A Preliminary Study on the Design of Inverter Circuit Based on PIC Microcontroller

　　In view of the requirements of variable frequency amplitude modulation of modern power supply, it is proposed to use PIC16F873 to generate SPWM wave to control IR2136 to trigger IGBT to generate PWM wave to act on the inverter to produce a standard sine waveform, thereby realizing variable frequency amplitude modulation. At the same time, the AD module is used to sample the inverter bridge output and filter it to realize the PI closed-loop control of the system. The simulation analysis is carried out through the SIMULINK component in MATLAB. The results show that this scheme has a fast dynamic response speed of output voltage, good precision control and real-time performance, small waveform distortion and high reliability.

　　With the advancement of science and technology, power quality has become the basis for the normal and good operation of various electrical equipment. A continuous research topic in the field of power technology is to study the reliability and stability of power supply, which is the lifeblood of the electronic information industry.

　　As the core part of the power supply, the modulation technology of the inverter largely determines the quality of the power supply output voltage. At present, the most commonly used modulation technology is sinusoidal pulse width modulation (SPWM). With the emergence and widespread application of single-chip microcomputers, intelligent control methods have gradually replaced traditional discrete component circuit generation methods or dedicated chip generation methods. The advantage of intelligent inverter power supply is that it can not only realize the output of modulation signals, but also provide an interface for monitoring, processing and displaying system data parameters. At the same time, it is better combined with modern computer technology to produce fault self-diagnosis and self-protection functions, which can improve the stability of the system.

　　Under the premise of fully considering the industrial control cost and stability requirements, this design uses PIC microcontroller as the control core, and then assists with related external circuits to form an inverter power control system with the advantages of stability and intelligence.

　　1. Specific circuit design

　　The single-phase bridge inverter circuit is shown in Figure 1. [1] When the circuit is working normally, the two pairs of switch tubes need two sets of driving pulses with opposite phases to control them respectively, so that VT1 and VT4 are turned on and off at the same time and VT2 and VT3 are turned on and off at the same time. The input DC voltage is 220VAC, and the load of the inverter is R. When switches VT1 and VT4 are turned on and VT2 and VT3 are turned off, the current flows through VT1, R and VT4, and the voltage polarity on the load is positive on the left and negative on the right; when switches VT1 and VT4 are turned off and VT2 and VT3 are turned on, the current flows through VT2, R and VT3, and the voltage polarity on the load is reversed, and the DC power is converted into AC power. If you want to change the output AC power frequency, just change the switching frequency of the two sets of switches, and then get an AC square wave voltage with symmetry in the positive and negative half cycles. When the load is pure resistance, the load current and voltage waveforms are the same and the phases are the same; when the load is inductive, the current lags behind the voltage, and the two waveforms are different. The output is equivalent to the superposition of three single-phase inverter circuits with a phase difference of 120°, that is, three-phase inverter, and its principle will not be repeated here.

　　Figure 1 Single-phase bridge inverter circuit

　　2. Select the chip that generates PWM wave

　　The circuit designed is a single-phase full-bridge inverter circuit, and its main circuit is a typical DC-AC inverter circuit. The single-chip microcomputer performs AD sampling on the voltage after LC filtering, and inputs the obtained data into the PIC16F873 single-chip microcomputer. The PIC16F873 single-chip microcomputer chip processes the data and outputs the corresponding SPWM signal to the IR2136 drive circuit to control the switch of the inverter circuit, thereby controlling the output of the inverter, adjusting the operating temperature of the current monitoring system, and protecting the control system circuit. In addition, a keyboard, control frequency and amplitude, and a display module are provided to display the working status of the system.

　　The PIC16F873 single-chip microcomputer circuit is the core control circuit of this system, and it mainly plays the following two roles: providing SPWM control signals to the drive circuit to control the on and off of the inverter bridge; and performing AD sampling on the output voltage.

　　The main function of the integrated circuit IR2136 chip is to generate the corresponding trigger level to control the on and off of the switch tube of the inverter circuit, thereby controlling the output of the inverter. In addition, since the system outputs not only SPWM waves, but also low-order and high-order harmonics, this design uses an LC filter circuit to achieve the purpose of inputting a standard sine wave.

　　ω=2R/L is its cut-off angular frequency, R is the nominal impedance, and if the cut-off frequency is fc, then:

　　3. System software design

　　The core part of the software design is the generation of SPWM signals. This design uses a classic symmetrical regular sampling method with a triangle wave as the carrier and a sine wave as the modulation wave, and obtains a series of rectangular waves with equal amplitudes but different widths. Then the duty cycle of the rectangular wave is calculated using an online calculation method:

　　Let N be the carrier modulation wave ratio, that is, N=fc/fr. Where fc is the carrier frequency and fr is the modulation wave frequency. The SPWM signal of this system is generated by the microcontroller, so the carrier frequency can be calculated by the following formula:

　　Where variable N represents the division factor (1, 8, 64, 256, or 1024) and fclki/o is the MCU clock.

