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Jun 21 2019

Design of Single-Phase Sine Wave SPWM Inverter Power Supply Based on SG3525

Introduction

The single-phase sine wave SPWM inverter power supply required in this article is designed to form a circuit by using operational amplifiers, diodes, power FETs, capacitors and resistors.

Inverter power supply is a device that uses power electronics to convert power. It obtains a constant-frequency AC output from an AC or DC input. Through the analysis of the circuit, the determination of the parameters selects the most suitable solution. The output frequency is controlled by voltage and the amplitude of the waveform is determined by the resistance.

This design takes the SG3525 driver chip as the core, completes the parameter design of single-phase sine wave SPWM inverter power supply, and uses the obtained results to complete the connection of the actual circuit. Through debugging and analysis, the correctness of the design is verified. 


Catalog

Article core

Single-Phase Sine Wave SPWM Inverter Power Supply

Overview

1.1 Development Background of Inverter Power Supply

1.2 Research Status of Inverter Power Supply

1.3 The Main Work of the Design

The Principle and Application of SPWM Inverter Power Supply

2.1 The Control Principle of SPWM

2.2 Development Prospects of SPWM Control

The Design of Hardware Circuit

3.1 Introduction of SG3525

3.2 Wien Bridge Oscillator Circuit

3.3 The Analysis of Shift Circuit

3.4 The Analysis of the Working Principle of the Inverter Circuit

System Detection and Analysis

4.1 Debugging of the Part of the Sine Generator

4.2 Inverter Part and Overall Operation Result


1. Overview

1.1 Development Background of Inverter Power Supply

Inverter power supply is a device that uses power electronics technology to convert power. It obtains a constant-frequency AC output from an AC or DC input. Inverter power supply technology is a comprehensive professional technology. It spans multiple fields such as power, electronics, microprocessor and automatic control. It is one of the hotspots in the power electronics industry and scientific research. Inverter power supplies are widely used in aviation, navigation, electric power, railway transportation, post and telecommunications and many other fields.

The development of inverter power supply is related to the development of power electronic devices. The development of devices drives the development of inverter power supplies. Inverter power supply appeared in the 1960s, where power electronics technology developed rapidly. So far, it has experienced three stages of development.

The first generation of inverter power supply is a switching device using a thyristor (SCR) as an inverter and it is called a thyristor inverter. Although the appearance of the thyristor inverter can replace the rotary converter unit, since the SCR is a device without self-shutdown capability, it is necessary to increase the commutation circuit to forcibly turn off the SCR. However, the commutation circuit is complicated, the noise is large, the volume is large, and the efficiency is low. All of these limit the further development of the inverter power supply.

The second-generation inverter power supply is a switching device that uses a self-shutdown device as an inverter. Since the late 1970s, various self-shutdown devices have emerged, including turn-off thyristors (GTOs), power thyristors (GTRs), power FETs, insulated gate bipolar transistors (IGBTs) and so on. The application of the self-shutdown device in the inverter greatly improves the performance of the inverter power supply.

The third-generation inverter power supply uses real-time feedback control technology to improve the performance of the inverter power supply. The real-time feedback control technology is proposed for the shortcomings of the second-generation inverter power supply with low nonlinear load adaptability and poor dynamic characteristics. It is a new power control technology developed in the last ten years and is still improving and developing. The adoption of real-time feedback control technology has made the performance of the inverter power supply a qualitative leap.

