## 2 General Considerations
### General Design Strategy and Cost Optimization  
In order to unveil the full potential of this secondary circuit, a detailed loss calculation of the silicon-based system is required. The critical area where the efficiency drops below, e.g., 90% of its peak value during operating under no-load/light-load or partial-load condition, has to be identified. The SiC MOSFET half-bridge branches have to be designed such, that their peak efficiency appears under the critical load condition (lowest efficiency at maximum operating hours in this specific area) of the original IGBT inverter. Dependent on parameters in particular occurring losses, operating load conditions, switching frequency of the WBG semiconductors, etc. either no additional cooling strategy is required or an existing heat sink of the IGBT inverter can be exploited. Therefore, positioning of the additional WBG switching components must be chosen such that it is fitting and complying with the existing IGBT design but optimized for fast switching to minimize switching losses of the WBG semiconductors. Besides various other factors and components, such as the adjacent gate resistance also the parasitic inductance of the current commutation path (DC-link capacitors \(\rightarrow\) high-side switch \(\rightarrow\) low-side switch \(\rightarrow\) DC-link capacitor) is limiting the switching speed of SiC and GaN devices. Hence, an optimized positioning of both WBG devices and IGBTs must be determined under the assumption that IGBT semiconductor losses under nominal load operation must not be impaired.  
Moreover, additional WBG semiconductor components also additional gate drive circuits are required and increase the cost of the adapted solution. However, dependent on the switching requirements such as turn-on and turn-off gate voltages, already existing hardware from IGBT driving circuits can be reused and exploited (e.g., DC/DC converters). In an optimized design, only one additional gate driver (cf., Figure 4) with, e.g., a bootstrap mechanism for high-side driving, is required for proper operation of the dedicated wide-bandgap half-bridge stage. Additional voltage level shifting, for example, a GaN application can be easily adapted via additional capacitors (capacitive voltage dividers), resistors, and/or Zener diodes. Those solutions in Figures 2 and 3 both consider one  
Figure 3: Three-phase IGBT-based t-type drive system with either (**a**) GaN half-bridge stages for 400 VDC or (**b**) SiC half-bridge stages for 800-VDC DC-link voltage levels.  
half-bridge topology for each grid phase. Therefore, these implementations will result in an additional number of 6 SiC/GaN semiconductors and six gate drivers with individual output control input or three gate drivers with PWM high-side/low-side configuration. To improve the payback time of the circuit extension the operating circuit can be reduced by 2 SiC-MOSFETs (as illustrated in Figure 5a) for unbalanced two-phase operation or 4 SiC-MOSFETS for unbalanced single-phase (four-phase connectivity) operation (as depicted in Figure 5b). It has to be noted that the single-phase solution does not benefit from a reduced current ripple (evoked due to the occurring common-mode voltage) anymore as the mains neutral is directly connected to the midpoint of the split DC-link capacitors.  
Figure 4: Auxiliary circuits for driving existing IGBTs (DC/DC converter(s), bootstrap, gate driver,...) and reutilization for light-load operating SiC/GaN extension.  
Figure 5: Grid-connected three-phase IGBT-based rectifier/inverter systems utilizing a low-cost version of the light load wide-bandgap solution by implementing merely either (**a**) 4 (two-phase operation) or (**b**) 2 (single-phase) SiC power semiconductors.