How to Incorporate SiC MOSFETs into Designs to Improve the Efficiency of Electric Vehicle Traction Inverters

“Engineers are faced with the trade-offs between the performance and range of modern electric vehicles (EVs). Faster acceleration and higher cruising speeds require more frequent and time-consuming charging stations. In addition, longer mileage requirements create uncertainty about progress. To increase range while providing drivers with higher performance, engineers need to design the drive system to ensure that as much battery power as possible is transferred to the drive wheels. Equally important is the need to keep the drive system small enough to fit within the vehicle’s constraints. These dual demands require components with high efficiency and high energy density.

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Author: Steven Keeping

Engineers are faced with the trade-offs between the performance and range of modern electric vehicles (EVs). Faster acceleration and higher cruising speeds require more frequent and time-consuming charging stations. In addition, longer mileage requirements create uncertainty about progress. To increase range while providing drivers with higher performance, engineers need to design the drive system to ensure that as much battery power as possible is transferred to the drive wheels. Equally important is the need to keep the drive system small enough to fit within the vehicle’s constraints. These dual demands require components with high efficiency and high energy density.

A key component of an electric vehicle drive system is the three-phase voltage source inverter (or “traction inverter”), which converts the DC voltage from the battery to the AC power required by the vehicle’s electric motor. Building an efficient traction inverter is critical to balancing performance and range, and one of the key ways to improve efficiency is the proper use of wide bandgap (WBG), silicon carbide (SiC) semiconductor devices.

This article first introduces the role of electric vehicle traction inverters. It then explains how, when designing the device with SiC power metal-oxide-semiconductor field-effect transistors (MOSFETs), it is possible to create a more efficient electric vehicle drive system than using insulated gate bipolar transistors (IGBTs). The article concludes with an example of a SiC MOSFET-based traction inverter and illustrates design techniques to maximize the efficiency of the device.

What is a traction inverter?

Electric vehicle traction inverters convert the DC power provided by the vehicle’s high-voltage (HV) battery to the AC power needed by the electric motor to generate the torque needed to move the vehicle. The electrical performance of the traction inverter has a large impact on the acceleration and range of the vehicle.

The high voltage battery drive system voltage of modern traction inverters is 400 volts, or to the nearest 800 volts. Equipment powered by an 800-volt battery system can provide more than 200 kilowatts (KW) of power with traction inverter currents of 300 amps (A) or more. As power has climbed, inverters have also been shrinking in size, greatly increasing power density.

Electric vehicles with 400-volt battery systems require traction inverters with power semiconductors rated at 600 to 750 volts, while 800-volt vehicles require semiconductors rated at 900 to 1200 volts. The power components used in traction inverters must also be able to handle peak AC currents in excess of 500A for 30 seconds (s) and maximum AC currents of 1600A for 1 millisecond (ms). In addition, the switching transistors and gate drivers used in the device must also be able to handle these large loads while maintaining high traction inverter efficiency (Table 1).

Table 1: Typical traction inverter requirements in 2021; compared to 2009, the energy density requirements shown in the table have increased by 250%. (Image credit: Steven Keeping)

Traction inverters typically consist of three half-bridge elements (high-side plus low-side switches), one for each motor phase, with gate drivers controlling the low-side switches for each transistor. The entire assembly must be galvanically isolated from the low-voltage (LV) circuits that power the rest of the vehicle’s systems (Figure 1).

Figure 1: Electric vehicles require a three-phase voltage source inverter (traction inverter) to convert high-voltage (HV) DC battery power into the AC power required by the vehicle’s electric motor. The high-voltage system, including the traction inverter, is isolated from the vehicle’s traditional 12-volt system. (Image source: ON semiconductor)

The switches in the example shown in Figure 1 are IGBTs. This type of inverter has been a popular choice for traction inverters because of their ability to handle high voltages, switch quickly, provide good efficiency, and be relatively inexpensive. However, as the cost of SiC power MOSFETs declines and they become commercially available, engineers are turning to these components because of their distinct advantages over IGBTs.

Advantages of SiC MOSFETs for High Efficiency Gate Drivers

The key performance advantage of SiC power MOSFETs compared to traditional silicon (Si) MOSFETs and IGBTs comes from the device’s WBG semiconductor substrate. The bandgap energy of the silicon MOSFET is 1.12 electron volts (eV), while the SiC MOSFET is 3.26 eV. This means that WBG transistors can withstand much higher breakdown voltages than silicon devices, and the resulting breakdown field voltages are about ten times higher than silicon devices. The high breakdown field voltage allows reducing the thickness of the device at a given voltage, lowering the “on” resistance (RDS(ON)), thereby reducing switching losses and improving current-carrying capability.

