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  • Examining GaN and SiC Applications in EVs

    SiC

    Source: EE Times


    Nearly every component of vehicle design, including chassis, powertrain, infotainment, connectivity and driving-assistance systems (ADAS), is undergoing rapid development and innovation in the automotive sector.

    Designers are looking to advanced technology to take the next step in innovation by switching from silicon-based solutions to power-semiconductor technologies that use wide-bandgap (WBG) materials, such as silicon carbide (SiC) and gallium nitride (GaN), in their quest for more power dense and efficient circuits for electric vehicles (EVs).

    Besides a high-voltage battery (ranging from 400 V to 800 V) and related battery management systems (BMS), EVs include at least four types of electronic units for energy conversion:

    • Onboard charger (OBC),
    • DC-DC converter, typically from high voltage to 12 V, for powering low-voltage electronics,
    • DC-AC traction inverter for driving the electric motor, which is typically a three-phase AC motor, and
    • AC-DC converters for recharging the vehicle’s batteries, both during braking energy recovery and from standard residential or high-power charging stations.

    The power of SiC

    As a WBG semiconductor, SiC boosts the power density of an electronics system, while also lowering overall size, weight and cost. Due to its properties, SiC has been a technology accelerator for EVs.

    Because of its wider bandgap, stronger breakdown electric field, and higher thermal conductivity, SiC has become increasingly popular in power electronics as silicon approaches its theoretical limits. Silicon carbide-based MOSFETs are more efficient than silicon-based ones in terms of losses, switching frequencies and power density.

    When attempts were made to raise the efficiency and range of EVs, while also lowering their weight and price to increase the power density of control electronics, the notion of employing SiC for EVs emerged.

    Because SiC devices offer several desirable qualities in comparison to commonly used silicon, they are increasingly used in high-voltage power converters with strict size, weight and efficiency requirements. Since SiC has nearly 3x higher thermal conductivity than silicon, components can dissipate heat more quickly.

    On-state resistance and switching losses are also significantly reduced thanks to SiC devices. This is significant because SiC dissipates heat more effectively than traditional silicon, making it more challenging to extract heat from electrical conversion processes as silicon-based devices get smaller in size.

    Figure 1 shows a comparison of some relevant properties among silicon, SiC, GaAs and GaN. It should be noted that, at present, 4H-SiC is the polymorphic crystalline structure generally preferred in practical power device manufacturing. Single-crystal 4H-SiC wafers of different diameters are commercially available.

    Figure 1: Properties of SiC and other materials. (Source: ROHM)

    In the case of EVs, the most power can be saved in traction inverters, where SiC FETs can replace insulated-gate bipolar transistors (IGBTs) for a noticeable increase in efficiency. Because the motor is the magnetic component and its size does not directly decrease as the inverter switching frequency rises, switching frequency is kept low—typically at 8 kHz.

    The circuit of a typical traction inverter, shown in Figure 2, includes three half-bridge elements (high-side and low-side switches—one for each motor phase—with gate drivers controlling the low-side switching of each transistor. For a long time, this topology has been based on discrete or power-module IGBTs, plus free-wheeling diodes.

    Today, six paralleled low RDS(ON) SiC FETs with an efficiency above 99% at 200 kW output might replace the IGBTs and their parallel diodes, resulting in a 3x reduction in power losses. The improvement is even better at lighter loads, where vehicles are used more frequently, with 5x to 6x lower losses than IGBT technology, as well as the benefits of much lower gate drive power and no “knee” voltage for better control at light loads. Lower losses translate into smaller, lighter and less expensive heatsinks, as well as improved range.

    Due to SiC’s higher defect density and substrate (wafer) manufacturing method, it is still significantly more expensive than silicon. Infineon chip makers, however, have been able to lower the overall production costs by using numerous substrates and lowering the fault density.

    Figure 2: A 3-phase inverter-based drive. (Source: ST)

    The benefits of GaN

    Another WBG material that is nearly 3x greater than silicon is GaN. Wide-bandgap means that greater power is needed to excite a valence electron in the semiconductor’s conductive band. Gallium nitride cannot be used in ultra-low-voltage applications due to this feature, but it has the advantage of permitting higher breakdown voltages and greater thermal stability.

