The Green Initiative continues to drive the transformation of power electronic system design in industrial, aerospace, and defense applications, particularly in the transportation industry. Silicon carbide (SiC) is the core technology leading this trend, providing a variety of new functions to continuously promote the electrification of various vehicles and aircraft, thereby reducing greenhouse gas (GHG) emissions.
Silicon carbide solutions support the replacement of aircraft's pneumatic and hydraulic systems with smaller, lighter, and more efficient electrical solutions to power onboard AC generators, actuators, and auxiliary power units (APU). This type of solution can also reduce the maintenance requirements of these systems. However, the most significant contribution of SiC technology lies in its mission to electrify commercial transport vehicles, which are one of the world's largest sources of GHG emissions. With the advent of 1700V metal oxide semiconductor field-effect transistors (MOSFETs) and configurable digital gate drive technology, today's SiC solutions enable designers to generate maximum productivity with minimal energy consumption for these systems.
Advantages of 1700V SiC MOSFET
After switching to 1700V MOSFETs, the power conversion advantages of SiC technology have expanded to the fields of electric commercial and heavy-duty vehicles, as well as light rail traction and auxiliary power. These devices support current and future automotive power systems and are rapidly replacing outdated silicon MOSFETs and insulated gate bipolar transistors (IGBTs). They can meet the high power and voltage demands of some of the world's largest sources of carbon dioxide (CO2) equivalent GHG emissions, including buses, rail vehicles, medium and heavy trucks, and charging infrastructure. Compared to silicon MOSFETs and IGBTs, these devices can also provide higher system efficiency and reliability, allowing designers to reduce the size of auxiliary power units (APU) and other critical vehicle systems.
The current 1700V SiC devices can significantly reduce switching losses, only a fraction of silicon IGBTs. In this way, designers can increase the switching frequency and reduce the size of the power converter. Unlike IGBTs, these devices do not have inflection point voltages, resulting in lower conduction losses for systems such as transport APUs (used for train doors that are mostly closed) operating under "light load conditions". The vast majority of applications operate under light load conditions during most of their lifespan, so designers can utilize the low switching and conduction loss combination of SiC MOSFETs to eliminate various thermal management measures such as heat sinks.
The current high-voltage SiC MOSFET not only simplifies the circuit topology and reduces the number of components, but also improves reliability while reducing costs. This type of device has a blocking voltage of 1700V, which can reduce the size of power converters and enable designers to replace the three-level circuit architecture with a lower complexity secondary circuit. This helps to reduce the number of devices by half or even more, while simplifying the control logic.
Important considerations for SiC MOSFET
When choosing SiC MOSFETs suitable for heavy transport vehicles and other multi megawatt level applications, designers need to consider several important factors, including whether to use modular solutions based on unit cells (also known as power electronic components or submodules).
In the past, the power semiconductor devices used in unit cells were silicon IGBTs ranging from 1200V to 1700V. Similar to low-power applications, deploying 1700V SiC MOSFETs at the unit cell level can improve their power processing capability and electrical performance. As mentioned earlier, the switching loss of 1700V SiC MOSFET is much lower, so it can increase the switching frequency and significantly reduce the size of each unit cell. In addition, a high blocking voltage of 1700V can reduce the number of unit cells required to achieve the same DC link voltage, ultimately improving system reliability while reducing costs.
Designers should also evaluate the robustness of the intrinsic diode of SiC MOSFET. In the drain source conduction state resistance (RDSon) test before and after applying stress, the device should not show significant changes. This is crucial to ensure that they do not degrade after several hours of constant forward current stress, as the devices conduct reverse current and reverse all remaining energy after switching cycles. There are significant differences between devices supplied by different suppliers, so designers must carefully examine the SiC MOSFET test results. Many devices exhibit at least some degree of degradation, while others may even become unstable. If SiC MOSFETs that do not degrade are selected, there is no need for external anti parallel diodes, and related chip costs and power module space can be saved.
There may also be some challenges related to the performance of body diodes with varying degrees of potential inconsistency, depending on the device. This can be solved by adjusting the conduction parameters of SiC MOSFETs using configurable digital gate drivers. These drivers can also be used to mitigate the secondary effects of faster switching speeds in SiC MOSFETs, including noise and electromagnetic interference (EMI), as well as limited short-circuit withstand time and overvoltage caused by parasitic inductance and overheating. The configurable digital gate drive technology has become the key to fully leveraging the capabilities of SiC technology.
