Traditionally, we have the impression that motor drive systems tend to use IGBTs as switching devices, while SiC MOSFETs as high-speed devices are often associated with applications that require high-frequency conversion, such as photovoltaics and EV charging. But in specific motor applications, SiCs still have incomparable advantages, they are:
Low-inductance motors have many different applications, including large air-gap motors, slotless motors, and low-leakage induction motors. They can also be used in new motor types that use PCB stators instead of winding stators. These motors require a high switching frequency (50-100kHz) to maintain the required ripple current. However, the use of insulated-gate bipolar transistors (IGBTs) for modulation frequencies above 50kHz cannot meet these needs, and in the case of 380V systems, the silicon MOSFET withstand voltage is not enough, which opens up new opportunities for wide bandgap devices.
Due to their high fundamental frequency, these motors also require high switching frequencies. They are suitable for applications such as high-power-density electric vehicles, high-pole motors, high-speed motors with high torque density, and megawatt-class high-speed motors. Again, the highest switching frequencies that IGBTs can achieve are limited, and these limitations may be overcome by using wide bandgap switching devices. An example is air compressors in fuel cells. The maximum speed of the air compressor exceeds 150,000 rpm, the output frequency of the air compressor motor controller exceeds 2500Hz, and the power device requires a high switching frequency (more than 50kHz), so SiC-MOSFETs are the preferred device for this type of application.
There are two interesting benefits to using wide bandgap devices in motor control inverters. First, they generate less heat than silicon devices, reducing the need for heat dissipation. Second, they can withstand higher operating temperatures – SiC: 600°C, GaN: 300°C, while silicon chips can withstand a maximum operating temperature of only 200°C. While there are currently some packaging-related issues with SiC products that prevent them from being suitable for operating temperatures above 200°C, research focused on addressing these issues is ongoing. As a result, wide bandgap devices are more suitable for motor applications that may face harsh operating conditions, such as integrated motor drives in hybrid electric vehicles (HEVs), subsea and downhole applications, space applications, and more.
In conventional motor drives, IGBTs are often used as switching devices. So, what are the advantages of SiC MOSFETs over SI IGBTs that make them more suitable for motor drive applications?
Firstly, from the perspective of switching characteristics, the switching loss of power devices is divided into turn-on loss and turn-off loss.
Turn-off loss
IGBT is a bipolar device, and electrons and holes participate in conduction together, but due to the holes during turn-off, they can only gradually disappear through recombination, resulting in tailing current, which is the main reason for the large shutdown loss of IGBT. SiC MOSFETs are unipolar devices with only electrons participating in conduction, and there is no tailing current during turn-off, making the shutdown loss of SiC MOSFETs much lower than that of IGBTs.
Turn-on loss
The current at the moment of IGBT turn-on tends to be overshoot, which is the reverse recovery current generated when the reverse parallel diode is commutated. The reverse recovery current is superimposed on the IGBT turn-on current, increasing the turn-on loss of the device. The anti-parallel diode of IGBT is often a SI pin diode, and the reverse recovery current is more obvious. The structure of the SiC MOSFET naturally integrates a body diode, eliminating the need for additional parallel diodes. The SiC body diode is involved in the commutation, and its reverse recovery current is much lower than that of the IGBT reverse parallel silicon PIN diode, so the turn-on loss of the SiC MOSFET is lower than that of the IGBT even under the same DV DT conditions. In addition, the SiC MOSFET allows the servo drive to be integrated with the motor, eliminating the limitation of DV DT on the cable, and the switching loss of the SiC is further reduced at high DV DT conditions, which is much lower than that of IGBTs. Even when the switching process is slow, the switching losses of silicon carbide are better than those of IGBTs.
In addition, the switching losses of SiC MOSFETs are largely independent of temperature, while the switching losses of IGBTs increase significantly as the temperature rises. Therefore, the losses of SiC MOSFETs at high temperatures are more advantageous.
Considering the limitation of DV DT, the total switching loss of the SiC MOSFET will be reduced by 50% to 60% at high temperature under the same DV DT condition, and the total switching loss of the SiC switch will be reduced by up to 90% if the DV DT is not restricted.
From the point of view of conduction characteristics:
There is no knee point when the SIC MOSFET is turned on, and a very small VDS voltage can make the SIC MOSFET on, so the on-voltage of the SIC MOSFET is much smaller than that of the IGBT at low current. The IGBT conduction loss is lower at high currents, which is due to the fact that as the voltage drop of the device rises, the IGBT of the bipolar device begins to turn on, and due to the conductance modulation effect, the electron injection excites more holes, the current rises rapidly, and the slope of the output characteristics is steeper. Corresponding to the motor operating condition, the SiC MOSFET has lower conduction loss under light load conditions. Under heavy load or acceleration conditions, the advantage of the conduction loss of the SiC MOSFET is reduced.
CoolSiC MOSFETs reduce conduction losses under a wide range of operating conditions
The following is a case study to verify the advantages of SiC MOSFETs in motor drives.
Compare the three devices under the following operating conditions:
IGBT IKW40N120H3, SiC MOSFETs IMW120R060M1H and IMW120R030M1H.
vdc=600v, vn,out=400v, in,out=5a–25a,fn,sin-out=50hz, fsw=4-16khz, tamb=25°c,cos(φ)n=0.9, rth,ha=0.63k/w, dv/dt=5v/ns
M=1, VDC=600V, FSIN=50Hz, rg@dv DT=5V NS, FSW=8kHz, cable length 5m, TAMB=25°C
It can be seen that based on the above operating conditions, the output current of the 30Mohm device is increased by 10A compared with the 40A IGBT under the same temperature conditions, and the output current can be increased by about 5A even if it is replaced by a 60Mohm SiC MOSFET with a lower level. At the same current, the temperature of the SiC MOSFT decreases significantly.
In summary, the benefits that SiC switching devices can bring to motor drive systems are summarized below:
Lower Losses Reduce power consumption and make people's lives more environmentally friendly and sustainable.
Superior performance Achieve higher power density and achieve more economical motor designs by achieving the same performance in smaller components.
Compact structure Enables more compact, space-saving motor designs with reduced material consumption and lower heat dissipation requirements.
Higher quality SiC inverters have a longer service life and are less prone to failure, allowing manufacturers to offer longer warranties.
Finally, Infineon's CoolSiC ensures the short-circuit capability of the 3U single tube and the 2Us Easy module, further ensuring the safety and reliability of the system.
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