1.2 Research Status of Inverter Power Supply

The original inverter power supply uses a thyristor (SCR) as a switching device and it is called a controllable inverter power supply. Since the SCR is a device without shutdown capability, it is necessary to force the shutdown of the SCR by adding a commutation circuit, and the SCR commutation circuit limits the further development of the inverter power supply. With the development of semiconductor technology and communication technology, power electronic devices with self-shutdown capability have emerged, and power transistors (GTR), turn-off thyristors (GTOs), power field effect transistors (MOSFETs), and insulated gate bipolar transistors (IGBT) appear one after another. The application of the shutdown device in the inverter greatly improves the performance of the inverter power supply and the switching frequency. Therefore, the frequency of the subharmonic wave in the output voltage of the inverter bridge is relatively high, so that the size of the output filter is reduced. Furthermore,the adaptability of the nonlinear load is improved. Initially, PWM control technology with output voltage RMS or average feedback is widely used in the control of inverters with fully controlled devices. The stability of its output voltage is achieved by the effective value of the output voltage or the feedback control of the average value. The method of using output voltage RMS or average feedback control has the advantages of simple structure and easy implementation, but it has the following disadvantages:

(1) The adaptability of linear load is not strong

(2) The presence of dead time will cause the PWM wave to obtain low-order harmonic wave that are not easily filtered, causing waveform distortion of the output voltage.

(3) The dynamic performance is not good, and the output voltage adjustment time is long when the load is abrupt.

In order to overcome the shortcomings of the single voltage RMS or the average feedback control method, the feedback control technology is applied. It is a new power control technology developed in the past 10 years and it is still improving and developing. The adoption of real-time feedback control technology makes a qualitative leap in the performance of the inverter power supply. Real-time feedback control technology is various and mainly includes the following types:

Principle of Harmonic Wave Control

When the load of the inverter power supply is a rectified load, since the load current contains a large amount of harmonic wave, the harmonic wave current is reduced in the internal resistance of the inverter power source, causing the output voltage waveform of the inverter power supply to be distorted, and the harmonic wave compensation control can be a better solution to solve this problem, especially the specific harmonics wave added to the PWM wave of the inverter bridge. It can cancel the influence of the harmonic wave in the load current on the output voltage waveform, and reduce the waveform distortion of the output voltage. And this method can only be implemented by a digital signal processor.

Dead-beat Control

In 1959, Kalman first proposed the theory of dead-beat control for state variables. In 1985, the GokhalePESC annual meeting proposed the application of dead-beat control to inverter control. The dead-beat control of the inverter caused widespread attention. The dead-beat control is a microcomputer-based control principle. The method calculates the switching time of the next sampling period according to the state equation of the inverter power system and the output feedback signal, so that the output voltage is equal to the given signal at each sampling point. The disadvantage of dead-beat control is that the algorithm is more complicated and difficult to be realized. It requires a high degree of accuracy in the system model and it is sensitive to the changes in load size and load characteristics. When the load size and the load properties change, the ideal sine wave output can’t be obtained.

Repetitive Control

In order to eliminate the influence of the nonlinear load on the inverter output, a repetitive control technique is introduced in the UPS inverter control. Repetitive control is a control method based on the internal model principle. It stores the deviation of a fundamental wave period for the control of the next fundamental wave period. After several cycles of the fundamental wave period, the high control frequentness can be achieved. In this control method, the input signal applied to the control object is superimposed with a past control deviation in addition to the deviation signal. This past control deviation is actually a control deviation of the fundamental wave period, and the previous fundamental wave is reflected to the present, and is added to the control object for control together with the current deviation. This control mode deviation seems to be repeated, so it is called repetitive control. It has good stability, strong control ability but slow dynamic response. Therefore, repetitive control is generally not used alone for inverter control, but combined with other control methods to achieve overall system performance.

Single Voltage Instantaneous Value Feedback Control

The basic idea of this control method is to compare the instantaneous feedback of the output voltage with a given sine wave, and use the instantaneous deviation as the control quantity to dynamically adjust the output PWM wave of the inverter bridge. Compared with the traditional PWM control method, this method can dynamically adjust the PWM wave, so the system's rapidity, immunity, adaptability to nonlinear loads, and waveform quality of the output voltage are improved comparing with the traditional PWM control method. The disadvantage of this method is that the stability is not good, especially when it is no load.

