“Wide band gap (WBG) semiconductors are being used in various power converters including electric vehicles. Its promised higher efficiency and faster conversion rate will save cost, size and energy. It is usually used in chargers and auxiliary converters, but it has not yet largely replaced IGBTs in traction inverters. This article will introduce the latest generation of SiC FETs, which are an excellent choice for new inverter designs because they offer lower losses than IGBTs and proven short-circuit robustness under high temperatures and multiple stresses.
Wide band gap (WBG) semiconductors are being used in various power converters including electric vehicles. Its promised higher efficiency and faster conversion rate will save cost, size and energy. It is usually used in chargers and auxiliary converters, but it has not yet largely replaced IGBTs in traction inverters. This article will introduce the latest generation of SiC FETs, which are an excellent choice for new inverter designs because they offer lower losses than IGBTs and proven short-circuit robustness under high temperatures and multiple stresses.
In 1900, 38% of cars in the U.S. were electric cars
Yes, you read that right, this is true… Of all American cars in 1900, 38% (33,842) were powered by electricity, 40% were powered by steam, and 22% were powered by gasoline. However, when Henry Ford mass-produced cheap gasoline-powered cars, the percentage of electric cars dropped sharply. Today, less than 1% of electric vehicles are on the road, but it is predicted that by 2050, 65% to 75% of light vehicles in the United States will be powered by electricity.
Since the Toyota Prius was launched in Japan in 1997, Hyundai Electric Vehicles (EV) have improved significantly. Now, advanced battery and motor technology can provide 300 miles or more of cruising range. However, the forecast of electric vehicle usage in 2050 also relies on certain assumptions: purchasing power, rising oil prices, stricter health and environmental regulations, and more advanced technologies to achieve more mileage and faster charging.
With a 59%-62% conversion rate from battery energy to wheel power, it seems that EV has room for improvement. Electrical engineers may roll their eyes and say that modern internal combustion engines are only trying to reach 21%! But with its new semiconductor switch used in the power transmission system, at least the EV has a possible blueprint for higher performance.
The key to gaining more mileage is the efficiency of power conversion. This is not only reflected in Electronic devices driven by motors, many auxiliary functions such as lighting, air conditioning and even infotainment systems also use a lot of energy. Many efforts have been made to reduce energy consumption in these areas through various measures, such as the use of LED lights. Various power converters need to reduce the main battery voltage from 400V to 12V or 24V for these auxiliary functions. At present, these converters use the latest topology and special semiconductor devices to achieve the best efficiency, and at the same time assume non-safety The inherent risks of new technologies that are relevant to the application’s acceptance (as shown in Figure 1).
Figure 1: Power conversion components of electric vehicles (Image source: US Department of Energy)
For the power transmission system, the electronic equipment controlled by the motor is considered to be life-critical, so designers have to uphold the principle of “safety first” and insist on using proven technology. In practice, this means that the use of IGBT switches has proven its robustness for more than 30 years. For example, behind the high-tech appearance of the Tesla model S are 66 IGBTs in the TO-247 package controlling the traction motor. The same IGBT was also very common in industrial process controllers in the 1980s. Newer models have just begun to appear, and this is the SiC FET.
Wide band gap semiconductors are becoming a strong competitor for motor control
In many modern applications, IGBTs have been replaced by newer technologies, such as silicon MOSFETs and wide band gap (WBG) semiconductors now made of silicon carbide (SiC) and gallium nitride (GaN) materials. The biggest advantage of WBG is the faster switching rate, which means smaller external components, like magnetic components and capacitors. This combination provides higher efficiency, smaller size and weight, thereby reducing overall costs. WBG devices can also work at high temperatures, usually 200°C for SiC, and the peak temperature is allowed to exceed 600°C (depending on the specific device).
SiC FET entry and its advantages analysis
A specific type of WBG device is the SiC FET, which is a composite or “cascode” body of SiC JFET and Si MOSFET, which is usually set to OFF, has no bias and supports nanosecond switching. Compared with SiC MOSFET and GaN devices, it is very easy to drive, and its good index RDSA and the normalized on-resistance of the chip area are very good (as shown in Figure 2). Due to the vertical structure of this device, it has extremely low internal capacitance, which makes the switching loss extremely low. SiC FET has a very fast body diode, which can reduce losses in applications such as motor drives, and does not require the use of external SiC Schottky diodes.
