SiC MOSFETs offer significant benefits in automotive and power applications

Abstract: Traditional silicon-based MOSFET technology is maturing and is approaching the theoretical limit of performance. Wide-bandgap semiconductors have better electrical, thermal and mechanical properties, which can improve the performance of MOSFETs and are an alternative technology that has received much attention.

Commercial silicon-based power MOSFETs have a history of nearly 40 years. Since their inception, 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 has also made MOSFET and IGBT realize the meaning of being harmed by success. As the overall performance of the product improves, especially the on-resistance and switching losses are greatly reduced, the application range of these semiconductor switches is becoming wider and wider. As a result, the market expects higher and higher performance requirements for these silicon-based MOSFETs and IGBTs.

Although major semiconductor R&D institutes and manufacturers make great efforts to meet market requirements and further improve MOSFET/IGBT products, at some point, the law of diminishing returns prevails. Over the past few years, despite a lot of investment, the results have been very small. It is not uncommon for technologies and products to eventually evolve to a stage where the payoff is not proportional to the payoff, laying the groundwork for new disruptive approaches and new products.

For MOSFET devices, this disruptive technological innovation cycle is the result of the development and mastery of new basic materials. Compared to pure silicon-based MOSFETs, silicon carbide (SiC)-based MOSFETs perform better. Note that the products used in this comparison test are not R&D samples or demonstration prototypes, but commercial SiC-based MOSFETs.

As an important fast-growing application area, the development of electric vehicles and hybrid vehicles (EV/HEV) has benefited from MOSFET technology advancements, which in turn have driven MOSFET R&D and manufacturing activities. Regardless of what consumers think, these fully-battered 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 lot of Electronic modules to drive the system Run, manage devices, and perform special functions, as shown in Figure 1.

Figure 1: Electric and hybrid vehicles are not just one large battery connected to several traction motors, but many smaller electronic subsystems and power supplies, as well as high-power subsystems that charge, discharge, and manage large battery packs.

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

In-wheel motor traction inverter (200 kW/up to 20 kHz);

AC input on-board charger (20 kW/50 kHz-200 kHz);

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

Power supplies for auxiliary functions: center console, battery management controls, air conditioning, infotainment system, GPS, network connection (on the order of 4 kW/50 kHz-200 kHz)

Why focus on energy efficiency? Range is clearly one of the most important considerations for consumers when shopping for electric and hybrid vehicles. Even a small improvement in inverter performance can lead to a significant improvement in the basic performance indicators of the car visible to consumers.

However, it is not just this one factor that requires high energy efficiency, there are many others:

·Reduce operating temperature and improve reliability;

Reduce thermal loads, reducing heat dissipation through radiators, fins, coolants and other technologies;

Reduce charging time and basic power consumption;

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

· Easier compliance with regulatory requirements.

　　SiC meets challenges

Fortunately, SiC offers a pathway to greater energy efficiency and associated performance improvements. How are SiC MOSFETs different from mainstream pure silicon MOSFETs in terms of structure and performance? In short, a SiC MOSFET is a SiC n-doped epitaxial layer (aka drift layer) on a SiC n+ substrate, as shown in Figure 2. The key parameter, the on-resistance, RDS(ON), is largely dependent on the channel resistance, RDrift, between the source/base and drift layers.

Figure 2: Unlike pure silicon MOSFETs, SiC MOSFETs form a silicon carbide epitaxial (drift) layer on top of an n+-type SiC substrate, with the source and gate placed on top of the SiC drift layer.

When the RDrift value is given and the junction temperature is 25⁰C, the actual area of ​​the SiC transistor die is a fraction of the die area of ​​the silicon superjunction transistor. If the chip area of ​​the two transistors is the same, the performance of the SiC transistor is higher. a lot of. 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 ST’s SiC MOSFETs is very small, about one-tenth of the best high-voltage silicon MOSFETs on the market (900V superjunction tubes).

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

Lower switching loss, lower conduction loss at medium and small power;

There is no PN junction voltage drop like IGBT;

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

Higher operating temperature (200⁰C), which reduces cooling and heat dissipation requirements while increasing reliability;

·With the same energy efficiency, the switching frequency is 4 times that of IGBT, and the weight, size and cost are lower due to less passive components and external components.

　　driver

Experienced engineers know that the power device is only one of many important components of the overall system. Making the design reliable, efficient, and cost-effective also requires selecting the right driver for the MOSFET. A suitable driver is one designed specifically for the current rate of change, voltage value, and timing constraints specific 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 whether the drivers are available in the market. Excitingly, driving a SiC MOSFET is almost as easy as driving a silicon-based MOSFET, driving an 80mΩ device with only 20V gate-source voltage and a maximum drive current of about 2A. Therefore, a simple standard gate driver can be used in many cases. STMicroelectronics and others have developed gate drivers optimized for SiC MOSFETs, such as the ST TD350.

