Detailed explanation of PWM control principle and circuit application

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Detailed explanation of PWM control principle and circuit application [Copy link]

 

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PWM (Pulse Width Modulation) is a technology that uses pulses to output analog signals. It modulates the width of a series of pulses to produce an equivalent target waveform. It is widely used in measurement, communication, switching power supply, motor control and other fields.

This article will share the basic control principles of PWM technology and common circuit applications.

1. Basic PWM control principle

1. Theoretical basis

Area equivalence principle: When narrow pulses with equal impulses but different shapes act on a link with inertia, their effects are basically the same. "Impulse" refers to the area of the narrow pulse, and "basically the same effect" means that the output response waveform of the link is basically the same.

The narrow voltage pulses shown in the figure are applied to the first-order inertia link (Figure (a) below), and the output current i(t) response waveform of each narrow pulse is shown in Figure (b) below.

It can be seen that in the initial transient state, their response waveforms are slightly different, but the subsequent response waveforms are completely consistent.

The narrower the pulse applied, the smaller the waveform difference of the output response. If the above pulse is applied periodically, the response is also periodic. After decomposing the response signal using Fourier series, it can be seen that the characteristics of the response in the low frequency band will be very similar, and only differ in the high frequency band.

2. Control principle

Based on the area equivalence principle, PWM modulates a series of pulse widths to generate a narrow pulse waveform with the same pulse impulse as the target waveform, thereby achieving the equivalence of the target waveform (including shape and amplitude).

Here we take the commonly used half-sine wave equivalent as an example to introduce the modulation process:

First, the sine half wave is equally divided into N connected pulses of equal width and different amplitudes. Then, it is replaced by N rectangular pulses of equal amplitude and unequal width. The midpoint of the rectangular pulse coincides with the midpoint of the corresponding sine wave pulse, and the areas (impulses) of the two are equal.

In this way, a series of PWM waveforms equivalent to half-sine waves can be obtained - SPWM waveforms, and the pulse width of the SPWM waveform changes according to the sinusoidal law.

In addition to sine waves, PWM technology can also perform equivalent processing on waveforms such as DC and non-sinusoidal AC. Its basic principle is the same as SPWM control, which is based on the area equivalence principle.

2. PWM Technology Application

PWM chopper circuit and PWM inverter circuit are the two most typical circuit applications of PWM technology.

At present, most of the inverter circuits in practical application are of PWM type, and PWM chopper circuits have been widely used in DC motor speed regulation. In addition, chopper-controlled AC voltage regulation circuits and matrix frequency conversion circuits based on PWM technology are also used in the field of AC-AC conversion.

The following is an introduction to typical DC chopper circuits and PWM inverter circuits.

1. DC chopper circuit

Commonly used DC chopper circuits include: Buck circuit, Boost circuit, Buck-Boost circuit, Cuk chopper circuit and Sepic chopper circuit.

Here we take the Buck DC chopper circuit as an example to explain, the circuit structure is shown in the figure below:

In the figure : V is a fully controlled device, and D is a freewheeling diode.

When the device is in the on state, the power supply supplies power to the load and charges the capacitor, and the diode voltage VD=Vin; when the device is in the off state, the capacitor supplies power to the load and the diode voltage is approximately 0.

If the switch device is periodically given an on and off signal, the output voltage waveform is shown in the figure on the right. The power supply is turned on during the on time ton and cut off during the off time T-ton, so it is also called chopping.

The average output voltage is:

It can be seen that the DC chopper circuit can adjust the output voltage average value by adjusting the on and off time of the switching device to obtain the required DC voltage waveform.

2. PWM inverter circuit

There are two commonly used PWM inverter circuit control methods: calculation method and modulation method. Among them, the calculation method is cumbersome and needs to be recalculated when the output sine waveform changes, so the modulation method is often used to control the PWM inverter circuit.

The following explains the SPWM modulation process of the single-phase bridge inverter circuit and the SVPWM modulation process of the three-phase bridge inverter circuit.

▍Single -phase bridge inverter circuit

The desired output signal is used as the modulation signal, and an isosceles triangle wave or a sawtooth wave is used as the carrier signal. The intersection of the two signals is used to control the on and off of the switching device, and a series of PWM pulses with a width proportional to the signal wave amplitude will be obtained.

The specific modulation process is as follows:

In the positive half cycle of the modulation signal ur:

The switch device V1 remains turned on, V2 and V3 remain turned off, and V4 is alternately turned on according to the relationship between the modulation wave and the carrier wave.