　　Let M = UR / UC, which is the modulation depth, and its value range is generally 0 ~ 1, where UC is the carrier amplitude and UR is the modulation wave amplitude. Changing the amplitude of the modulation wave can change the output fundamental voltage amplitude.

　　According to the principle of regular sampling method, assuming that there are N rectangular waves in one cycle, the duty cycle Di of the i-th rectangular wave is:

　　By setting the microcontroller, the duty cycle is calculated using the above formula and multiplied by the TOP value of the counter to form a sine table. Then the data is sent to the comparison register, the microcontroller I/O port register is configured, and the SPWM signal is output at the PD4 port. The entire SPWM generation program flow chart and real-time feedback diagram are shown in Figure 2:

　　Figure 2 SPWM generation program flowchart

　　Commonly used sinusoidal modulation methods are divided into synchronous modulation and asynchronous modulation. When the frequency of the modulation wave is very low, the synchronous modulation method is prone to produce harmonics that are difficult to filter out, and when the modulation wave frequency is too high, the switching elements cannot withstand it; the output waveform of the asynchronous modulation method has poor symmetry, and the pulse phase and number are not fixed. This software design adopts the segmented synchronous modulation method [4-6], which absorbs the advantages of the above two methods and overcomes their respective shortcomings to obtain a sine wave with better characteristics. The specific operation is: divide the modulation wave frequency into several frequency bands with different carrier ratios, keep the carrier ratio constant in each frequency band, and realize the change of the output fundamental frequency by configuring the carrier frequency inside the microcontroller, that is, change the TOP value of the counter to realize the frequency modulation function. The selection principles are:

　　The frequency band with high output frequency adopts low carrier ratio, and the frequency band with low output frequency adopts high carrier ratio. At the same time, the carrier ratio is selected as a multiple of 3 to obtain a strictly symmetrical bipolar SPWM signal. In this system, the frequency band is divided into five sections, as shown in Table 1:

　　Table 1 Frequency segment and carrier ratio values

　　Real-time feedback of output voltage is a key part of software design. Grid fluctuations or load changes may cause unstable output voltage. Therefore, in order to achieve dynamic stability of output voltage, PID incremental digital closed-loop control is added to the system. The formula is as follows:

　　Among them, Kp=1/σ is the proportional coefficient, Kl=KpT/Tl is the integral coefficient, and Kl=KpTD/T is the differential coefficient. Combining the A/D conversion function module in the microcontroller with the PID closed-loop control, the pulse width of each switching cycle can be well corrected to achieve the purpose of dynamic stability.

　　4. Inverter Simulation Results

　　In the simulation of the inverter part, this system uses the SIMULINK component in MATLAB. The circuit principle is to use the PIC16F873 microcontroller to output PWM waves to control the IR2136 and then control the gate conduction of the thyristor, thereby realizing variable frequency amplitude modulation.

　　In this three-phase inverter circuit, the output is obtained after LC filtering using a three-phase full bridge. At the same time, the system also includes a voltage negative feedback and a current negative feedback system. Such a design can resist some disturbances to a certain extent, making the output three-phase voltage relatively stable, with a good phase margin and a certain amplitude margin, but in the actual inverter process, two IGBTs in the same bridge arm may be turned on at the same time, resulting in a short circuit. After considering the above situation, the above circuit schematic diagram is improved, as shown in Figure 3 below, and a dead zone is added. The simulation results are shown in Figure 4:

　　Figure 3 Modulation wave with dead zone, triangle wave modulation circuit

　　Figure 4 Modulation wave with dead zone, triangle wave modulation circuit waveform

　　In FIG4 , the waveform is distorted at the lower peak. This is due to the introduction of dead zone nonlinearity in the lower bridge arm, which is an additional distortion.

　　V. Conclusion

　　The above experimental results show that the requirements for power supply under industrial conditions can be achieved by using the PIC16F873 microcontroller to output PWM waves to control IR2136 and then control the gate conduction of the thyristor. This method has the advantages of small harmonics and simple filter circuits. Therefore, it has broad application prospects in high-performance medium-frequency speed regulation, DC grid connection and other fields. At the same time, the use of a single-chip microcomputer to generate SPWM signals has incomparable advantages and is an inevitable development trend in the field of intelligent power supply.