# Emerging Technologies for Advanced Power Electronics and Machine Design in Electric Drives
## 2 Topologies of Electric Drives
### Series Power Stage Topologies
**The flying capacitor (FC) topology** (Figure 3c) is structurally relatively similar to the NPC topology, whereby the clamping diodes are replaced by a dedicated clamping capacitor [41; 42; 43]. The topology is suitable for higher voltages, but the main disadvantage remains the need for a higher number of clamping capacitors, which further increases the cost and size of the converter. In certain cases, however, such a solution is advantageous compared to the use of clamping diodes. Using the example of a flying capacitor converter, Figure 4 shows how an integrated multilevel solution with flying capacitors is formed by connecting a large number of power cells in series, which consist of transistor legs with corresponding capacitors. A similar approach is also possible for the T-type and NPC topology. Figure 3: Single phase schematics of (**a**) a 3-level T-type converter, (**b**) a 3-level neutral-point clamped converter, and (**c**) a 3-level flying capacitor converter. **The cascaded H-bridge (CHB) topology** (Figure 5) comprises several half-bridge or full-bridge converters, with their outputs in each phase being connected in series. Each submodule (SM) has its own isolated DC voltage source so that the instantaneous output voltage corresponds to the sum of the individual outputs, each set independently to \(+V_{\text{dc}}\) or \(-V_{\text{dc}}\). However, synchronized operation provides better results as switching losses can be minimized and distributed more evenly between the submodules. In addition, specific switching harmonics can be selectively suppressed as well [54; 55]. By using voltage sources with different voltage levels (e.g., 10 V, 30 V, and 90 V), different modulation schemes can provide additional output voltage levels and thus reduce total harmonic distortion factor of the voltage. Compared to other topologies, the main advantages of CHB topology are the lower number of additional components per additional voltage level and a highly modular design [23]. The voltage control of the individual DC link voltage levels is possible with relatively simple modulation methods [56; 57; 58; 59]. The main disadvantage of the CHB topology is the need for multiple isolated voltage sources, which, in combination with the need for additional DC link capacitors, can significantly affect the production price of the converter. Figure 4: Schematic diagram of a three-phase multi-level flying capacitor converter (FC), connected to a three-phase electric machine. Figure 5: Schematic diagram of a three-phase multilevel cascaded H-bridge (CHB) converter with a three-phase electric machine and multiple voltage sources (\(V_{\text{dc1}}\), \(V_{\text{dc2}}\), \(\dots\), \(V_{\text{dcN}}\)), where an ordinary H-bridge is utilized as a basic full-bridge submodule (SM). **The modular multilevel converter (MMC) topology** was proposed on the brink of this century [23; 60] and represents an improvement of the usual CHB converter topology. In addition to the possibility of using basic full-bridge submodules (Figure 6), alternative configurations with half-bridge, ANPC, cross-connected, single-clamped, and other submodules can be used [23; 33]. Compared to the common CHB topology, the main advantages are the reduced number of independent voltage sources, the possibility to reduce power losses, and capacitor voltage ripple [23]. Multiple output voltage levels are achieved by using multiple submodules and additional inductors in each branch, but this can lead to particular challenges when dealing with faults inside and outside the converter. The corresponding characteristics of most common multilevel converters are compared and summarized in Table 1. In general, with respect to the two-level B6 converter, the multilevel approaches require a higher number of switches and other components, as well as a higher number of associated trigger circuits and more complex control algorithms [44; 61], to achieve lower motor torque ripple and additional reduction in harmonic losses in the electric machine [25; 26].