Wide bandgap semiconductors (WBG), such as silicon carbide (SiC) and gallium nitride (GaN), provide superior performance compared to silicon: higher efficiency and switching frequency, higher operating temperature and operating voltage. EVs and HEVs include several power-conversion stages, with cumulative power losses that can reach 20% of the initially available power. WGB semiconductors greatly improve the efficiency of power conversion stages, acting as a valid substitute for silicon in the manufacturing of voltage converters, power MOSFETs and high efficiency Schottky diodes. Compared to silicon (Si) and gallium arsenide (GaAs), WBG semiconductors allow to obtain important improvements, such as: greater power efficiency, smaller size, lighter weight and lower overall cost.
Benefits of GaN and SiC
WBG materials have a relatively large energy bandgap, that is the energy gap existing between the upper limit of the valence band and the lower limit of the conduction band. Electrons can pass through the bandgap and enter the conduction zone by means of a thermal or optical excitation. Bandgap allows semiconductors to switch between conduction (ON) and interdiction (OFF) states based on electrical parameters controllable from the outside. WBG materials such as silicon carbide and gallium nitride have a bandgap equal, respectively, to 3.3 eV and 3.4 eV, values significantly higher than that of silicon (1.12 eV) and that of gallium arsenide (1.4 eV). A wider bandgap implies a greater electric breakdown field, but also the chance of operating at higher temperatures, voltages and frequencies. A wide bandgap means also higher breakdown electric field and, therefore, higher breakdown voltage. Overcoming the theoretical limits offered by silicon, WBG semiconductors such as GaN and SiC offer significant performance improvements and allow operating with efficiency and reliability even in the most severe conditions. Compared to silicon, the main benefits offered by these materials can be summarized as follows:
- lower on-resistance;
- higher breakdown voltage;
- higher thermal conductivity;
- operation at higher temperatures;
- greater reliability;
- near zero reverse recovery time;
- excellent high frequency performance.
SiC automotive applications
The main power devices which can be found in any electric or hybrid vehicles are shown in Figure 1: SiC-based devices can efficiently replace silicon-based devices for implementing those functionalities. The main inverter is a key component in the car. It controls the electric motor (regardless of its 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 has the task to provide the 12V power system bus, converting it from a high-voltage battery. Today, several kinds of high-voltage batteries, with different voltage levels and different power classes (usually in the range from 1kW to 5kW) are available on the market. Other optional components might be required, depending on whether the regenerative circuit shall support mono or bidirectional energy transfer. Auxiliary inverter/converter supplies power, derived from the high-voltage battery, to several auxiliary systems such as air conditioning, electronic power steering, PTC Heater, oil pumps and cooling pumps. The battery management system controls battery state during charging and discharging. This operation shall be performed in a smart way, so that the battery lifetime can be extended. As battery age increases, cells usage shall be optimized, balancing their performance during charging and discharging. The on-board battery charger plays a fundamental role, since it allows battery charging from a standard power outlet. This is an additional requirement for the designers, since different voltage and current levels shall be supported by the same circuit. A provision for future capabilities, such as bi-directional transfer of power (where the charger also feeds power from the car to the smart grid) shall be provided, as well.
GaN motor driver
Automotive applications require electric motors with increasingly compact size and increasingly higher performances. Motor driver circuits, traditionally based on MOSFET and IGBT silicon transistors, exhibit increasing difficulties in meeting this type of requirements. The silicon technology is in fact reaching its theoretical limits, with restrictions concerning, above all: power density, breakdown voltage and switching frequency which, in turn, impacts on the power losses.
The main effects of these limitations are manifested mainly in a sub-optimal level of efficiency, to which are added potential problems in operation at high temperatures and high switching rates. As an example, consider a silicon-based power device operating at a switching frequency equal to or greater than 40 kHz. Under those conditions, switching losses are greater than conduction losses, with cascading effects on overall power losses. To dissipate the heat produced in excess, it is necessary to use a suitable heat sink, a solution that, in addition to levitating costs and the device overall weight, can also be disadvantageous due to its excessive footprint. HEMT (High Electron Mobility Transistor) devices based on gallium nitride (GaN) offer superior electrical characteristics, offering themselves as a valid alternative to MOSFET and IGBT transistors in high voltage and high switching frequency motor control applications. Figure 2 shows the trend of the overall losses related to power devices built with silicon and gallium nitride technology, respectively. While the conduction losses can be considered constant, for both materials, a different behavior regards the switching losses. As the switching frequency increases, the switching losses of a GaN HEMT transistor are significantly lower than those of a silicon MOSFET or IGBT and this difference is even more marked the higher the switching frequency is.