A current inverter is a device that converts DC power into AC power. The size and direction of its output current are controlled by the voltage and phase of the input AC power. When DC power is input, the inverter performs a series of processes on it to make the output current show an inverter waveform, thereby converting DC power into AC power.

Inverters are widely used in home solar power system, working with off grid solar batteries. The output current of the inverter shows a certain waveform when the AC power is input, which is determined by its working principle. This article will give you a detailed introduction and comparison of inverter waveform.

1. Output principle of inverter waveform

The working principle of the inverter turning alternating current (AC) into direct current (DC) only requires one diode to form a simple rectifier circuit. There are three main types of output inverter waveform: square wave, modified wave and sine wave. So why is it square wave, and why is it sine wave?

First of all, the shape of the output inverter waveform is determined by several factors such as the characteristics and parameters of the components in the circuit. In a current inverter, the output inverter waveform is determined by the output voltage of the PWM converter, which will be processed according to the input AC power signal to make the output voltage show a certain shape of inverter waveform and frequency.

Secondly, the shape of the output inverter waveform is also related to the circuit topology of the current inverter. Different circuit topologies will have a great impact on the shape of the output inverter waveform.

2. Basic principles of PWM

Before we go for the principles of generating square and sine waves, we should first know the basic principles of PWM, which stands for Pulse Width Modulation, a technique that uses the digital output of a microprocessor to control an analog circuit.

Generally speaking, no matter what the shape of the inverter waveform, as long as the inverter waveform and the coordinate axis t surrounded by the same shadow area, the effect (average output voltage) is the same. This is the principle of area equivalence.

Generally, commonly used PWM is a rectangular pulse (square wave) form, the following figure shows a square wave with an amplitude of 5V and a frequency of 50Hz.

• Duty cycle: Refers to the high level of the proportion of the entire cycle, such as the above figure in the PWM, in this cycle, the proportion of high level is 50%, so the duty cycle is 50%. In the PWM frequency under certain conditions, by changing the size of the duty cycle, you can realize the size of the output voltage change. For example, when the duty cycle is 100%, the output voltage is 5V; when the duty cycle is 0, the output voltage is 0. If we want the output voltage to be 2.5V, we just need to change the duty cycle to 50%.
• Resolution: It means the smallest value that PWM can reach, which means how many parts a cycle time is divided into, if it is 10 parts, then the accuracy of the duty cycle is 10%. If it is divided into 1000 parts, then the accuracy of duty cycle is 0.1%.

3. How inverter generates square wave alternating current

The AC output of the old inverters was mainly in the form of square wave AC, which is suitable for the use of some equipment of lower requirements. Let's first look at how the DC is turned into a square wave AC. In the figure below, when both switches S1 and S4 are closed, the direction of the current is like this:

And when S2 and S3 are closed, the direction of the current is as below. We can see that the direction of the current on the load has changed and a square wave alternating current is generated.

The frequency of the mains power is 50Hz, which means that the switch is opened and closed 100 times per second. We can use semiconductor switches (MOS tubes) to control the on and off of the circuit, as shown below, at points A, B, C, by inputting a control signal, you can control the on and off of the MOS tube, thereby changing the direction of the load current and outputting square wave alternating current.

4. How inverter generates pure sine wave alternating current

Above we have learned the basic principle of the inverter generating square wave AC. The inverter with square wave output has high efficiency. Although it can be applied to many electrical appliances, some electrical appliances are not suitable.

Therefore, it is necessary to use a best solar inverter with pure sine wave alternating current output. Now let’s take a look at how the inverter generates pure sine wave alternating current.

As shown in the figure above, the duty cycle of PWM changes according to the sinusoidal law. Where the voltage amplitude is required to be large, PWM with a large duty cycle is generated, and where the voltage amplitude is small, PWM with a small duty cycle is generated.

In a short period of time, the average output voltage of PWM is shown as the red line. We can see that there has produced an inverter waveform similar to a sine wave. If the PWM is more accurate, the sine wave inverter waveform will be smoother.

The modulation pulse method in which the pulse width and time duty cycle are arranged according to the sinusoidal pattern is called SPWM. So how to generate this sinusoidal pattern of SPWM?

In the past, analog circuits were used to generate this modulation signal. A precise and high-speed voltage comparator compared the carrier wave and the modulation wave. When the two voltages were the same, the switching transistor was controlled in time to switch on and off. However, the analog circuit structure is complex and it is difficult to achieve accurate control.

There are now dedicated integrated circuits used to generate the above modulated signals. The microprocessor only outputs the output frequency, voltage and other parameters to generate high-precision control signals and outputs a perfect sine wave. The microprocessor can carry out detection, protection and other controls for the entire inverter. This method has a simple circuit, good effect and high reliability. It is a widely used control method at present.

