Why SiC and GaN power devices can play a leading role in the electronics industry? Here are the reasons
Advantages of WBG Semiconductors
To review, the band gap is the energy required to excite an electron from a material’s valence band to its conduction band, and the band gap of WBG materials is significantly wider than that of silicon (Figure 1). Si has a band gap of 1.1 eV, while SiC has a band gap of 3.3 eV and GaN has a band gap of .4 eV.
Compared to traditional silicon semiconductors, WBG semiconductor devices can operate at higher voltages, frequencies and temperatures. More importantly, switching and conduction losses are lower. The conduction and switching characteristics of WBG materials are about ten times that of Si materials. These capabilities make WBG technology a natural fit for power electronics, especially in the EV world, because SiC and GaN components are smaller, faster in response and more energy efficient.
However, despite the advantages of WBG devices, designers have to weigh them against the complexity of manufacturing and the high cost of high-volume production. Although the initial cost of WBG components may be higher, it is generally decreasing and generally reduces the total system cost. For example, using SiC devices in EVs may add hundreds of dollars in additional upfront costs, but ultimately reduces the total cost due to reduced battery costs and space requirements and simplified cooling measures (such as using small heat sinks or convection cooling).
SiC used in main inverter
The traction inverter that controls the traction motor in an EV is an example of a critical EV system that can benefit from WBG components. The core function of the inverter is to convert DC voltage into a three-phase AC waveform to drive the EV motor, and then convert the AC voltage generated by regenerative braking back into DC voltage to charge the battery. Since the inverter converts energy stored in the battery pack into AC to drive the motor, the lower the energy conversion losses, the higher the energy efficiency of the system. Compared to silicon, SiC devices have greater conductivity and faster switching frequencies, resulting in lower power consumption because less energy is lost as heat. Ultimately, SiC inverters are more energy efficient, which is reflected in the longer range of EVs.
High current power modules are usually IGBT type, which combines Si IGBT with Si fast recovery diode (FRD), which is a common configuration for automotive inverter modules. However, compared with existing Si IGBT devices, SiC devices operate at higher temperatures and switch faster. These features undoubtedly make it the best choice for traction inverters, because traction inverters need to transfer large amounts of energy into and out of the battery.
Here’s why: Since IGBTs are switching elements, switching speed (on time, off time) is one of the key parameters that affect energy efficiency (loss). For IGBTs, low resistance at high breakdown voltage is achieved at the expense of switching performance; there is “dissipation time” during device shutdown, which increases switching losses. Therefore, IGBTs have relatively low energy efficiency. If MOSFETs are used instead of IGBTs in the inverter module, higher energy efficiency can be achieved because MOSFETs have shorter off times and higher operating frequencies. However, Si MOSFETs also have problems, and their “on” resistance is greater than that of Si IGBTs.
SiC MOSFET takes full advantage of the favorable characteristics of SiC. The chip size is almost half that of IGBT, while having four ideal characteristics of power switches:
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high voltage
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Low on-resistance
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Fast switching speed
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Low switching losses (especially turn-off losses)
Additionally, the wider bandgap means that SiC devices typically operate in the 150°C to 175°C range, and can reach 200°C or higher if packaged properly.
For SiC Schottky barrier diodes (SBDs), SiC semiconductor-metal junctions are used to form Schottky barriers in SiC SBDs. However, unlike silicon FRDs, the advantages of SiC SBDs do not change significantly over a wide range of current and operating temperature. In addition, the dielectric breakdown field of SiC components is also ten times that of silicon devices. Therefore, SiC products with a rated voltage of 1200V are currently being put into mass production, and the cost has dropped accordingly. In addition, products with a rated voltage of 1700V are under development.
SiC diodes also have no forward and reverse recovery losses, only a small amount of capacitor charging losses. Studies have shown that the switching losses of SiC SBDs are 90% lower than those of Si fast recovery diodes, where the junction temperature affects the recovery current and recovery time. Therefore, the quality factor (FoM) (Qc x Vf) of SiC diodes is quite low compared to Si diodes. Lower FoM means lower power consumption and thus better electrical performance.
