#### Bridge inverter topology with suppression path C

**1. Half-bridge inverter topology**

In Figure 1, if the bridge arm 1 is a capacitor branch and the bridge arm 2 is a power tube, it constitutes a single-phase half-bridge inverter circuit. The driving logic is shown in Figure 3, that is, the upper and lower tubes of the bridge arm 2 are turned on at high frequency. Let the parasitic parameters

you can get the half-bridge topology as shown in Figure 2. During the entire working cycle, the value of v_{CM}+v_{CM-DM} is always maintained at U_{dc}/2 unchanged, and the circuit achieves leakage current suppression through path C.

**2. Double Buck half-bridge topology**

Figure 4 shows a conventional dual Buck inverter (DBI). The DBI circuit is obtained by paralleling two Buck DC-DC converters (Buck circuit 1: S_{1}, D_{1}, Li_{1}, C; Buck circuit 2: S_{2}, D_{2}, Li_{2}, C). DBI adopts half-cycle working mode, and the driving logic is shown in Figure 5. That is, when the output current is positive for half a cycle, Buck circuit 1 is regulated, and Buck circuit 2 does not work; when the output current is negative for half a cycle, Buck circuit 2 is regulated and Buck circuit 1 does not work. The specific common-mode voltage change is the same as that of a half-bridge inverter, to use path C to suppress high-frequency leakage current. Compared with the bridge topology, the advantages of this topology are: the circuit has no possibility of bridge arms through, and the body diodes of the power tubes S_{1} and S_{2} do not participate in the working process, which helps to improve reliability and efficiency. But like the half-bridge topology, the DC side input voltage is much higher than the full-bridge topology.

**3. 3L-NPC half-bridge topology**

NPC three-level topological circuit is shown as in Figure 6. Adopting a unipolar SPWM strategy, S_{1 }and S_{2} are turned on during the power transmission phase of the positive half cycle of the grid;

In the negative half cycle of the power grid, S_{3} and S_{4} are turned on; in the freewheeling phase, S_{2} and S_{3} are turned on, and current flows through D_{1} and S_{2} or S_{3} and D_{2}. The specific driving logic is shown in Figure 7.

Similar to the analysis of the half-bridge inverter, the levels of each level of the NPC three-level topology can be obtained. During the entire working cycle, the value of v_{CM}+v_{CM-DM} remains unchanged at U_{dc}/2, that is, the high-frequency leakage current is eliminated through path C.

The outer tubes S_{1} and S_{4} in the 3L-NPC circuit are high-frequency modulated during the working half cycle, and the inner tubes S_{2} and S_{3} are complementary switching with S_{4} and S_{1} during the non-working half cycle, providing conditions for the inverter to work at any power factor.

The 3L-NPC half-bridge inverter is one of the more mature multi-level inverter structures, but this topology has the problem of unbalanced capacitor voltage. In addition, this topology has the problem of uneven distribution of switching device losses.

**4. 3L-NPC improved topology**

Figure 8 is an improved 3L-NPC topology. Therefore, its leakage current suppression mechanism is the same as that of 3L-NPC, which uses path C to suppress high-frequency leakage current. The difference from the original topology is that the ground terminal of the power grid is not connected to the midpoint of the DC side voltage-balancing capacitor, but is connected to a new set of voltage-balancing capacitors. Through closed-loop control of the midpoint potential of the newly added voltage-balancing capacitor, the voltage unevenness problem at the midpoint of the 3L-NPC capacitor can be solved, and the DC component in the current into the grid of the 3L-NPC topology can be eliminated at the same time.

**5. 3L-ANPC half-bridge topology**

The ANPC half-bridge topology is shown in Figure 9. This topology can solve the problem of uneven distribution of 3L-NPC losses. This topology is evolved on the basis of the traditional 3L-NPC topology, using active devices S_{5} and S_{6} as the box position tube. Since the membership tube uses bidirectional switching devices, when the current flows in the forward direction, the 3L-ANPC half-bridge topology has two forward freewheeling paths composed of S_{5} parasitic diode-S_{2} and S_{6}-S_{3} parasitic diode respectively; when the current flows in the negative direction, there will also be two freewheeling paths, as shown in Figure 10. For the 3L-ANPC topology, different SPWM modulation strategies can be designed according to different freewheeling paths to realize freewheeling methods such as parallel freewheeling or alternating freewheeling. The principle of leakage current suppression of this topology is the same as that of 3L-NPC. The suppression path C is adopted, which theoretically completely eliminates the leakage current and solves the problem of uneven loss distribution, making the ANPC topology suitable for occasions with higher power levels.

**6. 3L-TNPC half-bridge topology**

The 3L-TNPC topology is a patent of the German Conergy company. As shown in Figure 11, this topology uses two switch tubes S_{3} and S_{4} and their anti-parallel diodes to achieve the midpoint registration function.

During the freewheeling phase of the positive half cycle of the power grid, the switching tube S_{3} is turned on, and the current is freewheeling through the parasitic diodes of the switching tube S_{3} and the switching tube S_{4} at this time. The reverse is true for the negative half cycle of the power grid. The value of v_{CM}+v_{CM-DM} is always maintained at U_{dc}/2, so this topology suppresses high-frequency leakage current through path C.

**7. 3L-SNPC half-bridge topology**

The 3L-SNPC half-bridge topology is shown in Figure 12. On the basis of the traditional 3L-NPC topology, this topology adds a four-quadrant switch between the capacitor bridge arm and the switch tube bridge arm. Due to the addition of a membership branch, the 3L-SNPC half-bridge topology has two positive freewheeling paths, D_{1}-S_{2} and S_{5} (parasitic diode)-S_{6}, and two negative freewheeling paths, S_{3}-D_{2} and S_{5}-S_{6} (body diode). The value of v_{CM}+v_{CM-DM} is always maintained at U_{dc}/2, so this topology suppresses high-frequency leakage current through path C.

**8. 3L-ASNPC half-bridge topology**

The 3L-ASNPC half-bridge topology is shown in Figure 13. The topology is a combination of 3L-ANPC and 3L-SNPC. Since the membership tube uses a bidirectional switch device to replace the diode in the 3L-SNPC, it has one more freewheeling path compared to the 3L-ANPC topology, so the 3L-ASNPC half-bridge topology has S_{7} (parasitic diode)-S_{2}, S_{8}-S_{3} (parasitic diode) and S_{5} (parasitic diode)-S_{6} three positive freewheeling paths and three corresponding negative freewheeling paths. The value of v_{CM}+v_{CM-DM} is always maintained at U_{dc}/2, so this topology suppresses high-frequency leakage current through path C.