Single Phase Sine Wave PWM Inverter Circuit Simulation And The Design Of Filter Based On Matlab

Inverter circuit is the most important application of PWM control technology. This paper mainly discusses the unipolar PWM ( pulse width modulation ) control mode of single-phase bridge inverter circuit, and uses MATLAB visual simulation tool Simulink to model the circuit. At the same time, a filter is designed to filter the waveform of the inverted output so that the output waveform can reach a smooth sine wave shape. This simulation not only deepens the understanding of PWM inverter circuit theory, but also lay a good experimental foundation for the study of modern power electronics.


Introduction
In power electronics technology, converting direct current into alternating current is called inversion. Inverter circuits are widely used, for example, inverter circuits are needed when DC power supplies AC loads. Inverter circuit is also the core part of power electronic devices such as frequency converter for AC motor speed regulation, uninterruptible power supply and induction heating power supply [1].
At present, the inverter technology mainly uses pulse width modulation ( PWM ). Because the characteristics of PWM wave determine that the output voltage of the power amplifier device contains more harmonic components [2], filters must be used in the inverter output to make the output waveform reach a smooth sine wave shape and reduce the influence of harmonics on the load.

Simulation of single-phase sine wave PWM inverter circuit
The simulation model of single-phase sine wave inverter circuit modulated by unipolar PWM is shown in figure 1. the DC power supply voltage in the main circuit is 200 v, which is connected to a resistive inductive load via a bridge inverter circuit formed by universal -bridge module [3]. The load inductance is 2mh and the resistance is 3Ω.   The parameter settings of universal -bridge module are shown in figure 2. The simulation waveform of the circuit is shown in figure 3. It can be seen from the waveform that the result is completely consistent with the real experiment result, which proves the correctness of the simulation result.

Parameter setting of filter
When designing filters, the value of inductance is usually designed first. The selection of inductance is related to the ripple current and the power consumption of the system. In general, the ripple current on the inductor l is selected to be 15 % -25 % of the rated current. In the case of unipolar modulation [4]: Under the condition that the switching frequency is far greater than the working frequency, according to the principle of unipolar PWM, the duty ratio of each switching period can be obtained as follows It can be obtained by substituting equation (2) As can be seen from equation (3), The value of L can be obtained from equation (5) and the value of C can be calculated from equation (6). The parameter settings for L are shown in figure 4 and the parameter settings for C are shown in figure 5 .

Establishment of simulation model
The simulation circuit of LC filter is shown in figure 6, and the output waveform is shown in figure 7.  Through the LC filter designed by the above method, the simulation experiment shows that the high frequency components can be filtered out and the relatively smooth sine wave can be output by further adjusting the parameters of L and C.

Harmonic analysis
The harmonic analysis of the output current obtained by the single-phase sinusoidal PWM inverter circuit without filter is carried out through the " power GUI". the result is shown in figure 8. THD = 13.62 %, which is much larger than the 8 % content of the national standard [5]. The output current obtained by the single-phase sinusoidal PWM inverter circuit is filtered by a filter and analyzed by " power GUI". the result is shown in figure  9. THD = 2.30 %, which greatly reduces the harmonic content and meets the national standard.

Concluding remarks
In order to make the PWM inverter have a better output waveform, the second order LC low pass filter network [6] is used in view of the high harmonic frequency of the PWM inverter.
From the simulation results, it can be seen that after LC filtering is added, the harmonic components in the output voltage are filtered out, which is closer to the sine wave shape, while the waveform of the load current is smoother and the sine degree is better. It shows that adding filter link can obviously improve the inverter output waveform.