SiC Hits the Performance Marks for xEV Systems

The market for plug-in hybrid electric vehicles (PHEVs) and fully electric vehicles (EVs) continues to grow. Vehicles in these categories, sometimes collectively referred to as xEVs, incorporate numerous power electronic devices, most of which are based on silicon technology today. The most advanced xEV designs, however, require power electronics solutions that can achieve higher efficiency and higher power density than conventional silicon structures can provide.

Silicon carbide (SiC) can overcome the structural limits of silicon, providing previously unattainable levels of performance. SiC’s advantages include low switching losses, low drain-source on-resistance (RDS(on)), high operating temperature, and high switching frequency. These features make SiC power devices suitable for meeting even the most stringent automotive requirements.

Power solutions based on SiC are more efficient, lighter, and more compact than traditional silicon-based solutions. Compared with the best silicon-based alternatives, SiC power transistors are able to switch at frequencies up to five times higher, with gate voltages up to two times higher. Therefore, the design of a gate driver based on SiC devices requires extreme attention, especially with respect to the effects produced during the transient state, the level of performance in the conduction state (on), and the parasitic capacitance.

KEY SiC CHARACTERISTICS

MOSFET power transistors built with SiC technology have a wider bandgap than silicon-based alternatives, which in turn implies a greater breakdown voltage. In fact, in silicon carbide, electrons require three times more energy than in silicon to move from the valence band to the conduction band. This allows SiC MOSFETs to withstand a breakdown field strength up to 10 times higher than silicon equivalents. A further benefit deriving from this property is the possibility of enormously reducing the thickness of the channel, with a consequent reduction in RDS(on).

SiC MOSFETs also behave differently in the saturation phase, and there is no well-defined transition between conduction and saturation states. Whereas a silicon MOSFET can be considered in the state of complete conduction (on state) when the gate-source voltage (VGS) exceeds a certain threshold (acting as a voltage-controlled current source), a silicon carbide MOSFET usually has a low transconductance and behaves like a voltage-controlled variable resistance. The described behavior is shown in Figure 1, which compares the output characteristics (at a given temperature) of a SiC MOSFET and a silicon-based IGBT. Note that in the SiC transistor, there is a smooth transition between on and off states, whereas in the IGBT this transition is much sharper.

Figure 1: Output-characteristics comparison between SiC and silicon devices (Image: ROHM Semiconductor)

As the gate voltage increases, the RDS(on) continues to decrease, until it reaches the limit set by VGS(max). Since VGS(max), depending on the specific device, is typically in the range of 18 to 25 V, the gate driver should be able to provide a voltage between 15 V and 20 to 22 V. Unlike IGBTs, SiC MOSFETs do not have any tail current in the off state and therefore are able to achieve higher switching rates without suffering power losses. The switching frequency of a silicon carbide MOSFET can be up to five times that of a silicon device, with the consequent possibility of using smaller passive components.

The higher switching frequency also imposes some extra requirements in the design phase of the gate driver, which not only has to generate a larger VGS to bring the device into conduction state (minimizing RDS(on)), but also has to provide a very fast output slew rate (several volts per nanosecond) so as to charge and discharge the capacitance of the gate circuit very quickly. Requirements of this type pose significant challenges for designers, who must contend with very fast rising edges and the quick movement of charges; these can lead to undesirable effects such as overshoot, ringing, and large voltage transients capable of generating spurious switching.

SiC BENEFITS FOR AUTOMOTIVE

SiC technology has many advantages that enable its use in automotive power electronics, characterized by essential requirements such as reliability, quality, and performance:

  • Higher power density. Smaller devices and systems can be realized, with a 3× to 5× volume reduction, depending on system conditions. By replacing silicon IGBT transistors with full-SiC devices, switching losses can be reduced by two-thirds, and the operating frequency can be increased by a factor of four to allow smaller peripheral components, leading to more-compact equipment.
  • Smaller battery. This is one of the most challenging requirements for an xEV. A reduction in battery size yields a series of derived benefits: reduced weight, longer range (or higher autonomy), and overall cost reduction. SiC technology can increase xEV autonomy by 5% to 10% over silicon-based solutions.
  • Scalability. SiC-based solutions allow the scaling of power devices such as inverters to support higher power requirements within the same footprint.
  • Higher efficiency. Higher-switching– frequency capability and reduced current harmonics make SiC devices more efficient than their silicon counterparts.