When |Ur|＞|Uc|, V4 is turned on and the load voltage Uo=Ud; when |Ur|＜|Uc|, V4 is turned off and the load current will continue to flow through the diode D3, at which time the load voltage Uo=0. The output voltage Uo is a waveform that alternates between 0 and Ud levels.

In the negative half cycle of the modulation signal ur:

The switch device V2 remains turned on, V1 and V4 remain turned off, and V3 is alternately turned on according to the relationship between the modulation wave and the carrier wave.

When |Ur|＞|Uc|, V3 is turned on and the load voltage Uo=-Ud; when |Ur|＜|Uc|, V3 is turned off and D4 continues to flow, at which time the load voltage Uo=0. The output voltage Uo is a waveform that alternates between 0 and -Ud levels.

In one complete cycle of the modulation signal wave ur, the PWM waveform output by the inverter consists of three levels: ±Ud and 0.

▍Three -phase bridge inverter circuit

In addition to the above-mentioned SPWM sinusoidal pulse width modulation technology, SVPWM space vector pulse width modulation technology is also very commonly used in the field of motor control.

SPWM generates a rotating magnetic force in space by passing a sine wave with a phase difference of 120° into the motor stator, which drives the rotor to rotate. SVPWM , on the other hand, generates a rotating magnetic force by setting the on and off of the switch tube to form a rotating voltage vector in the motor.

The specific implementation method of SVPWM is as follows:

The circuit is composed of six switching devices. The upper and lower tubes are grouped together to form three half-bridge circuits. The upper and lower arms of the same half-bridge cannot be turned on or off at the same time.

Define the state as 1 when the upper bridge arm is turned on and the lower bridge arm is turned off, and the state as 0 when the upper bridge arm is turned off and the lower bridge arm is turned on, then we can get 8 voltage states (000, 100, 110, 010, 011, 001, 101, 111). Among them, 000 and 111 are zero vectors, and the other six are non-zero voltage vectors, dividing the space voltage vector diagram into 6 sectors.

During the implementation of SVPWM:

First, the required Uα and Uβ values are determined according to the rotor position and the collected current data, and then the sector in which the voltage U synthesized by Uα and Uβ is located is determined.

Then, the voltage vector (U1~U6) required for synthesizing the voltage U is selected according to the determined sector, and the holding time of the switching device is calculated based on the relationship of the vector synthesis.

Finally, the on and off of the switching devices are controlled according to these calculation results to achieve the expected voltage U output.

3. Common topology modulation methods

If PWM control technology is applied to different power topologies, the modulation method of the control signal will be different.

Common modulation methods include: phase shift modulation, pulse frequency modulation, pulse width modulation, unipolar frequency multiplication modulation and bipolar modulation. These modulation methods have mature packaging in PPEC digital power control chips and can be directly applied, providing efficient, stable and reliable solutions for digital power research and development.

Next, we introduce the modulation methods of some commonly used power topologies:

▍Phase- shifted full-bridge topology: Using phase-shift modulation, by adjusting the phase difference (i.e., phase shift angle) of the PWM signal of the bridge arm switching device, the duty cycle of the primary output voltage is changed to achieve the purpose of regulating the output voltage.

▍ LC series resonant topology: adopts pulse frequency modulation to adjust the output voltage by controlling the frequency _{fs of the PWM signal. In practical applications, it often works in the mode of 0~0.5 times the resonant frequency fr} and the mode of switching frequency _fs higher than the resonant frequency _fr .

▍ LLC resonant topology: Pulse frequency modulation is often used to adjust the output voltage by controlling the frequency _{fs of the PWM signal. The converter often operates in under-resonant mode, quasi-resonant mode and over-resonant mode.}

▍Inverter /rectifier topology: unipolar frequency-doubled modulation and bipolar modulation are commonly used. Unipolar frequency-doubled modulation uses two fundamental signals (ug _, -ug ₎ to intersect with the carrier signal to obtain two modulation signals. The two signals interact to produce a unipolar frequency-doubled modulation signal. The bipolar modulation signal is generated by the intersection of a fundamental wave and the carrier, and its waveform is positive and negative within half a fundamental wave period.

▍ Buck-boost topology: Pulse width modulation (PWM) and pulse frequency modulation (PFM) are commonly used. PWM uses a constant switching frequency and adjusts the pulse width (duty cycle) to achieve output voltage regulation. PFM adjusts the switching frequency to achieve output voltage regulation.

