## 4 The Efficiency of Hybrid Power Stage under Light-Load Conditions
### 125 kW Si IGBT Converter vs. 10 kW SiC Power Stage under Light-Load Conditions
#### 4.1.3 Total Semiconductor Loss-Overview and Optimization Prospects  
Considering the equations from Sections 4.1.1 and 4.1.2, the total semiconductor losses of the three-phase setup with a switching frequency of 10 kHz results in 470 W for all 3 half-bridge IGBT modules and 39 W for 6 SiC-MOSFET semiconductors, respectively. Semiconductor standby losses during no-load operation could thus be improved by over 90% in this specific case. A summary of conduction, switching, and total losses are depicted in Figure 12.  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Value** \\ \hline \(k_{\text{off,MOS}}\) (800 V, 2 \(\Omega\), 50 \({}^{\circ}\)C) & 5 \(\upmu\)J/A \\ \(d_{\text{off,MOS}}\) (800 V, 2 \(\Omega\), 50 \({}^{\circ}\)C) & 50 \(\upmu\)J \\ \(k_{\text{off,IGBT}}\) (800 V, 0.63 \(\Omega\), 50 \({}^{\circ}\)C) & 120 \(\upmu\)J/A \\ \(d_{\text{off,IGBT}}\) (800 V, 0.63 \(\Omega\), 50 \({}^{\circ}\)C) & 4 mJ \\ \(P_{\text{sw,MOS}}\) (800 V, 2 \(\Omega\), 50 \({}^{\circ}\)C, 10 kHz)—no load & 1.84 W \\ \(P_{\text{sw,IGBT}}\) (800 V, 0.63 \(\Omega\), 50 \({}^{\circ}\)C, 10 kHz)—no load & 72.6 W \\ \hline \hline \end{tabular}
\end{table}
Table 5: Turn-off loss characteristics of FF600R12 KE4 IGBT and IMZ120R030M1H SiC–MOSFET under consideration of no-load operation.  
The dominant part of IGBT related losses is associated with switching losses of the implemented modules. Therefore, a general improvement towards higher efficiency during light- or no-load operation besides a WBG upgrade circuit could be the reduction of the applied switching frequency. This, however, would increase the input inductor volume if the similar current characteristics should adhere.  
Regarding the SiC-MOSFET, about 2/3 of all MOSFET related losses are related to conduction losses and 1/3 are originating due to switching losses. For a further loss reduction two different strategies are therefore proposed. First, an additional loss-reduction could be achieved by implementing a SiC device with a slightly larger chip area. Second, adapting (especially enhancing) the switching frequency during light or no-load condition leads to a lower current ripple (hyperbolic function). An investigation on total losses of the SiC-MOSFET at hand under no-load condition shows that the optimum switching frequency for minimum device losses is located around 25 kHz (illustrated in Figure 13).  
This phenomenon occurs due the fact that the same inductor of the 125 kW system is utilized for all different occurring load conditions. An enhancement of the switching frequency therefore results in a reduction of the occurring current ripple during light- and no-load operation. A lower current  
Figure 12: Comparison of conduction and switching losses of light-load SiC MOSFET and high-power IGBT modules under no-load condition.  
Figure 13: SiC–MOSFET loss optimization due to the adaption of switching frequency during no-load operation.  
ripple consequently results in lower conduction losses and increased switching losses (as the negative effect of the switching frequency is outperforming the positive impact of a lower ripple). The optimum switching frequency can be found at the local minimum of the sum of both functions.