Silicon carbide has some disadvantages. One of them is that it has a positive thermal coefficient, meaning that the higher the temperature, the higher the forward voltage (Vf). The higher the current through the diode, the higher the forward voltage. When the diode is subjected to high current, this conduction loss can lead to thermal breakdown.
However, combining SiC MOSFETs with SBDs enables system designers to improve energy efficiency, reduce the size and cost of heat sinks, and increase switching frequency to reduce the size of magnetic components, thereby reducing the final design cost, size and weight. Compared with Si-based devices, EV inverters using SiC devices can be 5 times smaller, 3 times lighter, and consume 50% less power.
For example, the BSM300D12P2E001 half-bridge SiC power module developed by ROHM Semiconductor integrates SiC MOSFETs with SiC SBDs in an integrated package, minimizing switching losses previously caused by IGBT tail current and FRD recovery losses (Figure 2).
Compared with IGBT, ROHM Semiconductor's SiC-based MOSFET has significantly reduced losses by 73%. The company's MOSFET series has a withstand voltage of up to 1700V, an on-resistance range of 45mΩ to 1150mΩ , and is available in TO-247N, TO-3PFM, TO-268-L, and TO-220 packages.
In addition, the SiC Schottky barrier diode launched by ROHM has passed the AEC-Q101 automotive grade standard. The device has a short recovery time, fast switching speed, low temperature dependence, low forward voltage, and can withstand a voltage of 650V and a current range of 6 to 20A.
The role of SiC devices in EV applications
Tesla, the first electric car manufacturer to integrate a full SiC power module in the main inverter, has adopted this technology in the Tesla Model 3 sedan. Previous Tesla models, such as the S and X models, all use IGBTs in TO-247 packages. Tesla worked with STMicroelectronics to assemble the SiC power module on the inverter's heat sink. Like STMicroelectronics' SCT10N120 , this MOSFET is rated at 650V and uses a copper base plate for heat dissipation.
The charging equipment for EVs is factory-installed and is called an “on-board charger” (OBC). The OBC for an EV or plug-in hybrid EV (PHEV) charges the battery from AC power, either at home or at a personal or public charging station. The OBC uses an AC/DC converter to convert 50/60Hz AC voltage (100 to 240V) to DC voltage to charge the high-voltage traction battery (typically around 400V DC). It also adjusts the DC level to the battery requirements, provides galvanic isolation, and AC/DC power factor correction (PFC) (Figure 3).
GaN is widely favored due to its high energy efficiency
The design of OBC requires maximizing energy efficiency and reliability to ensure fast charging while meeting the space and weight restrictions of EV manufacturers. OBC design using GaN technology can simplify EV cooling systems, shorten charging time, and reduce power consumption. In terms of automotive market share, commercial GaN power devices are slightly inferior to SiC devices, but are now rapidly gaining ground with their excellent performance. Like SiC devices, GaN devices have lower switching losses, faster switching speeds, higher power density, and can reduce system size and weight, reducing total cost.
For example, Transphorm’s TP65H035WSQA is qualified to the AEC-Q101 automotive standard. This GaN FET was qualified to 175°C (Figure 4). The device is housed in a standard TO-247 package and has a typical on-resistance of 35mΩ . Like its 49mΩ Gen-II TPH3205WSBQA predecessor , this device is suitable for AC/DC OBC, DC/DC converter, and DC/AC inverter systems in plug-in hybrid electric vehicles and battery EVs, enabling AC/DC totem pole bridgeless PFC designs.
Figure 4: Transphorm’s TPH3205WSBQA650V, 49mΩ GaN FET has passed automotive-grade qualification and AEC-Q101 stress testing for automotive-grade discrete semiconductor devices. (Image source: Transphorm)
While Si MOSFETs typically have a maximum dV/dt rating of 50V/ns, the TP65H035WS GaN FET switches with dV/dt of 100V/ns or more, minimizing switching losses. In this case, even the layout can significantly affect system performance. When laying out, it is recommended to minimize the gate drive loop, shorten the trace length between the switch nodes, and connect the power bus to ground with the shortest practical return trace. The cross-sectional area of the power ground plane should be large to achieve a uniform ground potential throughout the circuit. When laying out, be sure to separate the power ground and the IC (small signal) ground, connecting them only at the source pin of the FET to avoid any possible ground loops.