    Gallium nitride significantly boosts the efficiency of power-conversion stages, making it a desirable silicon substitute for the manufacture of Schottky diodes, power MOSFETs and high-efficiency voltage converters. The WBG material also offers significant advantages over silicon, including higher energy efficiency, smaller size, less weight and lower overall cost.

    While SiC can compete with IGBT transistors in high-power and extra-high-voltage (over 650 V) applications, GaN can compete with current MOSFETs and superjunction (SJ) MOSFETs in power applications with voltages up to 650 V. Gallium nitride FETs can switch at >100 V/ns and have zero reverse recovery. As a result, they experience very low switching power losses. For applications that require switching frequencies in the MHz range, GaN may be the best option.

    In EVs, GaN FETs are well-suited for:

    • AC-DC OBC,
    • High-voltage (HV) to low-voltage (LV) DC-DC converter, and
    • Low-voltage DC-DC converter.

    Gallium nitride has a lower gate and output charge than a comparable silicon device, which is one of its benefits. Gallium nitride-based designs may now achieve substantially faster turn-on and slew rates, while also minimizing losses. Thus, a GaN-based inverter decreases switching losses, as well as conduction losses in high-power applications. In EVs, this increased efficiency directly translates into a longer range or a range that is equivalent with a smaller battery.

    The best EV topologies that boost GaN efficiency include a phase-shift full-bridge and a hard-switching pulse-width-modulation converter for HV-to-LV DC-DC converters, as well as a totem-pole bridgeless power-factor correction (PFC), and CLLC and LLC resonant DC-DC converters for the OBC.

    The traction inverters in EVs demand power switches with several hundreds of amperes of current capability to support hundreds of kWs of power. As a matter of fact, these systems are a better fit for silicon IGBTs or SiC MOSFETs; on the other hand, OBCs and HV-to-LV DC-DC converters, which are rated up to 25 kW, are a great fit for GaN.

    Gallium nitride is a very fast, and therefore sensitive, device. As a result, it requires a package with very low inductance.

    Use case: the main inverter

    The traction inverter of an EV’s drivetrain transforms DC current from the battery to AC current to move the motor. The efficiency of the traction inverter must be increased to allow for:

    • Longer range, fewer charging cycles and extended battery life with the same battery cost.
    • Alternatively, the use of smaller and lower-cost batteries to achieve the same range.

    Efficiency, power density and cooling requirements all significantly depend on the semiconductors used in traction inverters for EVs. The three-phase AC motors found in modern EVs operate at switching frequencies of up to 20 kHz and voltages up to 1,000 V. This is quite near to the operational limits of silicon-based MOSFETs and IGBTs. Without a sizable technical advance, silicon-based MOSFETs and IGBTs will struggle to match the more stringent operational specifications of next-generation EVs.

    These restrictions result from both the physical limitations of silicon semiconductors and the design of the devices themselves. Large IGBTs and MOSFETs struggle to switch at high frequencies and endure switching losses because of their gradual transition from the ON to the OFF states.

    Although inverters are more effective at higher operating frequencies, these improvements are quickly offset by the devices’ inherent switching losses. Additionally, the inverter’s operating frequency has a limit above which operation is impossible due to the lengthy switching periods of the devices.

    Gallium nitride and SiC technologies complement one another well and will stay in use. Gallium nitride devices work well in applications ranging from tens to hundreds of volts, while SiC is better suited for supply voltages from roughly one to many kilovolts. They currently cover distinct voltage ranges. Gallium nitride has switching losses that are at least 3x less for mid- and low-voltage applications (below 1200 V) than SiC does at 650 V. Silicon carbide is available in some products at 650 V, but it is typically made for 1200 V or greater.

    From a system perspective, the benefits of GaN come from reductions in size, weight and cost, which includes BOM cost (the price of other system components like capacitors, heat sinks and inductors), consumption cost and cooling cost. For instance, switching a power supply’s power supply from silicon to GaN can reduce the size of magnetic components like transformers. All of this is possible while attaining increased efficiency, increased power density, or perhaps even both.

    Silicon is still competitive up to 650 V. At higher voltages, however, SiC and GaN allow effective high-frequency and high-current operation. All the devices are suitable for the 400 V EV bus voltage, while around 650 V is where the main conflict between silicon, SiC and GaN takes place. Although GaN is less developed than SiC, many experts agree that it has enormous promise in the automotive industry.

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