Classification analysis of PWM control

In the current rapid battle of science and technology, more and more PWM control methods have appeared on the market. As for the problem of PWM control, the basic principles of PWM control have been proposed a long time ago, and the problem is restricted by the development level of power electronic devices. . It has not been realized before the 1980s. Until the 1980s, with the emergence and rapid development of fully controlled power electronic devices.

PWM control technology has truly been applied. With the development of power electronics technology, microelectronics technology and automatic control technology, as well as the application of various new theoretical methods, such as modern control theory and nonlinear system control ideas, PWM control technology has achieved unprecedented success. Development. So far, a variety of PWM control technologies have appeared.

There is an important conclusion in the sampling control theory: when narrow pulses with equal impulses but different shapes are applied to links with inertia, their effects are basically the same. PWM control technology is based on this conclusion as a theoretical basis to control the conduction and conduction of semiconductor switching devices. Turn off the control, so that the output terminal gets a series of pulses with equal amplitude but unequal width, and use these pulses to replace the sine wave or other required waveforms. The width of each pulse can be modulated according to certain rules, which can be changed The size of the output voltage of the inverter circuit can also change the output frequency.

According to the characteristics of PWM control technology, there are mainly the following 8 types of methods so far:

Equal pulse width PWM method

The VVVF (Variable Voltage Variable Frequency) device was implemented using PAM (Pulse Amplitude Modulation) control technology in the early days. The inverter part can only output a square wave voltage with adjustable frequency but cannot regulate the voltage. The constant pulse width PWM method is It was developed to overcome this shortcoming of the PAM method and is the simplest one among the PWM methods. It uses a pulse train with the same width of each pulse as a PWM wave. By changing the period of the pulse train, the frequency can be modulated and changed. The pulse width or duty cycle can be adjusted, and the voltage and frequency can be coordinated with appropriate control methods. Compared with the PAM method, the advantage of this method is that it simplifies the circuit structure and improves the power factor at the input end, but at the same time, there are In addition to the fundamental wave, the output voltage also contains large harmonic components.

Random PWM

From the 1970s to the early 1980s, because high-power transistors at that time were mainly bipolar Darlington transistors and the carrier frequency generally did not exceed 5 kHz, the electromagnetic noise and harmonic vibrations caused by the motor windings attracted people's attention. .In order to achieve improvement, the random PWM method came into being. The principle is to randomly change the switching frequency to make the electromagnetic noise of the motor approximate to band-limited white noise (in the linear frequency coordinate system, the energy distribution of each frequency is uniform), although the noise The total number of decibels has not changed, but the intensity of colored noise characterized by a fixed switching frequency has been greatly weakened. Because of this, even today when IGBT has been widely used, random PWM still has a problem in situations where the carrier frequency must be limited to a lower frequency. Its special value; on the other hand, it shows that the best way to eliminate mechanical and electromagnetic noise is not to blindly increase the operating frequency. Random PWM technology provides a new way to analyze and solve this problem.

SPWM method

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The SPWM (Sinusoidal PWM) method is a relatively mature and currently widely used PWM method. An important conclusion in the sampling control theory mentioned earlier: when narrow pulses with equal impulses but different shapes are added to a link with inertia , the effect is basically the same. The SPWM method is based on this conclusion as the theoretical basis, and uses the PWM waveform whose pulse width changes according to the sinusoidal law and is equivalent to the sine wave, that is, the SPWM waveform to control the on and off of the switching device in the inverter circuit, so that the output The area of ​​the pulse voltage is equal to the area of ​​the desired output sine wave in the corresponding interval. By changing the frequency and amplitude of the modulation wave, the frequency and amplitude of the output voltage of the inverter circuit can be adjusted. The implementation of this method is as follows: plan.

equal area method

This plan is actually a direct explanation of the principle of the SPWM method. The same number of rectangular pulse sequences of equal amplitude but not equal width are used to replace the sine wave, and then the width and interval of each pulse are calculated, and these data are stored in the microcomputer. The PWM signal is generated by a table to control the on-off of the switching device to achieve the desired purpose. Since this method is based on the basic principles of SPWM control, it can accurately calculate the on-off time of each switching device, and the resulting waveform It is very close to a sine wave, but it has the disadvantages of cumbersome calculation, large data memory usage, and inability to control in real time.

hardware modulation method

The hardware modulation method is proposed to solve the shortcomings of the tedious calculation of the equal area method. Its principle is to use the desired waveform as the modulation signal, use the modulated signal as the carrier wave, and obtain the desired PWM waveform by modulating the carrier wave. Usually Using an isosceles triangle wave as the carrier wave, when the modulation signal wave is a sine wave, the resulting SPWM waveform is simple. The implementation method is simple. You can use analog circuits to form a triangular wave carrier wave and sine modulation wave generating circuit, and use a comparator to determine their intersection point. , By controlling the switching device on and off at the intersection moment, SPWM waves can be generated. However, this analog circuit has a complex structure and is difficult to achieve precise control.

software generation method

Due to the development of microcomputer technology, it has become easier to use software to generate SPWM waveforms. Therefore, the software generation method has emerged. The software generation method is actually a method of using software to realize modulation. It has two basic algorithms, namely natural sampling. method and regular sampling method.

natural sampling method

Using the sine wave as the modulating wave and the isosceles triangle wave as the carrier wave, the switching device is controlled on and off at the natural intersection point of the two waveforms. This is the natural sampling method. The advantage is that the resulting SPWM waveform is closest to the sine wave, but due to the triangle wave The intersection point with the sine wave is arbitrary, and the pulse center is not equidistant within a cycle, so the pulse width expression is a transcendental equation, the calculation is cumbersome, and it is difficult to control in real time.

regular sampling method

The regular sampling method is a widely used engineering practical method, generally using triangular waves as carrier waves. The principle is to use a triangular wave to sample a sine wave to obtain a step wave, and then use the intersection point of the step wave and the triangular wave to control the on and off of the switching device at all times, thereby realizing the SPWM method. When a triangular wave only samples a sine wave at its apex (or bottom) position, the pulse width determined by the intersection of the staircase wave and the triangular wave is symmetrical in position within a carrier cycle (i.e., the sampling period). This kind of The method is called symmetric rule sampling.

When a triangular wave samples a sine wave at both its peak and bottom points, the pulse width determined by the intersection of the staircase wave and the triangular wave generally has the same position within one carrier cycle (in this case, twice the sampling period). Asymmetric, this method is called asymmetric regular sampling. The regular sampling method is an improvement over the natural sampling method. Its main advantage is that it is simple to calculate and convenient for online real-time calculations. The asymmetric regular sampling method is closer to sinusoidal because of its large number of orders. Its disadvantage is that the DC voltage utilization rate is low and linear. The control span is smaller. The above are the major categories of PWM control methods. I hope that more and more designers will participate in this type of design.