Another key advantage of SiC is its thermal conductivity, which is about three times higher than Si. Higher thermal conductivity results in a smaller rise in junction temperature (Tj) for a given power dissipation. SiC MOSFETs can also tolerate higher maximum junction temperatures (Tj(max) ) than Si. Typical Tj(max) values ​​for silicon MOSFETs are 150˚C; SiC devices can withstand Tj(max) up to 600˚C, although commercial devices are typically rated at 175 to 200˚C. Table 2 provides a performance comparison between Si and 4H-SiC, the crystalline form of SiC commonly used to make MOSFETs.

Table 2: Breakdown electric field, thermal conductivity, and maximum junction temperature of SiC MOSFETs make them a better choice than Si for high current and high voltage switching applications. (Image credit: ON Semiconductor)

High breakdown voltage, low RDS(ON), high thermal conductivity, and high Tj(max) enable SiC MOSFETs to handle much higher currents and voltages than similarly sized Si MOSFETs.

IGBTs are also capable of handling high voltages and currents and tend to be less expensive than SiC MOSFETs – a key reason why they are favored in traction inverter designs. IGBTs also have drawbacks, especially when developers want to maximize energy density, with limitations on the maximum operating frequency due to their “tail current” and relatively slow turn-off speed. In contrast, SiC MOSFETs can handle the same high-frequency switching as Si MOSFETs, but with the voltage and current handling capabilities of IGBTs.

SiC MOSFETs are increasingly available

Until recently, their use was limited to traction inverters in luxury electric vehicles due to their relatively high price, but falling prices have made SiC MOSFETs a more diverse choice.

On Semiconductor: The NTBG020N090SC1 and NTBG020N120SC1 are two examples of this new generation of SiC power MOSFETS. The main difference between the two devices is that the former has a maximum drain-source breakdown voltage (V(BR)DSS) of 900V, a gate-source voltage (VGS) of 0V, and a continuous drain current (ID) of 1mA (mA), while the latter has a maximum V (BR)DSS of 1200 volts (under the same conditions). The maximum Tj for these two devices is 175˚C. Both devices are single N-channel MOSFETs in the D2PAK-7L package (Figure 2).

Figure 2: The NTBG020N090SC1 and NTBG020N120SC1 N-channel SiC power MOSFETs are both packaged in the D2PAK-7L, with the main difference being their V(BR)DSS values ​​of 900 and 1200 volts, respectively. (Image credit: Steven Keeping, using materials from On Semiconductor)

NTBG020N090SC1 has RDS(ON) of 20 milliohms (mΩ), VGS of 15 volts (ID = 60 A, Tj = 25˚C), RDS(ON) of 16mΩ, VGS of 18 volts (ID = 60 A, Tj = 25˚C). The maximum continuous drain-source diode forward current (ISD) is 148A (VGS = -5V, Tj = 25˚C) and the maximum pulsed drain-source diode forward current (ISDM) is 448A (VGS = – 5V, Tj = 25˚C). The NTBG020N120SC1 has an RDS(ON) of 28 mΩ at VGS of 20V (ID = 60A, Tj = 25˚C). The maximum ISD is 46A (VGS = −5V) , Tj = 25˚C), the maximum ISDM is 392 A (VGS = −5 volts, Tj = 25˚C).

Designing with SiC MOSFETs

Despite the advantages of SiC MOSFETs, designers wishing to incorporate SiC MOSFETs into their traction inverter designs should be aware of an important complication, namely the tricky gate drive requirements of such transistors. Some of these challenges arise from the fact that, compared to Si MOSFETs, SiC MOSFETs exhibit lower transconductance, higher internal gate resistance, and gate turn-on thresholds may be lower than 2 volts. Therefore, in the off state, the gate must be pulled below ground (usually -5 volts) to ensure proper switching.

However, the key gate drive challenge comes from having to apply a large VGS (up to 20 volts) to ensure low RDS(ON). Operating a SiC MOSFET at VGS that is too low can cause thermal stress or even failure due to power dissipation (Figure 3).

Figure 3: For the NTBG020N090SC1 SiC MOSFET, high VGS is required to avoid thermal stress due to high RDS(ON). (Image credit: ON Semiconductor)

Furthermore, since the SiC MOSFET is a low-gain device, the designer must consider the effect on several other important dynamic characteristics when designing the gate drive circuit. These characteristics include the gate charge Miller plateau and requirements for overcurrent protection.