5. Square wave vs. rectangular wave vs. modified sine wave vs. pure sine wave

① Square wave

A square wave is a periodic inverter waveform signal whose voltage alternates between two different levels. Square waves are characterized by instantaneous switching between positive and negative voltage values without smooth transitions. In other words, the rising and falling edges of a square wave are very steep, almost vertical. Square waves are simpler in shape than other inverter waveform such as sine and triangle waves.

Characteristics of square waves:

• Time duty cycle: The time duty cycle of a square wave refers to the ratio of the time the voltage is in a high state to the total cycle time in a cycle. An ideal square wave has a 50% duty cycle, that is, the high and low times are equal.
• Odd harmonics: The spectrum characteristics of the square wave show that the square wave only contains odd harmonics, and its amplitude gradually decreases as the frequency increases. This means that the square wave contains a large number of high-frequency components, which can cause problems such as electromagnetic interference and signal distortion.
• No even harmonics: Corresponding to odd harmonics, the square wave does not contain even harmonics. This is because the square wave is symmetric about the time axis, while the even harmonics are antisymmetric about the time axis, so their contribution to the square wave is zero.

Square waves are widely used in digital circuits, timers, logic control, switching power supplies and so on. However, for applications that require continuous, smooth AC power (such as driving motors or powering sensitive electronic equipment), square waves are not suitable because their high-frequency content can cause additional losses and interference. In this case, a sine wave or another option closer to a sine wave would be better.

② Rectangular wave

The rectangular wave is also a periodic inverter waveform. Its characteristic is that within a cycle, the inverter waveform will remain at one level for a period of time, and then remain at another level for a period of time. This inverter waveform can be used for PWM control and audio signal synthesis, etc. In PWM control, the rectangular wave can control the output voltage by changing the duty cycle, thereby controlling motors, lights and other equipment.

In audio signal synthesis, rectangular waves can be processed by adding filters to produce various timbres. Rectangular waves are widely used in electronic engineering, communication engineering, computer science and other fields.

③ Modified sine wave

A modified sine wave is an inverter waveform that has been adjusted or corrected. In the field of power electronics, the most common modified inverter waveform is the modified sine wave, which is improved on the basis of the square wave to make it closer to a pure sine wave. Modified sine waves are intermediate in shape between the inverter waveform of square waves and pure sine waves.

A modified sine wave usually consists of a brief phase of zero voltage followed by two phases of different voltage levels. During one cycle, the modified sine wave alternates between high level, zero level, and low level. Compared with square waves, modified sine waves contain fewer high-frequency components and are closer to the characteristics of pure sine waves, thereby reducing signal distortion and electromagnetic interference.

Compared with pure sine waves, modified sine waves have the advantages of low cost and simple structure. However, the AC inverter waveform output by the modified sine wave still has a certain degree of distortion, which may affect some sensitive electronic equipment. For applications with higher requirements for inverter waveform, a pure sine wave is usually a more suitable choice of inverter waveform.

④ Pure sine wave

A pure sine wave is a smooth, continuous, periodic inverter waveform whose voltage changes as a sinusoidal function over time.

Characteristics of pure sine waves:

• Smooth waveform: Compared with square wave and modified sine wave, pure sine wave inverter waveform is smooth and continuous, without sudden changes or sharp edges. This makes pure sine waves more suitable for applications that are sensitive to inverter waveform, such as driving motors and audio equipment.
• Single frequency: A pure sine wave contains only one fundamental frequency and no higher-order harmonics. This means that pure sine waves will not cause problems such as electromagnetic interference and signal distortion, and can provide more stable and cleaner power.
• Symmetry: A pure sine wave has symmetry about the time axis, that is, within a cycle, the waveforms of the positive half cycle and the negative half cycle are exactly the same. This helps reduce mechanical and electrical stress on the device.

The AC inverter waveform of pure sine wave output, for example, 2000w pure sine wave inverter or 3000w inverter is closer to ideal alternating current and is more suitable for application scenarios with higher requirements for inverter waveform. However, the manufacturing cost and complexity of pure sine waves are usually high.

6. How to change square wave into sine wave?

There are many ways to convert a square wave into a sine wave, including:

• Use a D/A conversion chip to convert digital signals into analog signals.
• Use function generation chip to convert square wave into sine wave.
• Use the Wien bridge oscillation circuit to convert the square wave into a sine wave.

The Fourier transform can express a function that meets certain conditions into a trigonometric function (sine sum or cosine function) or a linear combination of their integrals, which means that the square wave can be split into linear, cubic, and quintic.

The ending frequency is f=1/(2πRC), so if the square wave frequency is 100Hz, the RC=0.00159, and when the square wave frequency is 1000Hz, RC=0.000159. Just decide the size of the capacitor and resistor according to the on-site conditions (the multiplication is certain). Generally, it will be better to use more capacitors and resistors, that is, multi-order filtering.