- Heat sink

Generally, the heat sink is joined to the power module by silicone grease, and in this way, the heat generated by all kinds of losses dissipates to the air. The thermal resistance of silicone grease occupies 50% in the whole module, and thus thermal interface materials (TIM) become one of the critical factors to significantly reduce the radiator volume and increase converter power density [39]. Moreover, the grease will dry out and age as time goes on. Therefore, some innovative solutions to reduce the thermal resistance and optimize the power dissipation should be developed.

TIM research has focused on three primary areas of using high-performance fillers, studying the micro and nanoscale TIM, and developing carbon-allotropes-based TIM to enhance performance [40–42]. Carbon nanotube fillers are the most promising of all the carbon allotropes, and have become front research topics in TIM which can decrease thermal stress caused by CTE mismatch and can keep chemicals table over a wide range of temperatures.

The high-performance cooling methods are jet impingement cooling, spray cooling, and micro channels, and all of them can be designed as two-phase flow with the increased heat transport capability [43,44]. A model of single and two-phase liquid cooling using micro channels was established for GaN-on-SiC field effect transistor (FET) [45]. Furthermore, an additional method is the direct-immersion cooling technique, where the coolant liquid is in direct contact with the power electronics device. However, the notable problems of coolant leakage and compatibility requirements result in preference for indirect cooling.

## *2.3. High-temperature Gate Drives*

In ambient temperatures above 150 ◦C, a real challenge is to seek a proper gate drive circuit to match with attributions of heat resistance and high frequency of SiC power electronic devices. In theory, the junction temperature of SiC power electronic devices can reach up to 600 ◦C, which are commercially available for high-temperature applications. However, the integrated drive circuits for SiC devices are still in an early stage of research and development.

Based on the silicon-on-insulator (SOI) technology, a buried insulator layer in SOI structure shown in Figure 5 can effectively reduce the leakage current in high-temperature operation, improve the latch-up immunity, and suppress the threshold voltage variation to the temperature. The SOI-based integrated circuits can successfully operate at the temperature range from 200 ◦C to 300 ◦C [46], which is much higher than that of conventional bulk Si devices.

**Figure 5.** A buried insulator layer in a fully depleted SOI structure.

Compared to the SOI-based integrated circuit technology, the SiC-based integrated circuit technology is more attractive to extremely harsh environment [47]. However, the development of semiconductor technology presents many difficulties. Researchers have spent nearly half a century producing the first commercial SiC devices since the excellent characteristics of SiC material were discovered. However, the SiC-based integrated circuit remains at the stage of laboratory investigation, and is therefore unable to access the market in a short time. Regarding high-temperature gate drives of SiC devices, SiC-based integrated circuit technology is not mature enough yet, and Si-based integrated circuit technology is unavailable for high-temperature applications. Instead, Si-based discrete device technology may be a feasible solution when compensating temperature drift. In addition, the SOI-based integrated circuit is available for high-temperature applications when ignoring the high manufacturing cost [48].

High-temperature gate drives are required to have the input signal isolation, wide duty cycle range, static turn on/off, and low propagation delay time. Currently, no commercial opt couplers can be used in a high-temperature condition. As a result, the transformer becomes the best choice for high-temperature isolation of the gate drive. For a developed SiC MOSFET phase-leg power module, an input signal isolator is used to build an SOI gate drive chip. Even though SOI die, printed circuit board (PCB) and passive components can accommodate the high-temperature environment above 150 ◦C, but the SOI gate drive board cannot withstand such temperature. Its bottleneck is the commercial isolator, whose maximum operating temperature is 105 ◦C. Air-core transformer isolators for the SOI-based integrated circuit are under active study, with the hope to reduce the volume of natural chip-level integration and to eliminate the high-temperature ageing effects of magnetic materials [49].

As the SOI technology is well developed, companies, such as CISSOID, XREL, and Honeywell, have developed the high-temperature gate drive circuits. CISSOID provides an isolated gate drive circuit integrated with high-temperature SiC MOSFET devices in the shape of the intelligent power module (IPM), which dramatically reduces the effect of the parasitic parameters and allows the maximum ambient temperature up to 225 ◦C [50]. The gate drives manufactured by XREL can operate at the ambient temperature of 230 ◦C. The SOI technology can also be used to develop high-temperature drive ICs, but the manufacturing process is complicated and associated with the high cost. The gate drive ICs based on SiC technology can operate reliably at the temperature of 400 ◦C, and it has been reported in [51–54]. However, this technology is not mature and commercialized yet.

Due to the high cost of SOI ICs and SiC fabrication technologies, an alternative way to develop high-temperature gate drives is the utilization of commercial-off-the-shelf (COTS) discrete transistors and diodes. With the COTS discrete component, the operating temperature of gate drives can reach up to 180–200 ◦C [55]. The main drawback of this kind of gate drives is the large propagation delay due to the high number of SOI ICs, and the protection features of desaturation and under voltage lockout (UVLO) are not included. To solve these issues, a COTS gate drive is proposed with the integrated circuit of overcurrent and UVLO, which can operate under the ambient temperature of 180 ◦C [56]. The proposed COTS gate drive shows better performance than the commercial gate drive (EVK-HADES 1210) produced by CISSOID in the aspects of propagation delay and total power consumption. Figure 6 shows the prototype of the COTS gate drive and the test bench. In Figure 6a, the numbers shown from 1 to 5 represent on-state voltage monitoring diode, driving buffer stage, overcurrent detection circuit, UVLO with control logic, and isolated transformer, respectively.

## *2.4. High-temperature Passive Components*