Infineon’s AIDW20S65C5XKSA1 is part of the company’s fifth-generation CoolSiC automotive Schottky diode family, also developed for OBC applications in hybrid and electric vehicles. It complements the company’s IGBT and CoolMOS product lines and meets the requirements of 650V-class automotive applications.
Thanks to the new passivation layer concept, this product is one of the most durable automotive devices on the market, resistant to moisture and corrosion. The device is based on 110μm thin wafer technology, so the FoM performance is outstanding in its class, which is reflected in lower power consumption and thus better electrical performance.
Infineon’s CoolSiC Automotive Schottky Diodes improve OBC efficiency by one percentage point under all load conditions compared to conventional Si FRDs.
Using SiC and GaN devices
In addition to the careful layout mentioned above, another potential problem with SiC components is that the driving requirements are very different from those of IGBT devices. While most transistors are typically driven using symmetrical power rails (such as ±5V), SiC devices require a small negative voltage to ensure complete shutdown, so an asymmetrical power rail (such as -1V to -20V) is required.
In addition, although SiC has excellent heat dissipation characteristics and outstanding thermal conductivity compared to silicon, SiC components often use packaging designed for Si devices, such as chip bonding and wire bonding. Although this packaging method works well with SiC, it is only suitable for low-frequency circuits (tens of kilohertz). Once applied to high-frequency circuits, parasitic capacitance and inductance will increase accordingly, thus hindering the full potential of SiC-based devices.
Likewise, to fully exploit the benefits of GaN devices, the packaging must have very low parasitic inductance and excellent thermal performance. New packaging approaches, such as embedded die packaging (similar to multilayer printed circuit boards), achieve the required performance at a low cost while also eliminating wire bonding to avoid device reliability issues.
The gate driver is a key component that acts as an interface between the controller and the power device. Gate drive design is always a challenge for electronic designers using new devices, so it is important to understand how SiC and GaN power devices are driven. The specific requirements are:
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High supply voltage and high efficiency through low conduction losses
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High drive strength for low switching losses
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Fast short circuit protection
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Small propagation delays and variations enable energy-efficient and fast system control
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High dv/dt immunity
Some early GaN devices required special drivers to prevent gate overvoltage. Now, a new generation of E-HEMTs with large Vg tolerance are available on the market, which can be driven by many standard MOSFET drivers by simply changing the gate voltage. GaN FETs are lateral devices, so the optimal drive voltage required is relatively low. In summary, the gate drive requirements of GaN devices are similar to those of SiMOSFETs and IGBTs. Specific requirements include:
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Lower gate charge - lower drive losses, faster rise and fall times
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Lower gate voltage
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Negative voltage to improve gate drive robustness
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Using gate resistors to control slew rate
The advantage is that many SiC and GaN solution suppliers have added other electronic components within the package, so it can be a direct replacement for current designs.
Summarize
To meet the energy efficiency and power density requirements of EV systems such as inverters and on-board chargers, automotive power electronics designers can now use more advanced WBG semiconductors such as SiC and GaN. Compared with traditional silicon devices, WBG semiconductors have lower losses, faster switching frequencies, higher operating temperatures, higher breakdown voltages, and are more rugged in harsh environments.
GaN and SiC can operate at higher temperatures with the same expected lifetime as Si devices, or operate at the same temperature with a longer lifetime. This provides design engineers with different design options, depending on the application requirements.
In addition, using WBG materials allows designers to choose from a variety of strategies to suit their design goals: use the same switching frequency and increase output power; use the same switching frequency and reduce the system's thermal requirements and total cost; or increase the switching frequency but keep the same switching power consumption.
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