Design of a voltage-voltage SPWM controlled DC/AC circuit

Preface

Sine wave inverter power supply is widely used in various fields such as electricity, post and telecommunications, communications, aerospace, etc., and with the continuous development and popularization of microcomputer technology, the application of sine wave inverter power supply is becoming more and more extensive. In order to meet the user's requirements for power quality, the inverter power supply should keep the output voltage constant when the DC input voltage fluctuates. Traditional voltage single-loop control generally has the disadvantages of large output voltage fluctuations and slow dynamic response, and it is difficult to achieve precise control. In order to overcome the above shortcomings in the inverter circuit, voltage feedforward control technology is used to solve this problem. Based on the single-phase SPWM inverter, this paper adopts a voltage-voltage composite control scheme that combines feedforward adjustment of the triangular carrier and feedback adjustment of the sine wave, which better solves the problem of transient deviation of the output voltage and is simple to implement.

Basic idea of ​​voltage-voltage compound control

In the DC/AC inverter circuit, there is a certain linear relationship between the output voltage and the input voltage. When the input voltage changes, the output voltage changes accordingly. In order to keep the output voltage stable, the voltage output is generally collected for feedback closed-loop control. However, due to the high switching frequency of the inverter power supply and the presence of delay elements such as inductors and capacitors in the circuit, the change of the feedback voltage lags behind the change of the input voltage, and the system's response and adjustment are relatively slow, which can easily cause large transient deviations. If the control signal of the circuit can be adjusted by using the change of the input voltage before the output voltage changes, that is, the voltage feedback technology is supplemented by the voltage feedforward technology, this problem can be better solved. This voltage-voltage composite control can achieve the purpose of fast dynamic response, rapid adjustment, and small output voltage fluctuation.

Design and implementation of feedforward DC/AC control circuit

The control circuit of the DC/AC inverter can be composed of discrete components or integrated components.

Control circuit composed of discrete components

The control circuit composed of discrete components mostly adopts the modulation method, that is, the sine wave signal is used as the modulation wave, and the triangle wave is used as the carrier wave. When the triangle wave intersects with the sine wave, the switch device in the circuit is controlled. When the modulation method is used for control, the frequency of the triangle carrier wave is much greater than the frequency of the sine modulation wave. Therefore, in one switching cycle of the triangle wave, the sine wave Vc can be approximately considered as a constant, which is the same as the control law of the DC/DC converter, as shown in Figure 1. The duty cycle D = Vc/ Vr (Vc< Vt), and the output voltage can be changed by changing the amplitude of any one of Vc and Vr. When the triangle wave changes from Vr to Vr', the duty cycle changes from T1/T to T2/T.

In SPWM, the ratio of the sine wave peak value Vc to the triangle wave peak value Vr is generally called the modulation index a, that is, a = Vc/Vr. In this way, the input and output of the DC/AC conversion circuit have the following quantitative relationship: the fundamental effective value Uo of the output voltage is proportional to the modulation index a and the input voltage UI, that is, UOl = KaUI = KUIVc/Vr (K is a constant related to the main circuit structure). Therefore, when UI becomes n times the original, in order to keep the output voltage stable, the triangle wave peak voltage must also become n times the original accordingly.

The DC/AC inverter circuit constructed according to the above principle is shown in Figure 2. After the input voltage UI is divided and chopped by the chopper circuit, a square wave voltage Us with a voltage amplitude that changes with the input voltage is obtained. Us is integrated by the active integration circuit to obtain a triangular wave voltage Ur. The voltage waveforms before and after integration are shown in Figure 3. The peak value of this triangular wave voltage is proportional to the square wave amplitude voltage, so the triangular wave peak voltage changes proportionally with the input voltage, maintaining the stability of the output voltage. The control process of this circuit is independent and does not interfere with each other, making the design and actual debugging of the circuit simple. The design method of the control part is simple and easy to implement, achieving the purpose of fast and stable output.

The control circuit is composed of integrated components

With the continuous improvement of the cost performance of microprocessors, inverter power supplies have entered the intelligent stage, and integrated components can be used to easily form control circuits. At present, the control circuits realized by integrated components mainly include dedicated integrated chip method and microcomputer generation method. Dedicated chips for generating SPWM include SA838, SA868, HEF4752, SLE480, etc. The advantages of this method are high circuit integration and high reliability; the control circuit of the microcomputer generation method is generated by the single-chip microcomputer using software. At present, there are many cost-effective single-chip microcomputers on the market, such as the PIC series or the MC51 series single-chip microcomputers with few pins, which can generate SPWM with instructions and can easily realize the control, monitoring, management and protection of the inverter system.

In the single-phase DC/AC conversion circuit, in order to simplify the control circuit structure, a voltage-type PWM integrated control chip such as SG1525, TL494, etc. can also be used to form an SPWM controller, and the triangular wave generated inside the control chip is compared with the sine modulation wave input from the outside to obtain the SPWM waveform. At this time, the amplitude of the triangular wave cannot be adjusted, so the feedforward control idea of ​​adjusting the amplitude of the triangular wave by the change of the input voltage cannot be realized. In this case, the sampled value of the feedforward voltage can be normalized and converted according to the input-output relationship of the circuit UO1 = KUIVc/Vr, and then comprehensively compared with the feedback voltage and the given voltage. After PI adjustment, it is connected to the variable gain amplifier circuit to adjust the amplitude of the sine wave, thereby realizing voltage feedforward and voltage feedback control. The circuit principle of voltage-voltage control using integrated components is shown in Figure 4.

Simulation and Experimental Results

According to the above control method, the circuit shown in Figure 2 is simulated and tested. The inverter main circuit is an IGBT full-bridge circuit. The test parameters are: input DC voltage UI = 200 V, AC output voltage effective value is 100 V, design power is 5 kW, switching frequency is 50 kHz, L = "10" μH, C = 1 μF. When the input voltage jumps from 100 V to 150 V, as shown in Figure 5 (a), the simulation results of the output voltage of the traditional voltage feedback control and the feedforward voltage-voltage control are shown in Figure 5 (b) and Figure 5 (c), respectively, and the triangular wave change is shown in Figure 5 (d).

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The simulation results show that the feedforward voltage-voltage control is sensitive to input voltage fluctuations and can adjust the amplitude of the carrier in time to achieve the purpose of instantaneously stabilizing the output voltage.

in conclusion

This paper proposes a voltage-voltage composite control scheme that combines feedforward adjustment of triangular carrier and feedback adjustment of sine wave, and comprehensively explains the application method when the control circuit is composed of integrated components. The experimental results show that this control method solves the problem of output voltage transient deviation well and is simple to implement.