Solve design challenges while creating new business opportunities
The configurable digital gate driver is designed specifically to alleviate the secondary effects of faster switching speed in SiC MOSFETs. Compared with traditional simulation methods, in addition to reducing drain source voltage (VDS) overshoot by up to 80%, they can also reduce switch losses by up to 50% and shorten time to market by up to six months. These devices have a peak pull/sink current capability of up to 20A and are equipped with isolated DC/DC converters with low capacitance isolation layers, which can be used for pulse width modulation signals and fault feedback. In addition, they can achieve robust fault monitoring and detection while providing independent short-circuit response, enabling more accurate MOSFET on/off control compared to traditional analog gate drivers that only control the turn off ramp through gate resistance suitable for normal and short-circuit conditions. Even if standard analog gate drivers can be used with SiC MOSFETs after adjustment, they cannot provide these functions.
The configurable digital gate driver also adds enhanced switch functionality. This enables designers to explore various configurations and reuse them for different gate driver parameters (such as gate switch configuration files, system critical monitors, and controller interface settings), significantly reducing development time. Quickly customize gate drivers for various applications without any hardware changes, thereby shortening the development time from evaluation to production. During the design process, control parameters can be modified at any time, and designers can also modify the switch configuration file on-site based on application requirements and/or the degradation of SiC MOSFETs.
These enhanced switch functions are still being continuously improved. Compared with the single step control of traditional analog drivers, digital gate drivers can now provide up to two conduction control steps and up to three turn off control levels. This can achieve a 'soft landing' during the shutdown process, just like stepping on the brake of an anti lock braking system. Adding a fourth short-circuit setting level can more accurately control the secondary effects of SiC switch speed and solve the problems of variables such as overshoot, ringing, and turn off energy. By utilizing these features, designers can combine faster switching with finer dynamic multi-step on and off control to meet the growing demands of SiC applications.
Motor control is one example. If the voltage change rate (dV/dt) is too high, the expected service life of the motor will be shortened and the warranty cost will correspondingly increase. Before the emergence of higher frequency motors, reducing SiC switching speed was the only solution to the problem of analog gate drivers, but it would lower efficiency. Only with the configurable enhanced conduction function of digital gate drivers can dV/dt be fine tuned to quickly achieve the best compromise. Figure 1 summarizes the differences between analog gate drivers and the new generation of digital gate drivers.
Comparison of Traditional Analog Gate Driver and Two Generations of Configurable Digital Gate Driver Technologies
Complete solution
A comprehensive SiC ecosystem can meet various needs from assessment to production. The key components include gate driver core, module adapter board, SP6LI low inductance power module, installation hardware, as well as thermistor and DC voltage connector. Programming toolkits should be provided for configurable software.
The module adapter board is particularly important. They allow designers to quickly configure and reuse gate driver on/off voltages, thereby improving flexibility. This applies to SiC MOSFETs from many different suppliers, covering a wide range of positive and negative voltages without the need for any redesign. Even though SiC MOSFETs were previously used in conjunction with analog gate drivers. Simply reconfigure the digital gate driver, and designers can immediately put the solution into production. At the same time, they can continue to combine and match the gate driver core and module adapter board, and follow the same process to accelerate production. They can immediately start testing using the SP6LI low inductance power module and phase bridge arm connected to the laptop.
The combination of 1700V SiC MOSFET power management solutions and digital gate drive technology has had a huge impact on the electrification of everything, and more specifically, on heavy-duty transportation vehicles. This combination enables SiC technology to support the power conversion needs of such vehicles while improving efficiency and reliability. In addition, configurable digital gate drivers provide enhanced switching capabilities that help accelerate and simplify the entire process from design to production, while creating a range of new features, including on-site changes to switch configuration files based on application requirements and/or degradation of SiC MOSFETs.
After integrating SiC solutions into the entire system solution, power systems that meet current and future needs can be created, significantly reducing the size of APU in electrified subways and other heavy transport vehicles, thus freeing up more space to accommodate more paying passengers. One of the most popular advantages for designers is that using configurable digital gate drive technology for these devices eliminates the tedious process of soldering gate resistors to circuit boards to change behavioral parameters. Now, all these operations can be completed using buttons, which will help achieve "electrification of everything" faster.