Overview

Standard motor drive circuitry has traditionally been set aside in a separate enclosure from the motor, which adds weight and increases the overall size of the motor drive architecture. In addition, interface cables from the drive circuitry to the motor adds inductance to the circuit, possibly resulting in transient voltage overshoot to the motor from the high frequency drive. This can also lead to an inefficient power density design.

In today’s power management world, Wide Bandgap power transistors, like Silicon Carbide (SiC) and Gallium Nitride (GaN), enable designers to shrink the power electronics by increasing the switching frequency. And because SiC and GaN are far more efficient and dissipate less heat, smaller heat sinks are needed. This reduced size allows the drive components to be integrated into/onto the motor itself resulting in a lighter more compact system.

This architecture improvement is especially important in electric vehicle traction motors in the drivetrain, where weight and efficiency are most desirable. (Figure 1).

Figure 1

Figure 1: An electric vehicle (EV) drivetrain, using SiC MOSFETs, can be created in an Integrated Modular Motor Drive (IMMD) providing higher efficiency, faster switching speeds, and cooler operation, while meeting the stringent quality requirements of the automotive industry (Image courtesy of Wolfspeed)

Advantages of WGP power elements

The key advantages of WBG devices are due to their lower overall losses, fast switching capabilities, and high-temperature operation capabilities. SiC and GaN will be the leading WBG devices for power architectures. Considering the voltage ranges of WBG devices, GaN power transistors have voltage ratings up to 650V, while SiC starts at 650V and extends up to 10kV in modules.

Motor drive designs in aerospace, vehicle traction systems and others will benefit from WBG and IMMD. In addition to the size and weight reduction, the more expensive cost of the WBG devices can be offset by eliminating the separate cabinet enclosure and associated connectors and cables. Plus, no connecting cables lead to lower leakage current in the motor winding insulation that will increase the life of the motor and improve any Electro Magnetic Interference (EMI). Finally, installation, manufacturing, and maintenance costs will be lowered.

Considering the voltage ranges of WBG devices, GaN power transistors have voltage ratings up to 650V, while SiC starts at 650V and extends up to 10kV in modules.

SiC/Si hybrids

In addition to full SiC modules containing MOSFETs and Schottky Barrier Diodes (SBD), other perhaps less expensive SiC/Si hybrid options exist. Hybrid devices are typically Si IGBT’s or MOSFETs coupled with SiC SBD’s to improve Qrr for superior switching loss performance. Figure 2 shows the diagram of such a device and Figure 3 shows typical turn-on switching performance comparison of a Si IGBT and Si/SiC hybrid solutions.

Figure 2

Figure 2: A circuit diagram of a SiC-Schottky Barrier Diode (SiC-SBD) high-power, low-loss, reliable power module for Traction Inverters (Image courtesy of Mitsubishi)

Figure 3: High speed switching for SiC vs. Si (Image courtesy of Mitsubishi)

Reverse-recovery

The diode reverse-recovery current and the IGBT switching losses can be drastically reduced by replacing the silicon freewheeling PiN diode with a SiC Schottky barrier diode (SBD).

SiC Schottky Barrier diodes feature zero reverse-recovery charge (Qrr) for ultra-fast switching operations. The ultralow Qrr in SiC SBDs results in reduced switching losses in a typical hard-switched IGBT based application. This lowers the case temperature of the IGBT, improving the system efficiency and possibly allowing for a reduction in size of the silicon IGBT. (Figure 4)

Figure 4

Figure 4: Reverse-recovery charge (Qrr) of 650V (figure on the left) and 1200V (figure on the right) SiC Schottky diodes and silicon bipolar diodes at various temperatures. The shaded areas represent the wasted energy due to the recombination of the minority carriers of the silicon bipolar diode
(Image courtesy of CREE)

Reverse Recovery Time (Trr) is the total time which starts from the instant at which the reverse current starts to flow through the diode to the time instant at which it reaches to zero. This can be measured in Figure 4.

Applications for WBG devices in motor drives

WBG devices offer significant benefits for many applications including motor drives. Figure 5 diagrams low inductance, high speed and high temp motor applications, important requirements, and limitations of Si solutions.

Figure 5: Various WBG applications in motor drives (Image from Reference 1, below)

So we can see that WBG devices efficiently enable high-power, low-inductance motors which require a high switching frequency and high-bandwidth. And let’s not forget the Electric Vehicle which uses sophisticated power electronics to manage the energy flow between wheel demand, the combustion engine, and storage devices. Battery chargers are also an excellent application for SiC devices to perform their magic. More applications will appear from the needs of industry by the fertile minds of design engineers.

References
  1. Wide Bandgap Devices in AC Electric Drives: Opportunities and Challenges, Ajay Kumar Morya, Member IEEE, Matthew C. Gardner, Student Member IEEE, Bahareh Anvari, Member IEEE, Liming Liu, Senior Member IEEE, Alejandro G. Yepes, Member IEEE, Jesús Doval-Gandoy, Member IEEE, and Hamid A. Toliyat, Fellow IEEE, IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION, VOL. 5, NO. 1, MARCH 2019