Abstract: This paper mainly discusses the implementation of high-power high-voltage inverter H-bridge cascade inverters, and introduces the PWM control strategy from the system to achieve high voltage output. The implementation methods and performance of several control strategies are analyzed and compared. Keywords: H-bridge, PWM; 1 Introduction H -bridge cascade multilevel converters use multiple power units connected in series to achieve high voltage output. The output often uses multilevel phase-shift PWM control to achieve lower output voltage harmonics, smaller dv/dt and common-mode voltage, and smaller torque ripple. To achieve high voltage, simply increase the number of units; this implementation method has low technical difficulty. Each power unit is a separate DC power supply, independent of each other; controlling one unit will not affect other units. The main difference between H-bridge cascaded inverters and single-bridge inverters lies in the PWM control method. This article discusses the PWM control method of H-bridge cascaded inverters. 2. H-bridge Cascaded Inverter Structure Each power unit is an independent DC power supply, and its design is shown in Figure 1 below: [align=center] Figure 1 Power Unit Structure Block Diagram[/align] According to the description of the power unit in the figure above, this type of power unit can generate three voltage levels: +Udc, 0, and -Udc. When S1 and S4 are on and S2 and S3 are off, the load receives a voltage of +Udc; when S2 and S3 are on and S1 and S4 are off, the load receives a voltage of -Udc; when S1 and S3 (or S2 and S4) are on and S2 and S4 (or S1 and S3) are off, the load receives a voltage of 0 (Note: During the control process, it is strictly necessary to avoid the simultaneous conduction of two power devices on the same bridge arm; that is, the two control signals of the same bridge arm must be in opposite directions). Therefore, it can be seen that different PWM waveforms can be generated when different PWM control strategies are used. 3 Carrier phase shift control theory In general, N-level inverter modulation requires N-1 triangular carriers. In the carrier phase shift modulation method, all triangular waves have the same frequency and amplitude, but the phase of any two adjacent carriers must have a certain phase shift, the value of which is (1). The modulation signal is usually a three-phase sinusoidal signal with adjustable amplitude and frequency. By comparing the modulation wave and the carrier, the drive signal of the required switching device can be generated [1]. 4 PWM control strategy The frequency converter usually outputs in the form of a sine wave. For a single-phase bridge, its output can usually be divided into two types: unipolar modulation and bipolar modulation (due to space limitations, the specific implementation method is described in the references). The inverter based on the H-bridge method can also output a waveform similar to that of a single-phase bridge, and its PWM control strategy should be slightly adjusted. The waveforms output by unipolar modulation and bipolar modulation differ in performance. Since unipolar modulation can output three levels, while bipolar modulation can only output two levels, bipolar modulation has a larger dv/dt, resulting in greater impact on motor insulation. In product design, unipolar modulation waveforms are usually used as the final output waveform. This paper uses the structure of an H-bridge cascaded inverter and the SPWM generated by the carrier phase-shifting method as the control signal for each power unit to achieve the output of unipolar SPWM waveform. Several PWM control strategies are discussed and studied below: 1) Single-bridge arm chopper: The so-called single-bridge arm chopper method uses S1 and S2 as half-cycle control signals. During the positive half-cycle, S1 is turned on and S2 is turned off; during the negative half-cycle, S1 is turned off and S2 is turned on; the control signal of S3 is the SPWM signal. [align=center] Figure 2 Control signal waveform of S3 Figure 3 Control signal waveform of S1 Figure 4 Power unit output waveform[/align] The control signal of S4 is the opposite of the control signal of S3. Through such control, the waveform shown in Figure 4 can be output. Although its output waveform is similar to that of the single-phase bridge unipolar modulation output waveform, with a smaller dv/dt, this method leads to power imbalance between the two bridge arms. 2) Bipolar Modulation: The bipolar modulation of the H-type inverter is the same as that of the single-phase bridge bipolar modulation, and the generation method of the control signal is the same. The difference is that one is a single bridge arm and the other is a double bridge arm. To solve this problem, the control signal shown in Figure 2 is input to S1 and S4, and S2 and S3 are reversed with the signals of S1 and S4. This control method can only have two combinations of switching states, that is, S1 and S4 are on at the same time, and S2 and S3 are off at the same time; or S1 and S4 are off at the same time, and S2 and S3 are on at the same time. It can output a waveform similar to that of single-phase bridge bipolar modulation. Although the dv/dt of the output waveform is larger in this method, which will generate higher harmonics and increase the impact on the system, the power of the two bridge arms of the power unit is balanced, and its control method is simple and easy to implement. In high-voltage frequency converter systems, there is a distance between the control signal output unit and the power unit, which are connected by optical fiber. This method reduces the use of optical fiber, lowers product costs, and simplifies on-site wiring. 3) Unipolar Modulation: Although single-bridge arm chopping can achieve an output waveform similar to single-phase bridge unipolar modulation, this control method has inherent drawbacks. Here, we introduce another control method. As shown in Figure 1, S1 is controlled by the control signal shown in Figure 5, S3 is controlled by the control signal shown in Figure 6, and S2 and S4 are the inverse signals of the S1 and S2 control signals, respectively. The control signals shown in Figures 5 and 6 are symmetrical SPWM signals with a fundamental phase difference of 180 degrees. Since the fundamental phase difference is 180 degrees, the duty cycle of the corresponding carrier periods of the two control signals is 1, meaning they are complementary. The output waveform will have four combinations: S1 on, S3 off, output +Udc; S1 on, S3 on, output 0; S1 off, S3 off, output 0; S1 off, S3 on, output -Udc. See the left side of the dotted line in the figure, where the first three switch combinations will appear, and the right side of the dotted line, where the last three switch combinations will appear. This results in a PWM wave with a sinusoidal duty cycle, as shown in Figure 7. [align=center] Figure 5 Left bridge arm control signal Figure 6 Right bridge arm control signal Figure 7 H-bridge unipolar modulation output waveform[/align] This method of PWM control achieves the conversion from unipolar SPWM to bipolar SPWM, realizing power balance between the left and right bridge arms. Furthermore, the inverter output voltage harmonics obtained using this method are very low, eliminating the need for a filter, and is thus called a perfect harmonic-free inverter. In inverter control, DSP control is commonly used. However, since DSPs can only output two levels and cannot directly implement unipolar SPWM, external devices are required. This method utilizes the combinational logic relationship of power units (logic relationship as shown in Table 1) to replace the function of external devices, saving components, reducing development costs and difficulty, and simplifying control for easy implementation. Table 1: Logical Relationships 5. Conclusion In high-power high-voltage inverter technology, PWM control technology is one of its core technologies. A good PWM control strategy is essential for product performance. This paper mainly discusses the PWM control methods of H-bridge cascaded high-power high-voltage inverters, presenting three implementation methods and analyzing and comparing their implementation methods and performance. 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