Voltage Instantaneous Feedback Control with Current Inner Loop

The instantaneous voltage value feedback control method with current inner loop is developed on the basis of single instantaneous voltage value feedback control method. In this method, not only the instantaneous value feedback of the output voltage is introduced, but also the instantaneous value feedback of the filter capacitor current. The voltage loop is an outer loop, and the inner loop has the function of transforming the filter capacitor current or the filter inductor current into a controllable current source. In this way, a transfer function with a single pole is formed between the control input and the output voltage, so that the stability of the system is greatly improved, and the disadvantage that the single voltage instantaneous value feedback control system is easy to oscillate when it is no load is overcome. Due to the improved stability, the voltage regulator gain can take a relatively large value, so the dynamic performance of the output voltage is greatly improved when the load is suddenly added or unloaded, the anti-interference performance is greatly improved, and the adaptability to the nonlinear load is greatly improved as well. 

1.3 The Main Work of the Design

The design is divided into four phases to complete: idea analysis, architecture design, hardware connection and system debugging.

Firstly, the sine wave signal generator is designed. The sine wave signal generator is composed of two parts: the Wien bridge oscillation circuit and the shift circuit, as shown in figure 1-1.


Figure 1-1 Sine Wave Signal Generator 

Figure 1-1 Sine Wave Signal Generator

 

As shown in the figure, the 50Hz sine wave generated by the sine wave signal generator is sent to the sawtooth wave of pin 9 and pin 5 of the SG3525 chip, thereby obtaining the SPWM signal and changing the amplitude of the sine wavethat is, changing M. And then you can change the output voltage amplitude, normally M ≤ 1.

Design the SPWM drive circuit again as shown in Figure 1-2. A sine wave generator generates a 50Hz sine wave with variable amplitude, which is sent to the terminal 9 of the SG3525. After compared with the sawtooth wave of pin 5 of the SG3525, it outputs the modulated (the modulation frequency is about 10kHz) SPWM waveform. After the phase converter is inverted, two mutually inverting PWM drive signals are obtained, respectively drive the power FETs VT1, VT2 to make them alternately turned on. Therefor, the SPWM waveform is obtained on the secondary side of the high-frequency transformer. After LC filtering, a 50Hz sine wave is obtained, and the amplitude can be changed by the potentiometer RP.


 Figure 1-2 SPWM Inverter Circuit

Figure 1-2 SPWM Inverter Circuit

 

2. The Principle and Application of SPWM Inverter Power Supply

2.1 The Control Principle of SPWM

The ideal output voltage of the inverter circuit is shown in Figure 2-1(a). The sine wave u0=Uo1sinωt. The output voltage of the voltage-type inverter circuit is a square wave. If a sine wave half-wave voltage is divided into N equal parts, and the area surrounded by each sinusoid is replaced by an equal rectangular pulse of equal area, and the midpoint of the rectangular pulse coincides with the middle of the corresponding sinusoidal aliquot, the obtained pulse train shown in figure 2-1(b) is called PWM waveform. The other half of the sine wave can be equivalent in the same way. It can be seen that the pulse width of the PWM waveform changes according to a sinusoidal law, which is called a SPWM (Sinusoidal Pulse Width Modulation) waveform.


Figure 2-1 SPWM Voltage Equivalent Sinusoidal Voltage

Figure 2-1 SPWM Voltage Equivalent Sinusoidal Voltage


According to the sampling control theory, when the narrow pulses with the same impulse and different shapes are added to the link with inertia, the effect is basically the same. The higher the pulse frequency, the closer the SPWM waveform is to the sine wave. When the output voltage of the inverter is SPWM waveform, the lower harmonic wave will be well suppressed and eliminated, and the higher harmonic wave can be easily filtered out, so that the sine wave output voltage with extremely low distortion rate can be obtained.

The SPWM control mode controls the on and off of the switching device of the inverter circuit, so that the output terminal has a series of pulses with equal amplitude and unequal width, and these pulses are used to replace the sine wave or other required waveforms.

