## 4 The Efficiency of Hybrid Power Stage under Light-Load Conditions
### Further Loss Improvements Due to GaN-Based WBG Technology  
In Section 2.1, different topologies were presented, which also considered GaN transistor as a possible upgrade for a WBG-based extension circuit. As GaN HEMTs are currently only available up to 650 V maximum blocking voltage, integration into grid-connected power electronics inverters is only feasible in multilevel setups or for three-phase six-switch inverters with lower DC-link voltage (ideally around 400 V). Therefore, a three-phase six-switch setup with a DC-link voltage of 400 V is assumed (e.g., for electric drives). Two different types of switches are compared to each other. For the GaN transistor GaNSystems' GS66508T [18,19] 650 V/30 A was chosen. For the SiC-MOSFET, a ROHM SCT3080 AL [20,21] 650 V/30 A device has opted for a reference device. The component characteristics of both SiC and GaN semiconductors are presented in Table 6. In GaN transistor datasheets of mostly all manufacturers, at present, only very little information is included regarding switching losses during hard switched operation. These losses are currently omitted as they are highly dependent on the layout requirements. To get competitive results for a SiC vs. GaN performance comparison under light-load conditions, a double pulse test bench--as illustrated in Figure 14--was designed.  
Figure 14: (**a**) Testbench for benchmarking and extraction of turn-on and turn-off switching losses of a 650 V/30 A GaN transistor; (**b**) switching loss comparison of 650 V/30 A SiC–MOSFET and GaN transistor.  
A coaxial shunt was employed to minimize parasitic inductance of the commutation loop due to inductive coupling or wiring. Figure 14b shows the turn-on and turn-off energy loss comparison of datasheet values of the SiC-MOSFET in conjunction with a dedicated loss map of an equivalent GaN transistor. As can be observed from the data at hand, the GaN transistor will come with both, lower conduction (lower on-resistance compared to SiC MOSFET) and lower switching losses, even though a suboptimal coaxial shunt device was chosen (SDN-414 [22; 23]) which includes additional wiring considerably deteriorating parasitic within the measurement setup. Therefore, even lower values than the obtained characteristics can be expected in a final operating setup without a current measurement device.  
Care must be taken when junction temperatures beyond 30 \({}^{\circ}\)C are of interest. If both datasheets of SiC and GaN device are carefully compared to each other, it can be observed that the on-resistance of the SiC device is less sensitive to an increase in junction temperature (factor of 1.3 for \(T_{\rm j}\) = 25 \({}^{\circ}\)C\(\rightarrow\)150 \({}^{\circ}\)C) than the according to the parameter of the GaN-transistor, which is notably increasing (factor of \(>\)2.5 for \(\Delta T_{\rm j}\) = 25 \({}^{\circ}\)C\(\rightarrow\)150 \({}^{\circ}\)C). Therefore, a crucial aspect could be whether it is possible to attach the equipped WBG circuitry to the existing heatsink of the IGBT setup of the nominal load inverter or not. If a direct connection between the WBG upgrade and the initial heatsink is impossible, an independent cooling strategy must be designed. Due to existing space constraints, its \(R_{\rm th}\) will be accessing that one of the IGBT related heatsink and thus either a higher junction temperature of the additional WBG devices will occur (and result in higher conduction losses of a GaN transistor compared to the SiC device) or the critical limit of maximum power under light-load condition must be reduced to sustain low junction temperatures of WBG semiconductors.