*3.2. SiC-based Rectifier Unit*

The DC secondary power source is necessary for more electric aircraft in which the main power supply is an AC power source [67]. Here, the transformer rectifier or three-phase AC–DC converter can be used for rectifying 115 V AC bus voltage into 270 V DC, which supplies power to nitrogen generator system, engine starting system, environmental control compressor, hydraulic pump and other loads [68]. In a diode rectifier circuit, SiC-SBD with little reverse recovery current is used to reduce the switching losses, heat radiation requirements and high-temperature limitations for aviation rectifier unit. NATO is planning that SiC power electronic devices are applied to meet needs for converters to operate in a high-temperature environment in all-electric combat vehicles. Then, a 30 kW rectifier bridge is designed for the automobile application with 200 ◦C operation temperature.

For large-power high-temperature converters, the efficiency requirement is especially crucial except for reliability requirement in a harsh environment. The high-temperature converter with SiC power electronic devices can have a significant increase in efficiency due to the substantial reduction of switching losses. Figure 11 shows the main types of losses in a voltage doubling rectifying circuit and the efficiency comparison of the rectifier with Si IGBT and SiC MOSFET in same voltage level and control mode. The efficiency of SiC devices rectifier is 2.6% higher than that of rectifier equipped with Si devices. In addition, superior electric characteristics with low on-resistance at high-temperature and simultaneous high-speed switching conditions are also come with SiC devices rectifier.

Figure 12 shows the hardware configuration diagram of the high-temperature three-phase AC–DC converter with the integration of 1 SiC-JFET and 7 SiC-SBDs, which can be applied to more electric aircrafts with the 270 V DC output, and it can also operate reliably in 200 ◦C ambient environments. Each component in the rectifier system should be carefully designed. Regarding SiC power modules, the mismatch of parasitic parameters, the variance of device property, and nonidentity of junction temperature can result in the unbalanced electro-thermal stress. The imbalance of multichip in parallel can be reduced by optimizing the layout of a direct bonding base, adding the low series resistance and coupling inductance, designing the feedback control of the drive circuit [69]. Here, the SiC power module using the flat-packaged structure has advantages of small parasitic parameters, flexible line layout and, double side cooling characteristics. In switching individual tests, the parasitic inductance is reduced by 14 nH compared to the pin-packaged structure, which makes the drain-source spike voltage decrease from 295.7 V to 279 V and further lowers switching losses [70]. In temperature detection, the temperature rise of grid resistance and the voltage regulator is apparent, but the temperature rise is not more than 10 ◦C from the environment temperature, which benefits from the lower power losses and excellent heat-sinking capability of the high-temperature AC–DC converter.

**Figure 11.** Loss and efficiency comparison of the rectifier with Si IGBT and SiC MOSFET.

**Figure 12.** Hardware configuration diagram of the high-temperature three-phase AC–DC converter.

In [71], Virginia Tech developed a 15 kW 650 V dc/230 V ac three-phase rectifier with interleaving structure by substitution of all Si devices with SiC JFETs, and the SiC power modules in the rectifier can operate at the junction temperature of 250 ◦C. With the volumetric power density of 6.3 kW/L, it successfully achieved a 2 kW/L target in more electric aircraft. It gives a detailed design for each component, including the active component, passive component, and the system. Figure 13 shows the drawing and the prototype of the rectifier system.

**Figure 13.** Conceptual drawing and the assembled prototype rectifier system.

#### *3.3. SiC-based DC–DC Converter*

DC–DC converters are widely used in the electric vehicle, more electric aircraft, and renewable energy. For example, high power HEV, the electric drive system is made up of a storage battery, DC–DC converter, inverter, electric machine, and control circuit, in which DC–DC converter plays a role in boosting the DC voltage for the post inverter unit. The permissible operating temperature for main components in HEV is listed as 120 ◦C for the motor, 200 ◦C for a turbocharger, 200 ◦C for throttle, 145 ◦C for the gearbox, 175 ◦C for driving chain, and 650 ◦C for exhaust pipe [72]. Due to the electrification and electromechanical integration of HEV, as well as the limitation of self-cooling capacity, these harsh conditions give power converters characteristics of reliability when operating in a high-temperature environment. Another example is when the DC–DC converter is applied to the generator control unit (GCU) in more electric aircrafts. The converter provides di fferent levels of DC voltage for the various functional modules of GCU. The high voltage levels reach up to tens of thousands of volts, which are mainly used for communication, radars, transmitters of electronic warfare equipment, and a variety of cathode ray tube displays. The low voltage levels are classified as 24 VDC, ±12 VDC, ±5 VDC, ±6.3 VDC, and ±3.3 VDC. Due to the development tendency of the control system from traditional centralized engine mode to the distributed mode, the GCU would be placed closer to the generator, which makes converters operate in a harsh environment (– 55 to + 200 ◦C).