Theoretically speaking, after the sine wave frequency, the amplitude and the number of pulses in the half cycle are given in the SPWM control mode, the width and interval of the pulse waveform can be accurately calculated, Then, the result of the calculation controls the on and off of each switching device in the control circuit to obtain the required waveform. This method is called calculation. The calculation method is very cumbersome. When the frequency, amplitude or phase of the output sine wave changes, the result changes, and it is rarely used in practice.

In most cases, sine waves and isosceles triangle rubber are used to determine the width of each rectangular pulse. There is a linear relationship between the upper and lower width and height of the isosceles trianglesand are they are bilaterally symmetrical. When it intersects any smooth curve, a set of rectangular pulses whose pulse width is proportional to the value of the curve is obtained. This method is called modulation method. The output signal is a modulated signal, and the triangular wave that receives the modulation is called a carrier. When the modulated signal is a sine wave, the SPWM waveform is obtained.

According to the previous method analysis, the advantages of the SPWM inverter circuit are as follows:

 To get close to the sine wave output voltage to meet the load needs. 

 The rectifier circuit uses diode rectification to obtain a higher power factor. 

 Only one level of controllable power link is used so that the circuit structure is simple. 

 After the output pulse width control, the output voltage can be changed, which greatly speeds up the dynamic response speed of the inverter.  

2.2 Development Prospects of SPWM Control

In recent years, with the increasing application of inverter power supply in various industries, the use of sinusoidal pulse width modulation (SPWM) technology to control the inverter power supply to improve the control effect of the entire system becomes a problem that people are constantly exploring. There are many ways to realize the control of SPWM. One is to generate SPWM waveforms by using hardware circuits such as analog circuits and digital circuits. The waveform of this method is stable and accurate, but the circuit is complex, bulky and cannot be automatically adjusted. The second is to use the single-chip microcomputer. Microcontrollers such as DSP implement the digital control method of SPWM. Because of its internal integration of multiple control circuits, such as PWM circuit and programmable counter array (PCA), this method has the advantages of simple control circuit, fast running speed and strong anti-interference.

 

3 The Design of Hardware Circuit 

3.1 Introduction of SG3525

With the development of power technology, power MOSFETs have been widely used in switching converters. For this reason, Silicon General Semiconductor Corporation has introduced SG3525. The SG3525 is used to drive N-channel power MOSFETs, and its products have been widely praised as soon as they are introduced. SG3525 series PWM controller is divided into three categories: military products, industrial products and civilian products. The features, pin functions, electrical parameters, operating principles, and typical applications of SG3525 will be described below.

(1) Introduction of PWM Control Chip SG3525

The SG3525 is a current-controlled PWM controller. The so-called current-controlled pulse width modulator adjusts the pulse width according to the feedback ammeter. The signal flowing through the output inductor is directly compared to the error amplifier output signal at the input of the pulse width comparator, thereby adjusting the duty cycle to make the output inductor peak current changes in response to the error voltage variation. Due to the structure of double loop system(the voltage loop and the current loop ), the voltage regulation rate, load regulation rate and transient response characteristics of the switching power supply are improved, which makes it an ideal new controller.

(2) The Internal Structure and Working Characteristics of SG3525

 

Figure 3-1 Pin Diagram of SG3525

Figure 3-1 Pin Diagram of SG3525


Figure 3-2 Block Diagram of the SG3525 

Figure 3-2 Block Diagram of the SG3525

 

— Phase input (Pin 1): Inverting input of the error amplifier. The gain of the error amplifier is nominally 80dB. The magnitude of the error amplifier depends on the feedback or output load. The output load can be pure resistor or a combination of a resistive element and a capacitive element. The error amplifier has a common-mode input voltage range of 1.5 to 5.2V. This terminal is typically connected to a resistor divider connected to the output voltage of the power supply. In the case of negative feedback control, the power supply output voltage is divided and compared with the reference voltage.

