SiC MOSFET applications in automotive and power supplies

The continuous development of science and technology promotes the continuous innovation of electronic components. Traditional silicon-based MOSFET technology is becoming increasingly mature and is approaching the theoretical limit of performance. Wide-bandgap semiconductors have better electrical, thermal and mechanical properties and can improve the performance of MOSFETs. They are an alternative technology that has attracted much attention. Commercial silicon-based power MOSFETs have a history of nearly 40 years. Since their introduction, MOSFETs and IGBTs have been the main power processing control components of switching power supplies and are widely used in circuit designs such as power supplies and motor drives.

However, this success also made MOSFET and IGBT realize the meaning of suffering from success. With the improvement of the overall performance of the product, especially the significant reduction of on-resistance and switching losses, the application range of these semiconductor switches is becoming wider and wider. As a result, the market has higher and higher expectations for these silicon-based MOSFETs and IGBTs, and the performance requirements are getting higher and higher.

Although major semiconductor R&D institutions and manufacturers make great efforts to meet market requirements and further improve MOSFET/IGBT products, at some point, the law of diminishing returns dominates. Over the past few years, despite heavy investment, little results have been achieved. It’s not uncommon for technologies and products to eventually reach a stage where effort outweighs reward, laying the foundation for new disruptive approaches and products.

For MOSFET devices, this disruptive technology innovation cycle is the result of developing and mastering new basic materials. Compared with MOSFETs based on pure silicon, the performance of MOSFETs based on silicon carbide (SiC) is even better. Please note that the products used in the comparison test in this article are not R&D samples or demonstration prototypes, but already commercial SiC-based MOSFETs.

As an important and rapidly developing application field, the development of electric vehicles and hybrid vehicles (EV/HEV) has benefited from the advancement of MOSFET technology, which in turn has promoted MOSFET R&D and manufacturing activities. Regardless of what consumers think, these fully battery-powered cars are not just a large battery pack connected to several traction motors (hybrid cars also have a small gasoline engine to charge the battery), but require a large number of electronic modules to drive the system. Run, manage the device, and perform special functions, as shown in Figure 1.

Power switching conversion systems used in electric and hybrid vehicles include:

·Hub motor traction inverter (200 kW/up to 20 kHz);

·AC input car charger (20 kW/50 kHz-200 kHz);

·Optional fast charging function (50 kW/50 kHz-200 kHz)

·Auxiliary function power supply: center console, battery management control, air conditioning, infotainment system, GPS, network connection (4 kW/ 50 kHz-200 kHz level)

Why focus on energy efficiency? Range is obviously one of the important considerations for consumers when purchasing electric and hybrid vehicles. Even small improvements in inverter performance can lead to significant improvements in basic vehicle performance indicators visible to consumers.

However, this is not the only factor that requires high energy efficiency. There are many other factors:

·Reduce operating temperature and improve reliability;

Reduce heat loads and reduce the amount of heat dissipated through radiators, heat sinks, coolants and other technologies;

·Reduce charging time and basic power consumption;

Due to the inherent requirements and limitations of systems operating at higher temperatures, the overall package needs to have greater flexibility;

·Comply with regulatory requirements more easily.

SiC meets challenges

Fortunately, SiC offers a path to greater energy efficiency and associated performance improvements. In terms of structure and performance, how is SiC MOSFET different from mainstream pure silicon MOSFET? In short, SiC MOSFET adds a SiC n-doped epitaxial layer (also called a drift layer) on a SiC n+ substrate, as shown in the figure 2 shown. The key parameter, on-resistance RDS(ON), depends largely on the channel resistance RDrift between source/base and drift layer.

Figure 2: Different from pure silicon MOSFET, SiC MOSFET makes a silicon carbide epitaxial (drift) layer on the n+-type SiC substrate, and the source and gate are placed on top of the SiC drift layer.

When the RDrift value is given and the junction temperature is 25?6?2C, the actual area of ​​the SiC transistor die is a fraction of the area of ​​the silicon superjunction transistor die. If the chip areas of the two tubes are made the same, then the SiC transistor The performance is much higher. Another way to compare SiC and silicon is to use the familiar figure of merit (FOM), which is RDS(ON) × die area (the lower the figure of merit, the better). At a blocking voltage of 1200V, the FOM value of STMicroelectronics' SiC MOSFET is very small, about one-tenth of the best high-voltage silicon MOSFET (900V superjunction tube) on the market.