SiC APPLICATIONS IN xEVs

The main power devices found in the typical xEV model are shown in Figure 2. SiC-based devices can efficiently replace silicon-based devices for implementing any of the illustrated functions.

Figure 2: Applications for silicon carbide devices are found throughout the car.
(Image: ROHM Semiconductor)

The main inverter is a key component in the car. It controls the electric motor (regardless of type — synchronous, asynchronous, or brushless DC) and captures the energy released through regenerative breaking, giving it back to the battery. In EVs and HEVs, the DC/DC converter is tasked with obtaining the 12-V power system bus, converting to that voltage from a high-voltage battery. Today, several kinds of high-voltage batteries, with different voltage levels and different power classes (usually in the range of 1 to 5 kW), are available on the market. Other optional components might be required, depending on whether the regenerative circuit will support mono- or bidirectional energy transfer.

Auxiliary inverter/converters supply power, derived from the high-voltage battery, to auxiliary systems such as air conditioning, electronic power steering, positive temperature coefficient (PTC) heaters used in automotive HVAC systems, oil pumps, and cooling pumps. The battery management system controls the battery state during charging and discharging. This operation must be performed in a way that maximizes battery lifetime. As for battery aging, cell usage must be optimized to balance cell performance during charging and discharging. The onboard battery charger plays a fundamental role, since it allows battery charging from a standard power outlet. This is an additional consideration for designers in terms of the voltage and current levels to be supported.

GATE DRIVER CONSIDERATIONS

The control of a SiC MOSFET at high switching frequencies requires careful management of the gate current and, possibly, an asymmetrical behavior of the VGS gate-drive voltage: up to about 15 to 20 V in the conduction phase (on state) and between –5 V and –4 V in the off state. A VGS equal to 0 V could also be acceptable in single-switch topologies, avoiding the additional complexity of having to generate a negative voltage in the off state.

In any case, one of the most critical aspects related to the design of the gate driver is to avoid the spurious turn-on generated by dVDS/dt transients or by undesired gate currents. Techniques that can be used for this purpose include optimization of the gate-source voltage threshold of the MOSFET through accurate selection of the devices, application of a negative voltage in the off state, and clamping the gate to a low voltage in order to keep the MOSFET in the off state.

With its SCT3xxxxxxxxxHR series, ROHM Semiconductor says it offers the widest line available of AEC-Q101-qualified SiC MOSFETs, which guarantee the high reliability necessary for the onboard charger and DC/DC converters in automotive applications. ROHM’s SCT3160KL, meanwhile, is an N-channel SiC power MOSFET optimized for loads up to 17 A in a TO-247N package. It includes a thermal tab to ensure a good fit for current and power requirements.

Infineon Technologies has added two EasyPACK modules, in two topologies, to its 1,200-V family for energy-efficient charging. The Easy 1B and Easy 2B both integrate Infineon’s CoolSiC MOSFETs. The F4-23MR12W1M1_B11 is precisely suited for the DC/DC stage of the charging station. The F3L15MR12W2M1_B69 has a three-level configuration for the power factor correction (PFC) stage.

Figure 3: CoolSic MOSFET Easy 2B power module (Image: Infineon Technologies)

The CoolSiC Easy 2B power modules enable engineers to reduce system costs by increasing power density (Figure 3).

The major hurdle to EV adoption remains the vehicles’ relatively limited range. The trend for batteries is to increase their capacity, with associated shorter charging times. These objectives require onboard chargers characterized by high power and efficiency (11 kW and 22 kW, for example), driving the adoption of SiC MOSFETs as silicon MOSFETs and IGBTs approach their theoretical limits. Silicon carbide technology’s low on-resistance, high thermal conductivity, high breakdown voltage, and high saturation velocity all make it the most efficient solution for automotive requirements. ■