— Phase input (Pin 2): This terminal is usually connected to the voltage divider resistor of the reference voltage pin 16. The reference voltage of 2.5V is compared with the sampling voltage of pin 1.

— Synchronous terminal(Pin 3): It is used for external synchronization. When multiple chips need to work synchronously, each chip has its own oscillation frequency, which can be connected to their pin 4 and pin 3 respectively. At this time, the operating frequency of all chips is synchronized with the fastest chip operating frequency; A single chip can also operates at an external clock frequency.

— Synchronous output terminal(pin 4): Synchronous pulse output. It is used as a synchronous operation for multiple chips.

— Oscillation Capacitor terminal(Pin 5): One end of the oscillating capacitor is connected to pin 5, and the other end is directly connected to the ground.

— Oscillation resistor terminal(pin 6): One end of the oscillating resistor is connected to pin 6, and the other end is directly connected to the ground.

— Discharge terminal(Pin 7): The discharge of Ct is determined by the dead band resistance at both ends of 5 and 7.

— Soft start (pin 8): The inverting terminal of the comparator, that is, the soft starter control terminal (pin 8). Pin 8 can be connected to the soft start capacitor.

— A resistor and a capacitor are connected between the pin 9 of the output of the error amplifier and the pin 1 of the inverting input of the error amplifier to form a PI regulator, which compensates for the amplitude-frequency and phase-frequency response characteristics of the system.

— Lock terminal (pin 10): Pin 10 is an input terminal of the PWM latch, and an overcurrent detection signal is generally connected at this end.

— Pulse output terminal (pin 11, pin 14): The output stage uses a push-pull output circuit to drive the FET to turn off faster.

— Ground (Pin 12): All voltages on the chip are relative to pin 12, it is the power ground and also the signal ground.

— Voltage input terminal of push-pull output circuit (pin 13): As the voltage source of the push-pull output stage, it increases the output power of the output stage.

— Chip power terminal (Pin 15): The DC power supply is divided into two paths from pin 15: one is used as the operating voltage of the internal logic and analog circuit; the other is sent to the input of the reference voltage regulator to generate  the internal reference voltage of 5.1V ± 1.

— Reference voltage terminal (Pin 16): The voltage of the reference voltage terminal pin 16 is internally controlled at 5.1V ± 1. It can be used as a reference voltage for the error amplifier after voltage division.

(3) Features of SG3525 Pulse Width Modulator

· Operating voltage range: 8~35V.

· 5.1V ± 1% fine-tuned reference power supply.

· The frequency range of the oscillator: l00~400kHz.

· It has oscillator external synchronization function.

· The dead time is adjustable.

· Built-in soft start circuit.

· It has the function of under voltage lock out.

· It has PWM latch function and multi-pulse is prohibited.

· Turn off the pulse one by one.

· Dual output: 500mA (peak).

3.2 Wien Bridge Oscillator Circuit

The hardware circuit consists of three parts as shown in Figure 3-3.

 

Figure 3-3 Composition of Hardware Circuit

Figure 3-3 Composition of Hardware Circuit

 

The sine wave generator consists of two parts. The first half is the RC series-parallel sine wave oscillator, the second half is the shift circuit, and finally the sine wave signal is added to the input pin of the SG3525. Figure 3-4 is a circuit diagram of the selected sinusoidal signal generating device.


Figure 3-4 Sine Wave Signal Generator 

Figure 3-4 Sine Wave Signal Generator

 

As shown in figure 3-4, the left side of resistor R6 is a sine wave oscillating circuit composed of Ua741 and Wien bridge feedback network. R4, C1 and R5, C2 form the two arms of the Wien bridge, which form a positive feedback frequency selection network; the other two arms of the Wien bridge are composed of R1 and R2, R3, RP1, which is the negative feedback network of Ua741. Together with the integrated op amp, they form the amplification of the oscillating circuit. The entire oscillating condition is mainly determined by the parameters of the two feedback networks.

