**6. Experimental Results**

To further verify the feasibility of the proposed strategy and theoretical analysis, a lowpower prototype was established in the laboratory. Table 3 lists the electrical parameters of the prototype.


**Table 3.** Electrical parameters of the prototype.

Figure 16 shows the proposed control scheme for the soft start-up and modulation scheme. Vf is sensed for the balance of the flying capacitor voltage, and iL is sensed to judge the output current direction. Meanwhile, Vf is sensed for the soft start-up of the inverter. The proposed control scheme can be digitally realized in DSP or CPLD collaborative controllers.

**Figure 16.** Proposed control scheme for soft start-up and modulation scheme.

The voltage of the DC-bus is 400 V, output voltage is 100 Vac and the switching device is 650 V MOSFET IPW60R190Z. As shown in Figure 17, under the traditional modulation strategy, there exists an overvoltage issue in the dead time at the zero crossing point of the waveform. Under the 400 V DC-bus, the voltage stress is about 300 V due to voltage ringing, and if the device is used under a normal bus of 800 V DC, there will be a risk of breakdown.

**Figure 17.** The overvoltage stress issue. (**a**) Under fundamental frequency. (**b**) Under multiple switching cycles. (**c**) Under single switching cycle.

To solve the overvoltage stress issue, the driving signal shown in Figure 18 is applied. As shown in Figure 18, there will be no overvoltage stress or output voltage level jump during the same switching process.

**Figure 18.** (**a**) Traditional and (**b**) modified modulation strategy.

As shown in Figure 19, under the proposed modified modulation strategy, there is no overvoltage issue in the dead time at zero crossing point of the waveform.

**Figure 19.** The waveform under the proposed modified modulation strategy. (**a**) Under fundamental frequency. (**b**) Under multiple switching cycles. (**c**) Under single switching cycle.

Figure 20 shows the experimental waveforms of Vc\_up, Vc\_down and VFc during the soft start-up of the inverter under different charging resistances. As discussed in Section 5, the flying capacitor and DC-link capacitor are charged by the DC-link voltage source in the meantime until the flying capacitor reaches its reference voltage Vdc/4 and the DC-link capacitor is charged to the bus voltage continually by controlling several main switches.

The experimental results have demonstrated that, by using the proposed method, the switching frequency of S5–S8 is the same as the fundamental frequency while the switching frequency of the other switches will switch at carrier frequency. Moreover, the stress of power devices can be no more than Vdc/4. By controlling the main switches S3, S4, S5 and S8, the flying capacitor and bus capacitor are fully charged without the overvoltage problem of the switches.

**Figure 20.** Soft start-up under different charging resistance. (**a**) R1 = 500 Ohms; (**b**) R1 = 50 Ohms.

#### **7. Conclusions**

For the ANPC-5L inverter, the traditional modulation strategy has the security risk of the overvoltage of the power device in the switching dead time when the output current is inductive, which affects the commutation safety and leads to an overvoltage issue. In this paper, the overvoltage mechanism is deduced through the voltage stress analysis of different switching states. Meanwhile, a modified modulation strategy is proposed to solve this issue. In comparison with other modulations, this method provides several free degrees which are used to ensure the elimination of the voltage stress of power devices by choosing favorable circuit states and controlling current commutations. Additionally, it can realize the flying capacitor voltage balance in several carrier wave periods. The implementation of the proposed strategy in the digital system is rather simple. Meanwhile, a novel soft start-up method adapted to the ANPC-5L inverter is also proposed. In addition, an experimental prototype is also built to verify the issue of traditional modulation strategy and the validity and feasibility of the proposed modulation strategy.

**Author Contributions:** Conceptualization, G.C.; methodology, J.Y.; software, J.Y.; validation, G.C. and J.Y.; formal analysis, G.C.; investigation, G.C.; resources, G.C.; data curation, G.C.; writing—original draft preparation, G.C.; writing—review and editing, J.Y.; visualization, J.Y.; supervision, G.C.; project administration, G.C.; funding acquisition, G.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Shanghai Outstanding Academic/Technical Leaders Plan (20XD1430700).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

