(1) Capacitors

Capacitors are an irreplaceable component in the power electronics, and they need to be selected carefully for converters, since capacitance, insulation resistance, reliability, and service lifetime will be reduced to some extent with the increase of temperature [51]. The temperature dependency of these characteristics is related to capacitor structures and materials. Among various dielectric materials, only a few are available for high-temperature operation, such as ceramic, mica, and tantalum [57]. Mica-10 shows prominent characteristics even at 200 ◦C with an energy density of 11.27 J/cm<sup>3</sup> and efficiency of 94.7% at 500 MV/m, which is 30 times higher than the well-known polymer PI for high-temperature applications [58]. The tantalum capacitor is one option for high-temperature applications, and it can reliably work at elevated temperatures above 200 ◦C [59]. Considering the low capacitance of the mica capacitor and low voltage ratings of the tantalum capacitor, the ceramic capacitor is a preferable option for high-voltage and high-power power electronics.

C0G and X7R ceramic capacitors are rated for high-temperature operation. The capacitance of C0G decreases by 1% when the temperature varies from 25 ◦C to 250 ◦C. Although C0G capacitors present excellent temperature stability, capacitances are too low to meet the requirement of the high-power system. For X7R capacitors, capacitance is in the range of microfarad, which is almost an order of magnitude larger than C0G capacitors. However, the X7R capacitors present wicked capacitance stability, and the capacitance will be reduced 42% from 25 ◦C to 250 ◦C coming along with an excessive capacitance variation with operating voltage [60]. Moreover, the insulation resistance is only 210 kΩ at 200 ◦C, which is far below the 3 GΩ of C0G capacitors. This makes X7R capacitors unsuitable for most signal processing applications.

Alternatively, the stacked ceramic capacitors with XHT dielectric from Presidio components, Inc. present improved capacitance stability when DC bus voltage is very high, as shown in Figure 7. A capacitance reduction of 50% from 25 ◦C to 175 ◦C is quantified. This kind of capacitors is more to cracks when used under the conditions of shock and vibration. Thus, the de-rating and reliability should be considered when designing capacitors for a high-temperature converter. With the new materials and fabrication technologies, Si capacitors exhibit the promising temperature resistance up to 250 ◦C [60]. Table 6 shows some commercially manufactured capacitors for high-temperature applications, and few comments are also made in this table.

**Figure 7.** Capacitance stability with temperature and DC link voltage for the XHT stacked ceramic capacitors from Presidio.

**Table 6.** Commercially manufactured capacitors for high-temperature applications.