Figure 14 shows the circuit diagram of the boost DC–DC converter with SiC-MOSFET and SiC-SBD. Usually, Si-MOSFET has a large junction capacitance, which defines the switching frequency up to 100 kHz. Si-IGBT has current lag which limits the switching frequency up to 30 kHz. SiC-MOSFET can operate at a switching frequency of 200 kHz, or even MHz-level [73].

**Figure 14.** A topology of the boost DC–DC converter.

For Si power electronic devices, switching losses increase significantly with the increase of switching frequency. When it mentions SiC power electronic devices, the switching losses, and junction temperature are measured as shown in Figure 15, where the switching frequency of SiC-MOSFET changes from 100 kHz to 800 kHz in a 1 kW boost DC–DC converter [74]. It can be seen that both switching loss and junction temperature keep a linear relationship with switching frequency when the high-temperature and thinner layer solder is used for die attach. If the solder with a low melting point of 180 ◦C and low thermal conductivity is adopted for the die attach, the junction temperature can rise at an accelerating rate when the switching frequency is above 500 kHz. NASA has reported that a 100 kW DC/DC converter based SiC JFET can reach up to the operating temperature of 415 ◦C in 2006. In 2008, Mazumder [75] reported that the e fficiency of a DC/DC converter based on SiC-JFETs can reach up to 95% at 20 ◦C, while the e fficiency of 100 V/270 V 2 kW boost converter proposed by Kosai [76] can reach up to 90% at the temperature 200 ◦C, the design and performance of the boost converter were evaluated over the temperature range from 20 ◦C to 200 ◦C. The capacitance variation of the output filter is also presented in [74], reporting that a 1 kW all-SiC boost converter with the output voltage of 800V can work reliably over a switching frequency range of 100 to 800 kHz, and the steady-state working junction temperature of SiC MOSFETs has been extended to 320 ◦C. However, the high-frequency gate drive capability and high-temperature die-attachment technology can be the issues to develop SiC-based converter operating beyond 320 ◦C junction temperature.

**Figure 15.** Switching loss and junction temperature distribution diagram under different frequencies.

## *3.4. SiC-based MEMS Devices*

The excellent mechanical properties of SiC material, coupled with the good thermal stability at high operating temperature, offer new possibilities for developing MEMS devices for extremely harsh applications compared to those possible with Si devices.

The measurement and control technologies are also required for the high-temperature converters, and SiC devices allow the functionally integrated circuits (ICs) to operate in extreme environments. Many researchers have been working on the high-temperature ICs implemented by using SiC CMOS, JFET, and BJT, and developing the digital logic circuits, operational amplifiers, and memories. Reference [21] demonstrates that SiC makes high-temperature electronics possible up to 600 ◦C, reviewed the current technology performance and processing challenges relating to making ICs in SiC, and addressed that SiC devices should be commercially available in increasing quantities going forward, although the technology choice is unclear.

The first SiC-based power ICs were reported in 2008 [77]. Figure 16 shows the optical photo for 4H-SiC power integrated circuit after packaging, which includes a large power JFET and two buffer circuits. Reference [78] proposes an integrated bipolar OR/NOR gate based on 4H-SiC BJTs, and it can successfully operate up to 500 ◦C. References [79] and [80] report the differential amplifiers based on 4H-SiC JFET and 6H-SiC bipolar can reach up to the temperature of 500 ◦C and 600 ◦C, respectively. In [81], a 500 ◦C Schmitt trigger in 4H-SiC has designed and characterized, the proposed Schmitt trigger shows superior characteristics with a higher slew rate and almost independent temperature operation.

**Figure 16.** An optical photo for 4H-SiC power integrated circuit after packaging.

A linear voltage regulator based on the nMOS SiC has been successfully designed and tested under at 300 ◦C [82]. In [83], a bipolar SiC linear voltage regulator was developed to operate at 500 ◦C. Regarding the ICs structure, in [84], the authors propose a novel 4H-SiC lateral BJT design with symmetric and self-aligned structure, the simulation, and optimization are conducted to operate at the temperature range of 27–500 ◦C with an optimal current gain. is the study demonstrates that the self-aligned 4H-SiC lateral BJTs design is easier and less costly to produce, with >90% smaller than a conventional structure.

A monolithic SiC drive circuit for SiC BJTs was designed by Kargarrazi et al [85]. The performance was tested using a commercial power BJT under the resistive and capacitive conditions with the operating switching frequency up to 500 kHz. The SiC drive circuit has a good robust capability to the temperature range from 25 ◦C to 500 ◦C. In their latest publication, the controlled duty cycles from 0.5 to 0.7 are demonstrated with the operating frequency ranges from 160 to 210 kHz [86].

