# Emerging Technologies for Advanced Power Electronics and Machine Design in Electric Drives
## 2 Topologies of Electric Drives
### Series Power Stage Topologies  
In electric drives with series power stage topologies, the inverter and the machine typically remain three-phase [21; 22; 23]. Their power stage is adapted to the higher voltage, either in the form of (i) integrated single-cell multilevel converters, where more than two transistors per leg are combined with diodes and capacitors, or (ii) multi-cell converters with several cells of the same topology connected in series (Figure 2). The main features of multilevel topologies are (i) their ability to generate more than two voltage levels at the output of the converter and (ii) the possibility of using semiconductor switches and passive components with a lower than rated DC link voltage \(V_{\text{dc}}\). This results in (i) lower total harmonic distortion (THD) of the output voltage [24] and thus (ii) lower motor torque ripple and additional reduction of harmonic losses in the electric machine [25; 26], (iii) lower common-mode voltages, (iv) lower switching losses due to lower voltage slopes (\(dv/dt\)), and (v) improved operation in terms of EMI. These advantages are the reason why multilevel topologies are widely used not only in high-voltage applications but also in applications below 100 V. A similar shift can be observed in applications where high-voltage IGBT switches were traditionally used, which are often replaced by low-voltage MOSFETs, further reducing the power losses [27; 28]. Currently, multilevel converters are most commonly used in applications related to micro-grids and photovoltaics [29]. However, they (especially those with three output voltage levels) are also gaining interest in the supplies for electric machines [22; 30; 31]. In the last decades, several versions of multilevel converters [24; 32; 33] have been developed, such as symmetric and asymmetric cascaded designs [34; 35; 36], clamping diode version [37; 38; 39], T-version [37; 40], version with flying capacitors [41; 42; 43], generalized versions [44; 45], and several others [46; 47; 48]. Figure 2: Classification of series power stage topologies of electric drives. **The T-type multilevel topology** (Figure 3a) in its three-level version consists of four transistor switches and a split DC voltage, which is realized by two equivalent capacitors. Like a two-level B6 transistor leg, the T-type has two transistors located in the vertical transistor branch. Furthermore, two additional switches are used as an active bidirectional switch connecting the mid-point of the DC link with the output terminal [49]. The latter can thus produce three different voltage levels (0, \(V_{\text{dc}}/2\), and \(V_{\text{dc}}\)). The T-type topology is advantageous because the MOSFETs in the bidirectional switch can be designed for a lower maximum voltage (\(V_{\text{dc}}/2\)). However, the vertical MOSFETs must be able to withstand the entire source voltage \(V_{\text{dc}}\). Despite two additional MOSFETs, the sum of the conduction and switching losses in the T-type branch can be partially reduced, which improves the efficiency of the converter. **The neutral-point clamped (NPC) topology** (Figure 3b) is an upgrade to the T-type multilevel topology. It again comprises a modified DC link, four transistor switches in the vertical branch, and two additional clamping diodes that replace the bidirectional switch. Depending on the sign of the output current, both diodes clamp the potential in the MOSFETs mid-points to the potential of the split DC link in all switching modes. Consequently, transistors with half-rated voltages can be used, although complex modulation [50; 51] must be implemented to control the DC link's midpoint voltage. In some cases, clamping diodes can be replaced by active switches (transistors), resulting in an active neutral-point clamped (ANPC) topology, where the power losses of the converter are further reduced [21; 52; 53]. The NPC converters are generally characterized by a relatively high efficiency at higher switching frequencies.