extend driving range. TI's UCC5870-Q1 and UCC5871-Q1 gate drivers feature 30A drive strength, making it easy to implement adjustable gate drive solutions based on changing and optimizing the gate resistance. In addition, they feature galvanic isolation and 100kV/µs CMTI, so they can be easily used in high-voltage applications using fast-switching SiC technology.

The voltage level of the battery also affects the amount of dv/dt present in the system, which is also important when the designer needs to minimize EMI and the components selected need to meet various isolation safety standards while maintaining the same power density and area. Will bring challenges. SiC MOSFETs support high breakdown voltages in excess of 1,200V with a small die size, which enables high power density solutions for 800V electric vehicle battery applications.

Supporting the gate voltage requirements of high-voltage SiC MOSFETs becomes very challenging when the power supply needs to have isolation capabilities and good regulation capabilities. The influence of gate voltage can be clearly seen from the current-voltage characteristic curve of SiC MOSFET, as shown in the figure, where a higher gate-source voltage (VGS) will lead to a larger slope of the curve in the linear region. A steeper slope means that the drain-to-source on-resistance (RDS(on)) should be reduced to minimize conduction losses and avoid thermal runaway.

The isolated bias supply that provides power and voltage to the gate driver should maintain an appropriate positive gate voltage during fast transients and be able to support negative voltages to keep the SiC FET safely off. Isolated power is typically generated using a transformer with integrated semiconductor switching controller. However, the complex design of the transformer directly affects the performance of the power stage from an electrical efficiency and EMI perspective. Interwinding capacitance causes increased common-mode current, which in turn causes EMI, so the smaller the capacitor, the better, but there is a trade-off between size, voltage rating,
and efficiency, so more time is needed to design.

With integrated power modules such as UCC14241-Q1 and UCC1420-Q1, the primary-to-secondary isolation capacitance can be well controlled below 3.5pF, allowing the CMTI of fast-switching SiC MOSFETs to be greater than
150V/ns. HEV/EV subsystem designs are increasingly moving towards further integration, such as using traction inverters in conjunction with DC/DC converters. The UCC14241-Q1 reduces the bill of materials (BOM) area by approximately 40% compared to typical bias supply solutions using flyback converters, as shown in the figure. Its height is much lower than a discrete transformer design, resulting in a lower center of gravity and greater vibration tolerance. All these factors contribute to the reliability and longevity of the traction inverter system while being able to provide the correct voltage to drive the power transistors efficiently.

4 Conclusion

Electric vehicles are driving technological innovation across the board from processing to power semiconductors. Motor control and powertrain design directly affect the range and driving performance of electric vehicles. High-precision current sensors combined with smart MCUs with real-time control help reduce latency and improve the accuracy of the motor control loop, resulting in smooth speed and torque transitions. Electrical efficiency and range are improved due to reduced harmonic distortion; motor vibration is also improved, helping to prevent an uncomfortable driving experience.

Excellent traction inverter power density and efficiency achieved through the use of SiC MOSFET and 800V technology enable the integration of various powertrain functions, ultimately resulting in longer driving range per charge. TI's broad portfolio of integrated semiconductor technologies gives automakers and Tier 1 suppliers the flexibility to achieve high performance and low cost.

It can be seen from Xiaomi Auto’s recent press conference that 800V electric drive has become a mainstream product, and the silicon carbide market will become more and more mature.