MEMS switches have mainly been developed in a broad swath of RF and microwave applications, and they could possibly replace positive-intrinsic-negative (PIN) diode, mechanical, FET, and other types of switches [87]. When compared to traditional micromechanical switches, MEMS switches have several advantages, such as lower insertion loss, higher isolation, and better switching figure-of-merit. They are widely used to measure oil pressure, fuel pressure and tire pressure in automotive applications, electronics, and telecom. The Foxboro is the first company who is involved in MEMS switches with the invention of the first electromechanical switch patent in the world in 1984. Analog Devices, Inc. has been started the research of MEMS switches since 1990, with the first MEMS accelerometer product successfully launched in 1991, and the first integrated MEMS gyroscope was released in 2002. The newest MEMS products released by ADI are ADGM1304 and ADGM1004, the maximum operating frequency can reach up to 14 GHz and 13 GHz, respectively. The operation temperature ranges from 0 to 85 ◦C, with the peak reflow soldering temperature of 260 ◦C. In [22], it is shown that the SiC MEMS devices are well-developed for temperatures up to 500 ◦C for the sensing of motion acceleration and gas flow. The digital micro switches can also be used for wireless power transfer, but the maximum operating temperature has not been reported. Reference [88] reported nanoelectromechanical system switches based inverter can operate at the temperature as high as 500 ◦C with ultralow leakage current, and this achievement has created a pathway toward energy-e fficient high-temperature computation.

#### **4. Challenges in High-Temperature Power Electronics**

#### *4.1. Design of High-temperature Gate Drives*

Gate drives play an important role in the interface the control circuit to SiC-based devices, determining the performance of power electronics devices. Although SiC-based devices have high-speed switching capability, the drive circuit should also be matched to make full use of high-speed switching capability. Therefore, the SiC high-temperature drive circuit cannot follow the drive circuit based on conventional Si devices. Since the parasitic capacitance of a similarly-sized silicon carbide device is much lower than that of Si-based devices. A tradeo ff should be taken between the component's layout and the high-speed capability, so the drive circuit should be placed at a certain distance to the power electronic devices. This will increase the gate loop and introduce larger parasitic parameters, which decrease the high-speed switching capability of the SiC-based devices in practical application. Then, the switching frequency will be limited under this condition.

The performance of SiC power electronic devices might be degraded when employed in high-temperature condition. By testing, the threshold voltage of 1.2 kV SiC MOSFET devices at 200 ◦C is reduced to 2/3 under normal temperature, while the on-resistance increases to 2–3 times. The reduction of threshold voltage makes the crosstalk occur more easily in the bridge-arm circuit [89]. The method of gate negative bias voltage is adapted to suppress the crosstalk, in which the negative bias voltage usually arrives at −5 V and the minimum value is −9 V. Furthermore, the active miller clamping circuit is designed to avoid the bridge-arm shoot-through. On the other hand, the gate drive

board is allowed for a high-temperature environment only if the SOI die, PCB, passive components, packaging, as well as the input signal isolator, can endure the high temperature. This is expected to reduce the volume of easy chip-level integration, eliminate the high-temperature ageing e ffects of materials, and reduce the impact of parasitic parameters.

#### *4.2. Current Measurement in High Temperature*

The current divider and Hall sensor are usually employed to measure the current for converter control; however, they are challenging to work in the high-temperature environment. To tackle this problem, the saturated current sensor is developed for high-temperature application. While the B/H curve of magnetic materials can drift with the temperature variation, which will lead to a significant measurement error for the current sensor, a compensation algorithm or new measurement method is proposed for the high measurement accuracy. For example, the isolated DC and AC current measurement method based on a bidirectional saturated current transformer (Figure 17) can be applied to the high-temperature converter with SiC devices, which can suppress the e ffect of the coercive force of magnetic materials on detection precision [90].

**Figure 17.** Fast high temperature isolated DC/AC current measurement. (**a**) Photograph of the bidirectional saturated current transformer, and (**b**) PCB circuit.

#### *4.3. Parameters Matching within Wide Range of Temperature*

The parameters matching over a wide temperature range is also an issue to design the high-temperature power electronics. The performance of the SiC components will degrade under the wide temperature cycling and the high operating temperature. With the increase of the operating temperature, the junction capacitance of SiC devices is decreased, and the switching speed is increased. The performance of passive components, such as capacitors and resistors, is also reduced, and the withstand voltage, capacitance values, and resistance values are only about 40% to 50% of normal temperature. In addition, the coe fficient of thermal expansion between adjacent components should be similar; otherwise excessive mechanical stress will cause damage to the device.

Since SiC material defects and brittleness limit the size of the wafer to a small value of 4 inches in general use, the maximum current of a single chip is about 100 A. The cost of SiC chip grows exponentially with the chip current. For the application of high-power converters, the multichip power module is a cost-e fficiency solution. Among paralleled chips, the parameter mismatching due to the parasitic parameters can result in the uneven electro-thermal stress. For all kinds of active and passive components, their temperature stability di fference can result in the unmatched electric parameters and unbalanced mechanical stress, which significantly influence the performance and reliability of high-temperature converters.
