## 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  
In general, for evaluation of conduction losses of hard-switched AC/DC or DC/AC converter topologies, only losses during nominal load operation of the circuit are of interest as these are the worst-case losses under normal conditions. These losses are of interest for heatsink design and evaluation of system efficiency. As the current ripple has minor effects in terms of avg and rms values under these nominal conditions, it is often omitted to simplify relevant design equations. Besides the modulation index \(M\) and the phase shift angle (\(\Phi\)) of the fundamental current, these equations are only dependent on mains rms \(I_{\text{N,rms}}\) or peak \(I_{\text{N,pk}}\) current, respectively.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Design Parameter** & **Value** \\ \hline Switching frequency Si/SiC & \(f_{\text{sw}}=10\) kHz \\ Input inductance/phase: & \(L_{\text{inv}}=250\)\(\mu\)H \\ Input current ripple maximum & \(|\Delta I_{\text{Lmax}}|=80\) A \\ Input current ripple minimum & \(|\Delta I_{\text{Lmin}}|=27\) A \\ Input current ripple rms & \(\Delta I_{\text{L,rms}}=56.7\) A \\ IGBT module (3 ph./125 kW) & FF600R12 KE4 1200 V/600 A \\ SiC MOSFET (3 ph./10 kW) & IMZ120R030M1H 1200 V/30 m\(\Omega\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: System-level design specification of the three-phase converter (nominal rating).  
Furthermore, it must be noted that an IGBT can only conduct current in a positive direction when gated. Negative currents are processed via the supplementary reverse conducting diode. A MOSFET, on the other hand, can conduct currents in both (positive and negative) directions through its channel when exceeding its threshold voltage and thus turned ON. The diode of the MOSFET is thus only relevant for switching losses and only stressed during the specified dead-time of the gate driver. As this dead-time is usually much lower than the dedicated inverse switching frequency, conduction losses of the parasitic diode can be omitted. Therefore, it has to be distinguished between topologies utilizing IGBTs and MOSFETs.  
As depicted in Figure 6, it can be observed that for an IGBT-based three-phase six-switch inverter the high-side IGBT (IGBT\({}_{\text{P}}\); IGBT\({}_{\text{P}}\) turned ON) and low-side reverse conducting diode (D\({}_{\text{n}}\); IGBT\({}_{\text{P}}\) turned OFF) are stressed during negative input currents \(i_{\text{N}}(\varphi)\), whereas low-side IGBT (IGBT\({}_{\text{n}}\); IGBT\({}_{\text{n}}\) turned ON) and high-side reverse conducting diode (D\({}_{\text{n}}\); IGBT\({}_{\text{n}}\) turned OFF) are stressed during the positive half-wave of the input current \(i_{\text{N}}(\varphi)\).  
For a MOSFET-based three-phase six-switch converter, both high-side (MOS\({}_{\text{P}}\); MOS\({}_{\text{P}}\) turned ON) and low-side MOSFET (MOS\({}_{\text{n}}\); MOS\({}_{\text{P}}\) turned OFF) are conducting during one complete sinusoidal period \(0\leq\varphi\leq 2\pi\), with \(\varphi=\omega t=x\) (conduction during dead-time is neglected).