Compared with silicon-based IGBTs commonly used in traction inverters, SiC MOSFETs have the following main advantages:

·Lower switching losses and lower conduction losses at low and medium power;

·No PN junction voltage drop like IGBT;

SiC devices have a robust, fast intrinsic diode that eliminates the need for external diodes; the recovery charge of this intrinsic diode is so small that it is almost negligible;

·Higher operating temperature (200?6?2C), thereby reducing cooling requirements and heat dissipation requirements while improving reliability;

·Under the same energy efficiency, the switching frequency is 4 times that of IGBT. Due to fewer passive devices and external components, the weight, size and cost are lower.

driver

Experienced engineers know that power devices are just one of many important components of an overall system. To make the design reliable, efficient, and cost-effective, you also need to select the appropriate driver for the MOSFET. A suitable driver is one that is specifically designed based on the current slew rates, voltage values, and timing constraints unique to the target MOSFET and its load. Since silicon-based MOSFET technology has matured, there are many brands of standard drivers on the market to ensure that the driver/MOSFET combination works properly.

Therefore, it is normal for people to care about the difficulty of driving SiC MOSFETs and more about whether the drivers are available in the market. What’s exciting is that driving SiC MOSFETs is almost as easy as driving silicon-based MOSFETs. Driving an 80mΩ device requires only a 20V gate-to-source voltage and a maximum drive current of about 2A. Therefore, simple standard gate drivers can be used in many cases. STMicroelectronics and others have developed gate drivers optimized for SiC MOSFETs, such as the ST TD350.

Within this advanced gate driver, an innovative active Miller clamp feature saves negative voltage gate drive in most applications and allows the use of a simple bootstrap supply to drive the high-side driver; level and delay are adjustable The two-level shutdown function can prevent the shutdown operation from generating a large amount of overvoltage in case of overcurrent or short circuit. The delay set in the two-level shutdown function can also be used to control the turn-on operation of the switch to prevent pulse width distortion. (To further simplify the use of SiC MOSFETs, STMicroelectronics has released an application note titled "How to fine-tune SiC MOSFET gate drivers to minimize losses", which fully details the driver requirements and optimal performance solutions.)

Not just an inference, but a fact

Advances in manufacturing processes sometimes do not guarantee that new technologies will be industrialized and applied on a large scale, but SiC MOSFET is an exception. At present, SiC MOSFET has been mass-produced and adopted by hybrid and electric vehicles, achieving tangible results in energy efficiency, performance and working conditions, and transmitting them to the circuit level and system level.

We used the 80kW traction motor inverter power module of hybrid vehicles and electric vehicles to conduct a comparative test between SIC MOSFET and silicon IGBT. The results show that in many key parameters, 650V SIC MOSFET is far better than silicon IGBT. This three-phase inverter module uses a bipolar PWM control topology with synchronous rectification mode. Both devices are sized based on a junction temperature less than 80% of the absolute maximum rated junction temperature. The silicon IGBT solution uses 4 parallel-connected 650V/200A IGBTs and associated freewheeling silicon diodes of the same rating; the SIC MOSFET-based solution design uses 7 parallel-connected 650V/100A SiC MOSFETs without using any external diodes (only intrinsic diode); rated peak power 480Arms (10 seconds), normal load 230Arms. Other working conditions are:

·DC circuit voltage: 400Vdc

·Switching frequency: 16kHz

·SiC Vgs voltage +20V/-5V, IGBT Vge voltage ±15V

·Coolant temperature: 85℃

·RthJ-C(IGBT-die)=0.4℃/W; RthJ-C(SiC-die)=1.25℃/W

·Under any conditions, Tj ≤ 80% ×Tjmax℃

The following table lists typical power losses at rated peak power:

It is noted that compared with SiC MOSFET and silicon-based IGBT, almost all power loss parameters have been significantly improved. When MOSFETs are connected in parallel, the resulting RDS(ON) on-resistance is divided by the number of MOSFETs, causing the conduction loss to be close to zero. Therefore, the conduction loss of SiC MOSFET is lower than that of IGBT. In contrast, when IGBTs are connected in parallel, the resulting VCE(SAT) voltage does not decrease linearly, and the minimum on-voltage drop is limited to approximately 0.8 to 1 V.