Figure 2: SiC FET (cascode) RDSA-comparison of normalized on-resistance of chip area
SiC FET used in electric vehicle drive
So, since you want to promote higher-performance solutions, why haven’t these excellent devices entered the EV motor control market? In addition to the natural conservativeness of automotive system designers, there are some practical reasons: WBG devices are considered more expensive compared to IGBTs of similar levels; motor inductance will not be scaled down like a DC-DC converter, so Makes higher switching frequencies less attractive; high switching speeds mean high dV/dt rates, which may put pressure on the insulation of the motor windings. In addition, when the motor drive is under severe conditions or a general high temperature environment, WBG devices have potential short-circuit problems and back electromotive force (EMF), which also makes their reliability unavoidable.
The real temptation of WBG devices is the possibility of increasing efficiency. This means more available energy and longer mileage. The radiator can be smaller, which can reduce cost and weight, while also helping to expand the mileage. Compared with IGBTs with “knee point” voltages, WBG devices have particularly improved efficiency under typical operating conditions, thereby effectively achieving the minimum power consumption under all driving conditions. As shown in Figure 3 below, we will compare a 200A, 1200V IGBT module using two 1cmX1cm IGBT chips with a 200A, 1200V SiC FET module using two 0.6 X 0.6cm SiC stacked cascode chips.
Figure 3: The conduction loss of a 1200V SiC FET whose area is only 36% of the IGBT chip. In this 200A, 1200V module, for all currents below 200A at room temperature and high temperature, the turn-on voltage drop of the SiC FET is much lower than the voltage drop of the IGBT.
The unique properties of SiC FET enable it to provide the lowest conduction loss within a given module footprint. Of course, in the new design, the switching frequency of the WBG motor driver is also higher than the IGBT with sufficient EMI control design, thus reflecting all the advantages of WBG. Even if its cost is high, it should not be a concern in the future. For example, SiC FET chips are much smaller than IGBTs or SiC MOSFETs of the same level, which means that the production capacity of each wafer is higher. If you consider using smaller heat dissipation Filters and filters in order to save costs, then all this seems extremely economical and practical.
SiC FET reliability has been verified
Now all that remains is the worry about reliability, which is indeed a problem for some WBG devices. For example, SiC MOSFETs and GaN devices are extremely sensitive to gate voltage, and their absolute maximum values are very close to the recommended limits of operating conditions. But SiC FET allows a wider range of gate voltage, and its margin reaches the absolute maximum.
The short-circuit rating may be the focus of the EV motor driver, which is based on the robustness of the IGBT. Of course, GaN devices do not perform well in this regard, while SiC FETs perform well. Unlike SiC MOSFET or IGBT, there is a natural “pinch-off” mechanism in the vertical channel of the built-in JFET device, which can limit the current and make the short-circuit gate drive voltage relatively independent. The high peak temperature allowed by the SiC JFET also extends the duration of the short circuit. In automotive applications, it is generally expected that the short circuit should withstand the test of 5μs before the protection mechanism is activated. The 650 V SiC FET test from UnitedSiC shows that the 400 V DC bus can withstand at least 8μs short circuit test (Figure 4). After 100 short circuit events and high temperatures, there is no degradation of on-resistance or gate threshold.
Figure 4: Short-circuit performance of SiC FET
Another stress in motor drive applications is the back electromotive force from the motor. Similarly, GaN’s poor performance again loses points, but SiC FET has very good avalanche tolerance, and its built-in JFET turns on to clamp the voltage when its gate-drain junction is disconnected. More tests conducted by UnitedSiC show that after 1000 hours of avalanche test in a 150° environment, SiC FET components have no failures and 100% passed the avalanche tolerance production test.
Modern wide-bandgap devices, such as SiC FETs from UnitedSiC, are strong competitors for next-generation EV motor drivers that can provide better performance, overall cost savings, and proven robust operability in demanding environments. Therefore, SiC (Silicon Carbide) will most likely become the dominant power transmission system in the next decade.