The oscillating circuit is a frequency selective network with RC series and parallel connection, and the oscillation frequency can be calculated by f=1/2*pi*RC. In order to make the Wien bridge oscillation circuit meet the vibration condition, it is necessary to require A≥3, that is, R1≥2R2, that is, R2+R3+RP2≥2R1 in this circuit. Therefore, in the linear interval of the op amp, the circuit cannot satisfy the constant amplitude balance condition. Only when the op amp enters the nonlinear region, the circuit can satisfy the amplitude balance condition, and thus the output voltage signal will produce nonlinear distortion. In order to reduce the nonlinear

distortion, the amplification factor A of the circuit should be as close as possible to 3. However, this will make the oscillation circuit have a small margin for the vibration adjustment, and it may not vibrate when the operating conditions of the circuit are slightly changed. If the negative feedback network of the amplifying circuit uses a nonlinear component, it can ensure that A is large enough to make the circuit start to oscillate when the output signal is small; and as the output signal gradually increases, A can gradually become smaller, and it is also possible to make the circuit meet the amplitude balance condition before the op amp enters the nonlinearity. Therefore, a stable and undistorted sine wave output signal can be obtained.

Two diodes are added to the circuit for stabilization. It uses the nonlinearity of the diode to automatically adjust the strength of the negative feedback to maintain a constant output voltage. If the starting point A>3, the amplitude will gradually increase. During the oscillation process, VD1 and VD2 will be alternately turned on and off. There is always a diode in the forward conduction state connected in parallel with the resistor. Because the forward resistance of the diode decreases as the voltage increases, the negative feedback increases as the amplitude increases, that is, A decreases as the amplitude increases until the amplitude balance condition is satisfied, and a certain amplitude output is maintained. Therefore, adjusting RP1 can change the amplitude of the oscillation to obtain the minimum distortion. In general, the use of diodes for stabilizing circuits is simple and economical. Although the waveform distortion may be large, it is suitable for applications where such requirements are not high.

The Wien bridge sinusoidal oscillating circuit can easily change the oscillating frequency, and the frequency adjustment range is also very wide. Many oscillating circuits currently use this type of circuit. In addition, the oscillating frequency of the RC sine wave oscillating circuit is inversely proportional to the product of RC. If it is desired to add its oscillating frequency, it is necessary to reduce the values of R and C. However, reducing R will increase the load of the amplifying circuit, and reduce C cannot exceed a certain limit. Otherwise, the oscillation frequency will be unstable due to the influence of parasitic capacitance. In addition, the bandwidth of the general integrated operational amplifier is narrow, which also limits the increase of the oscillation frequency. Therefore, the oscillating frequency of the RC sine wave oscillating circuit composed of the integrated operational amplifier generally does not exceed 1 MHz. The output sine wave frequency of this circuit is 50 Hz, which is within the required range, so it is feasible to select the RC sine wave oscillating circuit. 

3.3 The Analysis of Shift Circuit

The SG3525 chip oscillates to produce a sawtooth wave and the apex of the sawtooth wave is about 3.3V and the lowest point is about 0.9V. The sine wave generated by the sinusoidal signal generator needs to be compared with the sawtooth wave generated by the SG3525, so the sine wave is shifted to the corresponding position.

In figure 3-4, the circuit on the right side including R6 is the displacement circuit. The resistor R6 and the varistor RP3 first reduce the amplitude of the sinusoidal signal output from the first half, adjust RP3 to change to the required amplitude range, and then output.

The functions of resistors R7, R8 and varistor RP2 are to shift the sinusoidal signal and adjust RP2 to shift the sine wave to the desired position of the circuit. This is followed by an op amp circuit with negative feedback. Moreover, there is a capacitor on it, indicating that there is a large negative feedback effect on a certain frequency segment. The potential of the non-inverting input terminal of the operational amplifier is zero. According to the principle of the virtual short circuit, the potential of the inverting input terminal is also zero, so the op amp has an output waveform when the input voltage is less than zero. 

