Silicon carbide (SiC) MOSFETs are rapidly gaining popularity in the power semiconductor market, as some initial reliability concerns have been resolved and the price level has reached a very attractive point. As more devices become available in the market, it is important to understand the characteristics of SiC MOSFETs so that users can make full use of each device. This article will show you the development trend of SiC MOSFETs and the product characteristics of the 1200V SiC MOSFET power module introduced by onsemi.
SiC is a high-quality WBG semiconductor material for high-voltage high-current power supply applications
SiC is part of the wide bandgap (WBG) semiconductor material series used to manufacture discrete power semiconductors, with a bandgap energy of 1.12 eV for traditional silicon (Si) MOSFETs and 3.26 eV for SiC MOSFETs. SiC and gallium nitride (GaN) have a wider bandgap energy, meaning that moving electrons from their valence band to the conduction band takes about three times as much energy, which makes the materials behave more like an insulator than a conductor. This allows WBG semiconductors to withstand higher breakdown voltages, with its breakdown field robustness being 10 times that of silicon.
For a given rated voltage, a higher breakdown field reduces the thickness of the device, which translates into lower on−resistance and higher current capacity. Both SiC and GaN have the same order of mobility parameters as silicon, which makes both materials very suitable for high−frequency switching applications. The thermal conductivity of SiC is three times that of Si and GaN, and for given power consumption, higher thermal conductivity translates into lower temperature rise.
RDS(ON) for a specific breakdown voltage is an important part of the MOSFET characteristic and is inversely proportional to the mobility multiplied by the cubic of the critical breakdown field. Even though SiC’s mobility is lower than silicon, its critical breakdown field is 10 times higher, resulting in a much lower RDS(ON) for a given breakdown voltage. Commercial SiC MOSFETs have a guaranteed maximum operating temperature of 150°C < TJ < 200°C and can achieve a SiC junction temperature of up to 600°C. However, subject to bonding and packaging techniques, SiC has become a high-quality WBG semiconductor material for high-voltage, high-speed, high-current and switching power supply applications, as well as high-temperature switching power supply applications.
SiC MOSFETs are generally available within the range of 650 V < BVDSS < 1.7 kV, and although a SiC MOSFET’s dynamic switching behavior is very similar to standard silicon MOSFETs, the unique gate drive requirements determined by its device characteristics must be taken into consideration.
SiC has great advantages in fast DC charging applications
Taking the direct current fast charging (DCFC) electric vehicle (EV) charging application as an example, the current market demand for shorter charging times has led to the need for quick charging of high-power EVs close to 400 kW. This application requires active rectifier three-phase PFC boost topologies and three-phase power factor correction (PFC) systems, which can also be called an active rectification or active front-end systems, and the demand has increased dramatically in recent years.
Three-phase power factor correction (PFC) topologies are key to effectively supplying fast DC charging. By incorporating SiC power semiconductors into a three-phase PFC topologies, the normally conflicting challenge of reducing power losses and increasing power density can be solved. Front-end three-phase PFC boost stage can be implemented in multiple topologies, and multiple topologies can meet the same electrical requirements.
Another important factor affecting power device design and rated voltage is the number of levels in the architecture. The 6-switch topology is a 2-level architecture that typically uses 900 V or 1200 V switches for DCFC EV chargers. SiC MOSFET modules with low RDS(ON) (6-40 m) are suitable for higher power ranges above 15 kW per block. This integrated solution exhibits superior power performance over discrete solutions, improving efficiency, simplifying the design, reducing overall system size, and maximizing reliability.
Starting at 15 kW, the full bridge is a very suitable and common solution in the DC-DC power conversion level using the SiC modules, and enabling higher frequencies helps reduce the size of transformers and inductors, thereby reducing the form factor of the complete solution.
SiC MOSFET with 1200 V rated voltage supported
onsemi’s M1 SiC MOSFET has a rated voltage of 1200 V and a maximum zero-gate-voltage drain current (IDSS) specified in the datasheet for each specific device. However, a SiC MOSFET’s blocking voltage capability decreases as the temperature rises. Taking the 1200 V 20 m SiC MOSFET power module as an example, the typical derating in blocking voltage (VDS) at -40°C is about 11% compared to the value at 25°C. Usually, onsemi’s devices have some margin, especially when they will run at extremely low temperatures, and a VDS drop should also be considered during design.
One of the key differences of SiC MOSFETs compared to Si relatives products is the correlation between the drain to source voltage (VDS) and the gate source voltage (VGS) of a particular drain current (ID), and there is no exception in this 1200 V SiC MOSFET from onsemi. Traditional Si MOSFETs shows a clear transition between linear (ohmic) and active region (saturation). On the other hand, SiC MOSFETs do not have this problem. Actually, they have no saturation region, which means that SiC MOSFETs behave more like a variable resistance rather than a non-ideal current source.
An important aspect to consider when selecting an appropriate VGS is that SiC MOSFETs will continue to show significant improvements in RDS(ON) even at relatively high voltages compared to Si, so most Si MOSFETs are usually driven by VGS ≤ 10 V. If a Si MOSFET is replaced with SiC, it is recommended to modify the driving voltage, and although 10 V is above the typical threshold voltage of SiC MOSFETs, conduction loss at such a low VGS is likely to result in thermal runaway of the device, so it is recommended that VGS ≥ 18 V be used to drive the 1200 V M1 SiC MOSFET from onsemi.
SiC MOSFET power module with low thermal resistance
onsemi’s NXH020F120MNF1 is an M1 SiC MOSFET power module containing a 20 mohm/1200V SiC MOSFET full bridge and an NTC thermistor in F1 module. NXH020F120MNF1 has recommended gate voltage of 18V – 20V, a 4-PACK full bridge topology with low thermal resistance, and an option with or without pre-coated thermal interface material (TIM), supports Press-Fit pins, and is a Pb-free, halide free device and is RoHS compliant.
NXH020F120MNF1 improves RDS(ON) at higher voltages, improves efficiency or power density, is a flexible solution for high-reliability thermal interfaces, and can be widely used in solar inverters, uninterruptible power supplies (UPS), electric vehicle charging stations, industrial power supply, AC-DC conversion, DC-AC conversion, DC-DC conversion. Common end products include electric vehicle chargers, energy storage system (ESS), three-phase solar inverters, uninterruptible power supplies, etc.
Conclusion
More and more power applications are moving towards higher power, especially fast charging applications for EVs and ESS, which need to save precious charging time and improve charging efficiency. SiC MOSFETs are an ideal solution for fast charging applications. onsemi’s 1200V SiC MOSFET power module will provide better charging efficiency and higher power density, and will be the ideal choice for related high-voltage and high-current applications.
Courtesy: Arrow Electronics
