*4.3. E*ffi*ciency Evaluation*

Figure 9a shows the efficiency dependencies versus the input power. It corresponded to the different irradiance levels of the PV-string. It means that different operation points corresponded to the different input voltages and input currents. Since PV inverters are not operating at a nominal power point constantly, the important characteristics of the PV inverter performance is the weighted CEC efficiency [46]. The value of the weighted CEC efficiency was obtained by assigning a probable percentage of time the inverter resides at a certain operating point. If we denote the efficiency at 50% of the nominal power by "Eff50%", the average EU (European) and CEC efficiency values weighted appropriately are defined as:

$$\eta\_{\rm EI} = 0.03\text{-Eff}\% + 0.06\text{-Eff}10\% + 0.13\text{-Eff}20\% + 0.10\text{-Eff}0\% + 0.48\text{-Eff}0\% + 0.20\text{-Eff}00\%,\qquad(6)$$

$$\eta\_{\rm EC} = 0.04\text{-Eff}10\% + 0.05\text{-Eff}20\% + 0.12\text{-Eff}0\% + 0.21\text{-Eff}0\% + 0.53\text{-Eff}5\% + 0.05\text{-Eff}10\%,\qquad(7)$$

The CEC efficiency measurement and calculation results corresponding to Figure 9a are shown in Table 4. For both inverters, it exceeds 96%. However, for the 2L QZSI it supersedes by 0.4%. The EU efficiency was also measured and for both converters it exceeded 95%.

**Figure 9.** Efficiency evaluation of the 2L QZSI and the 3L NPC QZSI under different solar irradiance (power) levels (**a**) and within input voltage operating range (**b**).


**Table 4.** The CEC efficiency of 2L QZSI and 3L NPC QZSI.

It should be mentioned that efficiency curves for the 2L QZSI and the 3L NPC QZSI are characterized by different shapes. Figure 9b shows efficiency dependence versus different input voltage. This case illustrates the efficiency of the converters with different numbers of PV panels or at shadowed conditions. It can be seen that the 2L solution had higher peak efficiency, which corresponds to the nominal operation point, but more significant efficiency decrease occurred at low input voltage. In general, for the zero ST duty cycle, the 2L QZSI demonstrated 0.4 ... 2.3% higher efficiency than the 3L NPC QZSI. The situation changed when a non-zero ST duty cycle was utilized. It can be explained by higher conduction losses in SiC transistors, which take force under higher current rates in ST mode.

#### *4.4. Evaluation of Temperature Behavior of Semiconductors and Heatsinks*

The temperature of semiconductor devices and heatsinks was controlled by an infrared thermal camera Fluke Ti10. The results are presented in Figures 10 and 11. It should be mentioned that clamping diodes and inverter power switches in the 3L NPC QZSI had four common heatsinks, each one intended for two MOSFETs and one clamping diode. One more heatsink was used for two QZS-stage diodes, as can be seen from Figure 5. At the same time, two heatsinks were used in the 2L QZSI for inverter switches and one heatsink for the QZS-stage diode.

Figure 10a shows the temperature of the QZS-stage diode and its heatsink in the 2L QZSI under different power levels. These points correspond to Figure 9a. It can be seen that the diode and the heatsink temperature rose under the power increase. The maximum temperature corresponded to the maximum power and fully corresponded to the theoretical assumptions. Figure 10b shows the temperature of the QZS-stage diodes and their heatsink in the 3L NPC QZSI under different power levels. It can be seen that the diode and heatsink temperature was slightly higher than in the 2L solution. The thermal images of the QZS-stage diode D1 at close to nominal power level (1850 W) are shown in Figure 11a,c. For the 2L QZSI and 3L NPC QZSI, the hottest points were the temperatures 130 ◦C and 140 ◦C, respectively.

Figure 10c,d show similar diagrams for SiC power switches in the 2L QZSI and Si power switches in the 3L NPC QZSI. The maximum temperature of the SiC power switches in the nominal mode was not higher than 95 ◦C in the 2L QZSI, while the maximum temperature of the Si power switches in the

3L NPC QZSI was much lower and did not exceed 75 ◦C. It should be mentioned, that under the ST states application in the boost mode, the temperature of the bridge power switches rose significantly in both cases.

**Figure 10.** Temperatures of the QZS-stage diode and heatsink in the 2L QZSI (**a**) and 3L NPC QZSI (**b**), temperatures of the SiC bridge power switches and heatsinks in the 2L QZSI (**c**), and temperatures of the Si bridge power switches and heatsinks in the 3L NPC QZSI (**d**).

**Figure 11.** Thermal images in the nominal operation mode: QZS-stage diode (**a**) and SiC bridge power switches (**b**) in the 2L QZSI and QZS-stage diode in the 3L NPC QZSI (**c**); thermal images in the boost mode under ST states application: QZS-stage diode (**d**) and SiC bridge power switches (**e**) in the 2L QZSI and QZS-stage diode in the 3L NPC QZSI (**f**).

In the case of the 2L solution, the SiC temperature reaches 160 ◦C, while in the 3L NPC solution the Si temperature did not exceed 120 ◦C at the maximum power points. Figure 11b,e show the SiC

thermal pictures for two modes at the same power point of 1850 W. It can be seen that under the transient from nominal to boost mode, the temperature of the QZS-stage diodes changed insignificantly in both solutions.

The SiC power switches temperature in the 2L solution rose from 90 ... 110 ◦C to 130 ... 160 ◦C, while in the 3L solution based on the Si power switches, the temperature rose from 70 ... 80 ◦C to 100 ... 110 ◦C only. Finally, the SiC semiconductor devices can safely operate with higher temperature. It means that the size of the heatsinks in the case of full-SiC design can be smaller.