3.4 The Analysis of the Working Principle of the Inverter Circuit

The main function of the inverter circuit is to invert the direct current into an AC with a certain frequency or variable frequency and supply load. The inverter circuit selected in this article is shown in Figure 3-5. Ud=15 is the DC input voltage. When the switch turns VT1 on and VT2 is turned off, the inverter output voltage U0=Ud; when the switch turns VT2 on and VT1 is turned off, the inverter output voltage U0=-Ud. When VT1 and VT2 are alternately switched at the frequency fs, an alternating voltage waveform obtained on the output is shown in figure 3-6 , and the period Ts=1/fs. In this way, the DC voltage Ud becomes the alternating voltage U0. U0 contains each harmonic, and what this article wants is to get the sine wave voltage, which can be obtained by LC filter.

 

Figure 3-5 SPWM Inverter Circuit

Figure 3-5 SPWM Inverter Circuit


Figure 3-6 Alternating Voltage Waveform 

Figure 3-6 Alternating Voltage Waveform

 

4 System Detection and Analysis

4.1 Debugging of the Part of the Sine Generator

The test results are as follows: Table 4-1 shows the relationship between the Wien oscillator circuit potentiometer RP1 and the output voltage Uo.

 

Table 4-1 Relationship between Output Voltage and Potentiometer RP1

Table 4-1 Relationship between Output Voltage and Potentiometer RP1

 

The sine wave generated by the oscillation during the operation and the sine wave after the displacement are shown in figures 4-1 and 4-2. The vibration amplitude of the sine wave is 3V, and the RP1 is 1.74K during the vibration. The maximum undistorted amplitude is 6V and the RP1 is 5.20K.

The pulse width modulated SG3525 oscillator produces a sawtooth wave apex of approximately 3.3V and a lowest point of approximately 0.9V. The sine wave after displacement should be adjusted to be close to it. Finally, the adjusted value of RP3 is 5.28K, and the adjusted value of RP2 is 2.03K.

 

Figure 4-1 Wien Oscillation Circuit Waveform

Figure 4-1 Wien Oscillation Circuit Waveform


Figure 4-2 Shift Circuit Waveform 

Figure 4-2 Shift Circuit Waveform

 

4.2 Inverter Part and Overall Operation Result

A 50Hz sine wave with variable amplitude is generated by the waveform generator, sent to terminal 9 of the SG3525, and compared with the pin 5 (sawtooth wave) of the SG3525, and the modulated SPWM waveform (modulation frequency is about 10 kHz) is output. After being inverted by the phaser, two PWM driving signals which are mutually inverted are obtained, respectively driving the power FETs VT1 and VT2, so that VT1 and VT2 are alternately turned on, thereby obtaining an SPWM waveform on the secondary side of the high frequency transformer. After LC filtering, a 50Hz sine wave is obtained, and the amplitude can be changed by the potentiometer RP.

The waveform is shown in figure 4-3. table 4-2 shows the relationship between the potentiometer RP and the output voltage Uo in the inverter circuit.

 

Table 4-2 Relationship between Output Voltage and Potentiometer RP

Table 4-2 Relationship between Output Voltage and Potentiometer RP

 

The sawtooth waveform of the pin 5 of the SG3525 chip is shown in Figure 4-3.


 Figure 4-3 Sawtooth Waveform of Pin 5

Figure 4-3 Sawtooth Waveform of Pin 5

 

The sine wave pulse width modulation signal waveform of the pin 13 of the SG3525 chip is shown in figure 4-4.


Figure 4-4 Pulse Width Modulated Sine Waveform 

Figure 4-4 Pulse Width Modulated Sine Waveform

 

The output single-phase sine wave inverter power supply signal waveform is shown in figure 4-5.

 Figure 4-5 Output Sine Wave Inverter Power Supply Signal Waveform

Figure 4-5 Output Sine Wave Inverter Power Supply Signal Waveform


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