It is easy to see that SiC-based MOSFET solutions have much lower power losses over the entire load range. Due to the lower conduction voltage drop, the conduction losses of these MOSFETs at 100% load are also reduced from 125 W to 55 W, as shown in Figures 3a and 3b.

Figure 3: a) The power consumption of the SiC-based design (red line) is much lower than that of the silicon-based IGBT (blue line) over the entire load range (left image). b) The energy efficiency of the SiC system (red line) is significantly higher than that of the pure silicon solution (blue line), especially at lower load ratios.

At low loads, the energy efficiency of SiC devices is up to 3% higher than that of silicon IGBTs; over the entire load range, the total energy efficiency is at least 1% higher. Although 1% may not seem like a lot, for this power level, 1% represents a lot of power consumption, power dissipation, and heat dissipation. Engineers know that high temperatures are the enemy of long-lasting performance and reliability. In addition, high energy efficiency can also extend the driving range of electric vehicles, which is a value proposition that car manufacturers and consumers value. At 16 kHz switching frequency, comparing the junction temperatures of SiC and IGBT, from low load to full load, SiC is obviously the winner. The coolant temperature of both is 85?6?2C, as shown in Figure 4. The data shows that the IGBT cooling system must be more efficient because of the high losses.

Figure 4: The junction temperature determines the switching frequency, reliability and other performance; in terms of reliability, the SiC solution (red line) is better than the silicon solution (blue line), maintaining a low Δ(Tj-) until 100% load Tfluid) temperature difference.

The SiC device junction temperature is at a lower level almost throughout the entire switching frequency range, as shown in Figure 5. Even at a switching frequency as low as 8 kHz, the temperature is lower than that of the IGBT. The silicon-based IGBT has exceeded the rated junction temperature at 46 kHz. temperature range.

Figure 5: Low junction temperature is also a major advantage of SiC devices over the entire switching frequency range; the two solutions have approximately the same junction temperature at 8 kHz, but thereafter SiC (red line) gradually outperforms Si (blue line), which It increases significantly with increasing switching frequency.

Under peak power pulse conditions, the conduction loss of SiC MOSFET is higher than that of IGBT. In order to keep the junction temperature below the maximum junction temperature (usually 80% of Tjmax of 200?6?2C), we limit the size of SiC MOSFET. At this time SiC MOSFET has the following advantages:

·Small chip area, suitable for more compact solutions;

·Much lower power loss at medium and low loads;

·Longer battery life and extended car cruising range;

·Lower losses at full load, suitable for smaller cooling solutions;

·In the entire load range, the temperature difference between the junction temperature Tj and the coolant temperature Tfluid is small, which can improve reliability.

These features and advantages bring tangible benefits to users, such as at least 1% improvement in energy efficiency (75% reduction in losses); a smaller and lighter inverter-side cooling system (approximately 80% reduction); smaller power modules, Lighter (50% less).

cost considerations

When discussing technological advancements and the benefits they bring, any discussion that does not take cost into account is one-sided. Currently, the cost of SiC MOSFET is 4-5 times that of silicon IGBT. However, the savings of SiC MOSFET in bill of materials, cooling system and energy consumption reduce the total system cost and can usually offset the cost difference of these basic components. In the next 2-5 years, as the industry shifts to large-diameter wafers and STMicroelectronics has begun its transformation, this price difference should be reduced to 3 times or even 2.5 times, the quality factor RDSON × area will also be improved, and the output will increase . In the long term, over the next 5-10 years, as these parameters improve, costs will continue to decrease.

SiC power switches bring the promise of improved performance while also turning those hopes into reality, with virtually no design compromises in application and installation. As automakers ramp up research and development of hybrid vehicles, electric vehicles and many related power modules, as well as other high-power motor-centric applications, SiC power switches can play an important role in successful designs, even if the improvement is small. Bringing huge improvements at the system level. The above is the application of SiC MOSFET in automobiles and power supplies. I hope it will be helpful to everyone.