These complex designs require dedicated gate drivers with the following properties:

・Able to provide VGS drive from -5 to 20 volts to take full advantage of the performance benefits of SiC MOSFETs. To provide enough overhead to meet this requirement, the gate drive circuit should be able to withstand VDD = 25 volts and VEE = -10 volts.
• VGS must have fast rising and falling edges, on the order of nanoseconds (ns).
• The gate drive must be capable of delivering high peak gate currents of several amps across the entire MOSFET Miller plateau area.
・ The sink current rating should exceed the current required to discharge only the input capacitance of the SiC MOSFET. For high performance half-bridge power supply topologies, the 10 A class minimum peak sink current rating should be considered.
・ Low parasitic inductance for high-speed switching.
・ Small driver package can be placed as close as possible to the SiC MOSFET to improve energy density.
• Desaturation (DESAT) function enables detection, fault reporting and protection for long-term reliable operation.
• A VDD undervoltage lockout (UVLO) level to match the requirement of VGS > 16 volts before switching begins.
• Provides VEE UVLO monitoring capability to ensure that the negative voltage rail is within acceptable limits.

On Semiconductor has introduced a gate driver designed to meet the above requirements for traction inverter designs. The high level of integration of the NCP51705MNTXG SiC MOSFET gate driver makes it compatible not only with its SiC MOSFETs, but also with products from many manufacturers. The device includes basic functionality common to many general-purpose gate drivers, but also has specialized requirements necessary to design a reliable SiC MOSFET gate drive circuit with minimal external components.

For example, the NCP51705MNTXG integrates a DESAT function that can be implemented using only two external components. DESAT is a form of overcurrent protection for IGBTs and MOSFETs to monitor faults, whereby VDS can rise to maximum ID. This can affect efficiency and, in the worst case, can damage the MOSFET. Figure 4 shows how the NCP51750MNTXG monitors the VDS of the MOSFET (Q1) through the DESAT pins of R1 and D1.

Figure 4: The DESAT function of the NCP51705MNTXG can measure the abnormal behavior of the VDS during maximum ID and implement overcurrent protection. (Image credit: ON Semiconductor)

The NCP51705MNTXG gate driver also features programmable undervoltage lockout. This is an important function when driving SiC MOSFETs because the output of the switching element should be disabled until VDD is above a known threshold. Allowing the driver to switch the MOSFET at low VDD can damage the device. The programmable UVLO of the NCP51705MNTXG not only protects the load, but also verifies to the controller that the applied VDD is above the turn-on threshold. The UVLO turn-on threshold is set by a resistor between UVSET and SGND (Figure 5).

Figure 5: The UVLO turn-on threshold of the NCP51705MNTXG SiC MOSFET is set by the UVSET resistor RUVSET, which is selected according to the desired UVLO turn-on voltage VON. (Image credit: ON Semiconductor)

Digital Isolation for Traction Inverters

To complete the design of a traction inverter, engineers must ensure that the low-side electronics of the vehicle are isolated from the high voltage and current passing through the inverter (Figure 2 above). However, since the microprocessor controlling the high voltage gate driver is on the low side, any isolation must allow digital signals to pass from the microprocessor to the gate driver. On Semiconductor also offers a component that does this, the NCID9211R2, a high-speed, dual-channel, bidirectional ceramic digital isolator.

The NCID9211R2 is a galvanically isolated full-duplex digital isolator that allows digital signals to pass between systems without ground loops or hazardous voltages. The device features a maximum operating isolation capability of 2000 volts peak, a common mode rejection of 100 kilovolts per millisecond (kV/ms), and a data throughput of 50 megabits per second (Mbit/s).

Figure 6 shows the isolation barrier formed by off-chip ceramic capacitors.

Figure 6: Block diagram showing the single-channel structure of the NCID9211R2 digital isolator. Off-chip capacitors form the isolation barrier. (Image credit: ON Semiconductor)

Digital signals are transmitted across the isolation barrier using ON-OFF On-Off Keying (OOK) modulation. On the transmitter side, the VIN input logic state is modulated by a high frequency carrier signal. The resulting signal is amplified and transmitted to the isolation barrier. The receiver detects the isolation barrier signal and demodulates it using envelope detection techniques (Figure 7). When the output enable control EN is high, the output signal determines the output logic state of VO. When the transmitter is powered off, or the VIN input is disconnected, VO defaults to a high-impedance low state.

Figure 7: The NCID9211 digital isolator uses OOK modulation to transmit digital information across the isolation barrier. (Image credit: ON Semiconductor)

Epilogue

SiC power MOSFETs are a good choice for high-efficiency and high-power-density EV traction inverters, but their electrical characteristics also present unique design challenges in gate driver and device protection. In addition to the design challenges, engineers must ensure that their traction inverter designs provide a high level of isolation from the vehicle’s sensitive low-voltage electronics.

As mentioned above, to facilitate engineering development, On Semiconductor has introduced a range of SiC MOSFETs, dedicated gate drivers and digital isolators to meet the requirements of traction inverters, and in between the long range and high performance requirements of modern electric vehicles A better balance was achieved.