Design and implementation of a multifunctional inverter power supply

1 Introduction

With the development of modern science and technology, inverter power supplies are widely used in all walks of life, which puts forward higher requirements for their performance. Traditional inverter power supplies are mostly control systems that combine analog control or digital control. A good inverter power supply voltage output waveform mainly includes high steady-state accuracy and good dynamic performance [1]. The current inverter structure and control can obtain a good sinusoidal output voltage waveform, but the effect is not very ideal for waveforms with rapid mutations.
Function signal generator is a commonly used device in experimental teaching. It can generate waveforms of different frequencies and voltage levels: square wave signals, triangle waves, and sinusoidal signal waveforms. A new DDS technology, namely direct digital frequency synthesis technology, has emerged in recent years. However, they are all small signal waves, have no power output, and cannot carry a certain load.

The multifunctional inverter power supply proposed in this paper adopts a double single-phase full-bridge inverter structure in the main circuit. The output voltage waveform tracks the given reference waveform, has power output, and can carry a certain load. The control adopts hysteresis control with differential link, which fully realizes digital control.

2 Main circuit design

The principle of the multifunctional inverter power supply is shown in Figure 1, which consists of two parts: the main circuit and the control part. The reference signal of the main circuit can be obtained by communicating with a computer or other circuits.

Figure 1 Principle of multifunctional inverter power supply

The design of the main circuit draws on the structure of multiple inverters and adopts a double single-phase full-bridge inverter connection [2]. The schematic diagram is shown in Figure 2. The DC side voltages of the two inverters are different. The DC side voltage of the main inverter is Udc, and the DC side voltage of the slave inverter is 3Udc. The transmission voltage waveform consists of 9 levels: ±4Udc, ±3Udc, ±2Udc, ±Udc, 0. Since the number of output levels is more than that of a single inverter, the output waveform is better. The main inverter works at a higher frequency, and the slave inverter works at a lower frequency, which greatly reduces the switching loss. In the stage where the reference waveform changes slowly, only the main inverter bridge needs to work to track the reference signal well; when the reference signal changes very quickly, the auxiliary inverter bridge and the main inverter bridge need to work at the same time to quickly and accurately track the reference signal.

Figure 2 Double cascaded single-phase full-bridge inverter topology

3 Control Design

Hysteresis control is used in the control part to achieve full digital control. Hysteresis control has the characteristics of fast response speed, high accuracy, high tracking precision, and output voltage does not contain harmonic components of specific frequencies. It can be digitally controlled using DSP [3]. Hysteresis control is used for the master inverter and slave inverter of the main circuit [4].

Figure 3 Hysteresis control principle

As shown in Figure 3, the hysteresis width of the main switch is h, the hysteresis width of the slave switch is hs, and hs>h. The main inverter is always working, and the switch tubes V1 and V4; V2 and V3 are turned on and off alternately. The slave inverter has three working states. At t1~t2, the error voltage does not exceed the hysteresis width of the slave inverter. Only the main inverter needs to work, and the four switch tubes are turned off; at t3, the error voltage △u＞hs, the switch tubes VS2 and VS3 are turned on, and the switch tubes VS1 and VS4 are turned off; at t4, the error voltage -△u＜-hs, the switch tubes VS1 and VS4 are turned on, and the switch tubes VS2 and VS3 are turned off.
Considering the difficulty of following the sudden change signal, a differential link is introduced before the hysteresis controller, as shown in Figure 4, to improve the following effect [5][6].

Figure 4 Hysteresis control with differential link

After the differential link is introduced, according to Figures 1 and 2, the hysteresis control strategy for the main inverter is:

Where: T is the differential time constant.

If the above inequality sign is equal, the actual ring width h′ is:

When the voltage is in steady state or the rate of change is not large, the differential link is very small and can be ignored, and h' is large; when the voltage suddenly changes, the differential link will be very large and cannot be ignored, h' is small, and u quickly tracks Uref. Adding a differential link actually changes the hysteresis loop width. The same principle is also used in the inverter hysteresis control.

3 Simulation

Using Matlab, a model is established based on the proposed main circuit and control design. The double cascaded single-phase full-bridge inverter in Figure 1 is simulated, and the load is a resistive-inductive type.

The reference signal is a sine wave with a period T of 0.02s and a maximum value of 50V. The output voltage waveform is shown in Figure 5.

Figure 5 Reference signal is a sine wave output voltage

The reference signal is a triangle wave with a maximum voltage of 70V. The output voltage is shown in Figure 6.

Figure 6 Reference signal is a triangular wave output voltage

As can be seen from FIG5 and FIG6, when the reference signal is a sine wave and a triangle wave signal that does not change very quickly, the output voltage of the inverter power supply can be accurately tracked.

The reference signal is a step wave, and the output voltage waveform is shown in Figure 7.

Figure 7 Reference signal is square wave output voltage

When the reference voltage signal is a square wave, the voltage value is 70 V. The output voltage waveform is shown in Figure 8.

Figure 8 Reference signal is square wave output voltage

When the reference signal is a step wave or a square wave, and the square wave and the step wave have a sudden change moment, the output voltage of the inverter power supply can also track the reference signal well. As can be seen from Figures 7 and 8, the output voltage is a step wave and a square wave of good quality, which can be used as a voltage source.

4 Conclusion

The multifunctional inverter power supply adopts a double cascade single-phase full-bridge inverter structure in the main circuit. The output voltage waveform tracks the given reference waveform, has power output, can carry a certain load, and can be used directly as a voltage source. The control adopts hysteresis control with differential links to fully realize digital control. Finally, through Matlab simulation, it is proved that the designed multifunctional inverter power supply is feasible.

References
[1] Liu Chunrui. Research on digital control technology of inverter power supply: [Master's degree thesis]. Xi'an: Xi'an University of Technology, 2008.
[2] Hongfa Ding, Xianzhong Duan, Qingchun Zhu. A multifunctional series power quality conditioner based on asymmetry cascade multilevel inverter and its strategy. 2005 IEEE/PES Transmission and Distribution Conference & Exhibition: Asia and Pacific Dalian, China, Page(s): 1-6.
[3] Wang Zhaoan, Yang Jun, Liu Jinjun, et al. Harmonic suppression and reactive power compensation [M]. Beijing: Machinery Industry Press, 2005.
[4] GH Bode, DG Holmes. Implementation of three level hysteresis current control for a single phase voltage source Inverter [J]. Powerelectronics specialists conference, 2000 IEEE 31st annual volume 1, 18-23 June 2000 Page(s): 33 – 38.
[5] Mao Yufang, Yang Zhenyu. Application of voltage hysteresis control technology with differential link [J]. Jiangsu Electrical Engineering, 2007, 26(1): 21-24
[6] Gao Jun, Li Hui, Yang Xu, Wang Zhaoan. Research on sinusoidal inverter power supply based on PID control and complex control [J]. New Technology of Electrical Engineering and Power, 2002, 21(1): 1-4.