Inside this advanced gate driver, an innovative active Miller clamp function 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 adjustable The two-stage shutdown function can prevent the shutdown operation from generating a large amount of overvoltage in case of an overcurrent or short-circuit condition, and the delay set in the two-stage 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 published an application note titled “How to Trim SiC MOSFET Gate Drivers to Minimize Losses”, which provides comprehensive details on the driver requirements and solutions for optimal performance.)

　　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, and SiC MOSFETs are an exception. Currently, SiC MOSFETs are mass produced and adopted in hybrid and electric vehicles, delivering tangible results in terms of energy efficiency, performance and operating conditions, and conduction down to the circuit and system levels.

We did a comparison test of SIC MOSFETs and silicon IGBTs with 80kW traction motor inverter power modules for hybrid and electric vehicles, and the results showed that 650V SIC MOSFETs far outperformed silicon IGBTs in many key parameters. This three-phase inverter module uses a bipolar PWM control topology with synchronous rectification mode. Both devices are sized for a junction temperature less than 80% of the absolute maximum rated junction temperature. The silicon IGBT solution uses 4 parallel 650V/200A IGBTs and associated freewheeling silicon diodes of the same rating; the SIC MOSFET based solution is designed with 7 parallel 650V/100A SiC MOSFETs without 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:

Note that almost all power loss parameters are significantly improved for SiC MOSFETs compared to silicon-based IGBTs. When MOSFETs are connected in parallel, the resulting RDS(ON) on-resistance is divided by the number of MOSFETs, resulting in close to zero conduction losses, so SiC MOSFETs have lower conduction losses than IGBTs. Conversely, when IGBTs are connected in parallel, the resulting VCE(SAT) voltage does not drop linearly, and the minimum on-voltage drop is limited to approximately 0.8 to 1 V.

It is not difficult to see that the power loss of the SiC-based MOSFET solution is much lower over the entire load range. Due to the lower on-voltage drop, the conduction losses of these MOSFETs are also reduced from 125 W to 55 W at 100% load, as shown in Figures 3a and 3b.

Figure 3: a) Over the entire load range, the SiC-based design (red line) dissipates much lower power than a silicon-based IGBT (blue line) (left). 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 duty ratios.

At low loads, SiC devices are up to 3% more efficient than silicon IGBTs; over the entire load range, the overall energy efficiency is at least 1% higher. Although 1% may not seem like a lot, for this power class, 1% represents a high amount of power dissipation, 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 extend the range of electric vehicles, which is a value proposition that automakers and consumers value. Comparing the junction temperature of SiC and IGBT at a switching frequency of 16 kHz, from low load to full load, it is clear that SiC is the winner, and the coolant temperature of both is 85 °C, as shown in Figure 4. The data shows that because of the high losses, the efficiency of the IGBT cooling system must be higher.

Figure 4: Junction temperature determines switching frequency, reliability, and other properties; SiC solution (red line) outperforms silicon solution (blue line) in terms of reliability, maintaining lower Δ(Tj- Tfluid) temperature difference.

The junction temperature of the SiC device is at a low level almost over the entire switching frequency range, as shown in Figure 5, even when the switching frequency is as low as 8 kHz, the temperature is lower than that of the IGBT, and 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; both schemes have roughly the same junction temperature at 8 kHz, but then SiC (red line) gradually outperforms Si (blue line), which It increases substantially with increasing switching frequency.

In peak power pulse conditions, SiC MOSFETs have higher conduction losses than IGBTs. In order to keep the junction temperature below the maximum junction temperature (typically 80% of Tjmax at 200⁰C), we limit the size of the SiC MOSFET so that the SiC MOSFET has the following Advantage:

Small chip area, suitable for more compact solutions;

The power loss of medium and low loads is much lower;

Longer battery life and extended car cruising range;

Lower losses at full load, suitable for smaller cooling schemes;

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

cost considerations

When discussing technological advancements and the benefits they bring, discussions without cost considerations are one-sided. Currently, SiC MOSFETs cost 4-5 times more than silicon IGBTs, however, the savings in bill of materials, cooling systems, and energy consumption of SiC MOSFETs reduce overall system cost and often offset the cost gap for these base components. In the next 2-5 years, as the industry shifts to large-diameter wafers, STMicroelectronics has begun to transform, this spread should be reduced to 3 times or even 2.5 times, the quality factor RDSON × area will also be improved, and the yield will increase . In the long run, costs will continue to decrease over the next 5-10 years as these parameters improve.

SiC power switches hold promises of improved performance and turn those promises into reality, with few design tradeoffs in application and installation. SiC power switches can play an important role in successful designs as automakers ramp up development of hybrid vehicles, electric vehicles, and many related power modules, as well as other high-power motor-centric applications, and even small steps of improvement can add value to The system level brings huge progress.