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FIGURE 2: 1.2-KV SiC POWER MOSFET MODULE (SOURCE: MICROCHIP TECHNOLOGY)
anti-parallel diode. With SiC power devices, power losses can be reduced and efficiency can be increased through two major considerations.
First, a SiC power device switches faster and has lower switching losses than [a comparable] IGBT. As a unipolar device, a SiC MOSFET doesn’t have turn-off tail current like IGBTs [do], and its turn-on and turn-off speeds are highly controlled by the gate driver strength. With high peak current from the gate driver, a SiC MOSFET’s switching loss can be further reduced. In addition, a SiC MOSFET has a linear RDS(on) I-V curve starting from 0 V. An IGBT I-V curve has a ~0.4-V knee voltage, which makes it have higher conduction loss for mid- to low current ranges [assuming the IGBT has a lower VF for the peak current range]. Therefore, a SiC MOSFET will have fewer losses for most of the load conditions for motor control applications.
Additionally, a SiC MOSFET can operate in third-quadrant mode with a linear I-V curve, whereas the anti-parallel diode co-packaged with the IGBT also has a similar, ~0.4-V knee conduction voltage, which will generate more conduction loss. With reduced switching and conduction loss, the demand for thermal management systems will be reduced to achieve smaller size or keep the system cooler for a longer time.
Kevin Speer, senior manager, SiC Solutions, Discrete and Power Management, Microchip Technology: Efficiency in motor drives is limited by two sources of loss: conduction losses, which dominate at low switching frequencies, and switching losses, which dominate at higher switching fre- quencies. Compared with silicon IGBTs, SiC has 80% lower switching losses and similarly reduced conduction losses at light/medium load. The answer to how SiC can increase efficiency in motor drives therefore depends on the motor’s speed and usage.
One might imagine that motors with lower fundamen- tal frequencies have little need for high switching speeds,
and therefore, these drives work reasonably well with sili- con solutions. Where SiC can help is for motors with higher fundamentals — up to 2 kHz — that will necessitate higher pulse-width–modulation [PWM] frequencies and thus ben- efit from the slashed switching losses of SiC. At the same time, any motor that runs most often at light load, like ser- vos, or those whose power sources are a battery, such as an electrified vehicle, stands to benefit from the decimated conduction losses offered by SiC under light- and medium- load conditions, making more use out of the same battery, as one option, or by reducing the size and weight of the battery required by silicon IGBTs.
SiC MOSFETs are known to outperform IGBTs in high-frequency power applications, such as switch-mode power supplies. What are the main benefits of SiC-based motor control, another key application for SiC MOSFETs?
Speer: For the use cases we’ve talked about previously — namely, high-speed motors and motors that operate most of their service lifetime in light load — SiC can create major monetizable benefits to go with the performance enhance- ments. For high-speed motors, there is the possibility to increase PWM frequency and dramatically shrink the size [of] the motor and drive system. And then for any speed-rated motor that operates primarily under light- or medium-load conditions, the reduced losses could mean smaller, lower- cost thermal management. Or in the case of battery-powered loads, the size, weight, and expense of the batteries can either be reduced or used to extend travel range.
Zhang: The main benefits of SiC-based motor control include, first and foremost, higher efficiency for less con- duction loss and switching loss. This loss savings is higher at a light or medium load when the IGBT has a knee effect and higher switching loss, thus extending drive range [miles per kilowatt-hour] for battery electric vehicles. Battery- powered EVs are mostly designed to handle very high peak power but work only in light or medium loads for the major- ity of use cases. Studies show that the drive range for EVs can be extended by 3% to 6%, which translates to huge bat- tery savings.
Moreover, SiC supports higher motor drive switching fre- quency, in the range of 30 kHz to 100 kHz, compared with a <10-kHz IGBT-based inverter, and high switching frequency is critical for motor drives that need high rotation speed [such as for a high-speed fan motor]. In this scenario, SiC can sig- nificantly save the switching loss and help reduce EMI filter size and ripple current for the motor.
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Technology Analysis Motor Control Design with SiC Power Devices