The difference between pwm and spwm and the principles and advantages of PWM

What are pwm and spwm? What is the difference between them? Pwm and spwm have been widely used, but many people still do not understand the difference between them. This article first introduces the difference between pwm and spwm, and secondly explains the principle of PWM and advantages, and introduces what the principle of spwm is.

1. The difference between pwm and spwm

1. Difference 1

PWM is the abbreviation of Pulse Width Modulation in English. It is a modulation method that changes the pulse width of the pulse sequence according to certain rules to adjust the output volume and waveform. PAM is the abbreviation of Pulse Amplitude Modulation in English. It is a modulation method that changes the pulse amplitude of a pulse sequence according to certain rules to adjust the output volume and waveform.

2. Difference 2

PWM is pulse width modulation and is composed of continuous square waves with a certain pulse width. SPWM uses sine waves to modulate and synthesize square waves with regular changes in sine waves based on PWM. Principle of SPWM The signal wave of sinusoidal PWM is a sine wave, which is equivalent to a series of rectangular pulse waveforms of equal amplitude and unequal width. The pulse width is generated by the natural intersection of sine waves and triangular waves. There are many ways to generate sine waveforms, but the three main typical ones are: symmetrical rule sampling method, asymmetrical rule sampling method and average symmetrical rule sampling method.

In the first method, because the generated PWM pulse width is too small, the output voltage of the inverter cannot reach multiple times of the DC side voltage;

The second method requires sampling sine waves twice in one carrier cycle. Obviously the output voltage is higher than the former, but for the microprocessor, the data processing volume is increased. When the carrier frequency is higher, the requirements for the microcomputer are higher;

The third method is the most widely used and takes into account the advantages of the first two methods.

2. PWM

Introduction to pwm

PWM, Pulse Width Modulation, is the abbreviation of "Pulse Width Modulation" in English, referred to as pulse width modulation. It is a very effective technology that uses the digital output of a microprocessor to control analog circuits. It is widely used in everything from measurement to Communications into many areas of power control and conversion.

PWM principle

With the development of electronic technology, a variety of PWM technologies have emerged, including: phase voltage control PWM, pulse width PWM method, random PWM, SPWM method, line voltage control PWM, etc., and the PWM technology used in nickel-metal hydride battery smart chargers The pulse width PWM method uses a pulse train with equal pulse width as a PWM waveform. By changing the period of the pulse train, the frequency can be adjusted. By changing the pulse width or duty cycle, the voltage can be adjusted. Using appropriate control methods, the voltage can be adjusted. Coordinate changes with frequency. The charging current can be controlled by adjusting the PWM period and PWM duty cycle.

The value of an analog signal can vary continuously with no limits on its time and amplitude resolution. A 9V battery is an analog device because its output voltage is not exactly 9V, but changes over time and can take on any real value. Similarly, the current drawn from the battery is not limited to a set of possible values. The difference between analog signals and digital signals is that the values ​​of the latter can usually only belong to a predetermined set of possible values, such as values ​​in the set {0V, 5V}.

Analog voltages and currents can be used directly to control, for example, the volume of a car radio. In a simple analog radio, the volume knob is connected to a variable resistor. When the knob is turned, the resistance value becomes larger or smaller; the current flowing through the resistor also increases or decreases, thereby changing the current value driving the speaker, making the volume larger or smaller accordingly. Like a radio, the output of an analog circuit is linearly proportional to the input.

While analog control may seem intuitive and simple, it's not always very economical or feasible. For one, analog circuits tend to drift over time, making them difficult to adjust. The sophisticated analog circuits that can solve this problem can be very large, cumbersome (like old home stereo equipment) and expensive. Analog circuits can also become very hot, with power dissipation proportional to the product of voltage and current across the working components. Analog circuits can also be sensitive to noise, and any disturbance or noise will certainly change the magnitude of the current.

By digitally controlling analog circuits, system cost and power consumption can be significantly reduced. In addition, many microcontrollers and DSPs already include PWM controllers on the chip, making the implementation of digital control easier.

Advantages of PWM

One advantage of PWM is that the signals from the processor to the controlled system are all in digital form, without the need for digital-to-analog conversion. Keeping the signal in digital form minimizes the effects of noise. Noise can only affect digital signals if it is strong enough to change a logic 1 to a logic 0 or a logic 0 to a logic 1. Enhanced immunity to noise is another advantage of PWM over analog control, and it is also the main reason why PWM is sometimes used for communications. Moving from analog signals to PWM can greatly extend communication distance. At the receiving end, an appropriate RC or LC network can filter out the modulated high-frequency square wave and restore the signal to analog form.

In short, PWM is economical, space-saving, and has strong noise immunity. It is an effective technology worthy of being used by engineers in many design applications.

3. SPWM

Introduction to SPWM

The full name of PWM is Pulse Width Modulation (Pulse Width Modulation), which changes the equivalent output voltage by changing the duty cycle of the output square wave. It is widely used for motor speed regulation and valve control, such as electric vehicle motor speed regulation.

The so-called SPWM changes the modulation pulse method on the basis of PWM. The pulse width time duty cycle is arranged according to the sinusoidal law, so that the output waveform can be output as a sine wave after appropriate filtering. It is widely used in DC and AC inverters, such as advanced UPS. Three-phase SPWM uses SPWM to simulate the three-phase output of the mains, and is widely used in the field of frequency converters.

SPWM principle

The signal wave of sine PWM is a sine wave, which is equivalent to a series of rectangular pulse waveforms of equal amplitude and unequal width. The pulse width is generated by the natural intersection of sine waves and triangular waves. There are many ways to generate sine waveforms, but the three main typical ones are: symmetrical rule sampling method, asymmetrical rule sampling method and average symmetrical rule sampling method. In the first method, the generated PWM pulse width is too small, so the output voltage of the inverter cannot reach twice the DC side voltage; in the second method, the sine wave needs to be sampled twice in one carrier cycle, so the output voltage is obviously higher than the former. , but for the microprocessor, the data processing capacity is increased. When the carrier frequency is higher, the requirements for the microcomputer are higher; the third method is the most widely used, and it takes into account the advantages of the first two methods.

Although SPWM can obtain three-phase sinusoidal voltage, the voltage utilization rate on the DC side is low, and is up to three times the DC side voltage. This is the biggest shortcoming of this method. The above are the advantages, disadvantages and differences between pwm and spwm. I hope it can help you.