The basic control principle of PWM technology and common circuit applications are shared here. It should be noted that although PWM control technology simplifies the power conversion process and has advantages such as good stability, high efficiency, and high reliability, the implementation of PWM technology has high requirements for switching devices and high circuit noise. Therefore, in the application, everyone should choose the appropriate control method according to actual needs.

Jacktang

The picture in the article is very classic. Otherwise, it is difficult to understand the relationship between these two brothers, PWM and PFM, just by text description.

UUC

The introduction is very detailed and thorough. You can study it in depth. Thank you for sharing the technical dry goods.

盛世辉煌电子科学

It's really good, worth exploring and learning

振动试验仪器

PWM (Pulse Width Modulation) is a technology that uses pulses to output analog signals. It modulates the width of a series of pulses to produce an equivalent target waveform. It is widely used in measurement, communication, switching power supply, motor control and other fields.

This sentence is wrong.

振动试验仪器

The last figure on the first floor, the unipolar frequency doubling modulation and bipolar modulation, are the working principle diagrams of PWM.

The analog signal sine wave and high-frequency triangle wave pass through a high-frequency comparator such as LF311, and the comparator outputs a high-frequency square wave, and the duty cycle of the square wave is different. It can be said that the signal characteristics of the sine wave, the frequency and size become the periodic changes of the duty cycle of the square wave.

The right side of the first picture on the first floor is bipolar modulation.

振动试验仪器

Look up on the Internet, Chinese Baidu Chinese dictionary explanation

to

The alteration of a characteristic (such as amplitude, frequency, or phase) of a periodically or intermittently varying carrier or signal in order to convey information (as in telegraphy, telephone, radio broadcasting, or television)

Modulation_Words_Idioms_Baidu Chinesehttps
://hanyu.baidu.com/zici/s ?wd=%E8%B0%83%E5%88%B6

振动试验仪器

Pulse Width Modulation is the literal translation of PWM. PWM does not modulate the width of a pulse, but modulates a signal to generate a square wave with varying pulse widths. This square wave is a digital quantity.

Therefore, PWM is a signal conversion technology that can convert analog quantities into digital quantities.

After the analog signal is converted into a square wave using PWM, one of the most important uses is to control high-power switching tubes such as MOS tubes.

The high-power power supply is converted into power supply of different voltage levels through high-frequency switching, voltage transformation, rectification, filtering, etc. of the high-power switching tube.

zxhgll1975

Good information, I am still learning, thank you for the word analysis, very good learning material.

振动试验仪器

There are two ways to transform the waveform. The first one is the third picture, which uses a half-sine wave to transform.

Dividing the half-sine wave into N equal parts along the time axis means dividing the half-sine wave into N pulses with different areas. According to the formula of the sine wave, the size of the N areas and the size of the amplitude can be calculated, that is, N pulses with equal time but unequal amplitudes.

According to the principle of area equivalence, N pulses of unequal amplitude are converted into pulses of equal amplitude and unequal width.

This converts the half-sine wave analog signal into a series of square waves with equal amplitude and unequal width. Such a series of square waves is digitized.

振动试验仪器

The second conversion method is to directly convert the signal to be converted using discrete components.

The analog signal to be converted and the high-frequency triangular wave are connected to the two input ends of the high-frequency comparator, and the output end outputs a square wave of equal amplitude but unequal width.

t5cn

Very detailed PWM information, collection reference

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The current response waveform generated by the square wave signal passing through the inductor. If the power supply is changed to a 200V high-power power supply, and a switch tube is added to control the G pole of the switch with a PWM wave, when the G pole is at a high potential, it is instantly connected and then disconnected, and a high-voltage square wave pulse is output. Isn't this the working principle of the switch circuit? If the position of one component in the circuit is changed,

Figure 1-2 is the working principle diagram of a series switching power supply with rectification and filtering functions.

The working principle diagram of the series switching power supply with rectification and filtering function in Figure 1-2 is based on the circuit in Figure 1-1-a, with the addition of a rectifier diode and an LC filtering circuit.

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The preamplifier of the vibration table's power amplifier amplifies the sinusoidal signal output by the controller, and then converts it into a positive and a negative sinusoidal wave output in the dual operational amplifier, which are then transformed into two PWM square waves, as shown in the figure below.

振动试验仪器

The vibration table amplifier should use bipolar modulation, without the bottom positive and negative pulse output. The two square waves have a high-voltage overlap period on the time axis. The two square waves are transformed in the front part of the vibration table amplifier and finally enter the amplifier part. The two PWM square waves in the drive circuit are inverted and there is no high-voltage overlap period.