Design of digital sinusoidal inverter power supply based on AVR microcontroller

Abstract: A design method for a high-performance DC-AC digital sinusoidal inverter is proposed. SG3525A and AVR series single-chip microcomputer AT90PWM2 are used as controllers for design. SG3525A generates PWM wave to perform push-pull boost control at the front end. Through the negative feedback of the high voltage on the DC bus, the input voltage of the full-bridge inverter is kept stable. The waveform generator of AT90PWM2 generates SPWM waveform to control the subsequent full-bridge inverter. By sampling the output current and output capacitor voltage after the full-bridge inverter, double closed-loop control is achieved, so that the inverter power supply can have good output characteristics and responsible effects under various types of load conditions. Experimental results show that the performance of the 1 kW prototype is stable, the inverter efficiency is greater than 90%, and the voltage accuracy can be maintained at 220 V±1%, the frequency accuracy is 50 Hz±0.1%, and the THD is less than 1% under different types of full-power load conditions.
Keywords: digital; inverter; AT90PWM2; dual closed-loop control; SG3525A

Inverter power supply is widely used, especially precision instruments have higher requirements for inverter power supply performance. A good inverter power supply not only requires stable operation, high inverter efficiency, good output waveform characteristics, and good transient response characteristics, but also requires the inverter power supply to be miniaturized, intelligent, and scalable. Therefore, a digital sinusoidal inverter power supply based on the AVR series microcontroller AT90PWM2 is proposed here. The front-stage SG3525A adopts PWM control boost circuit to realize input and overheating protection. The rear-stage microcontroller AT90PWM2 uses unipolar frequency multiplication SPWM control method for full-bridge inversion and output protection.

1 Overall design and working principle
The overall block diagram of the inverter power supply system is shown in Figure 1. The main circuit of the system adopts a two-stage structure of front-stage push-pull boost and rear-stage full-bridge inversion, which can avoid the use of power frequency transformers, effectively reduce the volume and quality of the power supply, and improve the inverter efficiency. Its working principle is: the 12 V DC input voltage is filtered, pushed-pull boosted and full-bridge rectified to a DC bus voltage of 350 V, and then converted into 220 V/50 Hz industrial frequency AC power through a full-bridge inverter circuit.

In order to reduce the influence of input voltage, the output voltage after full-bridge rectification is sampled as feedback voltage to control the controller of the front-stage push-pull boost circuit, and the duty cycle of the SG3525A output PWM waveform is adjusted to keep the voltage of the DC bus at 350 V. At the same time, the voltage on the output filter capacitor and the current on the output filter inductor are sampled, and the voltage and current dual-loop control is adopted to adjust the output of the SPWM of the rear-stage full-bridge inverter circuit, so as to obtain good output waveform characteristics and stable load characteristics. 2 System Hardware Design
2.1 Generation of Push-Pull Boost PWM Waveform
The DC boost circuit adopts push-pull type, which has a simple structure and high efficiency. The power tubes VQ1 and VQ2 in the circuit are controlled by the two complementary PWM waves generated alternately by SG3525A outputA and outputB to control the on and off. SG3525A can generate PWM waveforms of different frequency bands between 100Hz and 500kHz by adjusting external resistors and capacitors. The relationship between the frequency fPWM of the PWM wave and the external resistors RT, RD, and external capacitors CT is as follows:

SG3525A can automatically adjust the duty cycle of the PWM wave through the feedback DC bus voltage to make the output stable. At the same time, SG3525A has the functions of input undervoltage lockout and PWM lockout of voltage. Once overcurrent or overvoltage occurs, PWM can be quickly shut down to ensure the safety of the entire circuit.
2.2 Generation of SPWM waveform of full-bridge inverter
The SPWM waveform is generated by AT90PWM2 of the AVR microcontroller series. AT90PWM2 has two 12-bit waveform generators PSC0 and PSC2, which respectively generate two complementary SPWM signals, which are isolated and amplified by the drive circuit and then drive the power tube in the full-bridge inverter. Among them, PSC0 generates the control signals of VQ3 and VQ4, and PSC2 generates the control signals of VQ5 and VQ6.
Select PSC to work in center alignment mode, and PSCO and PSC2 to work in synchronization state. Then the PSC counter starts counting from the maximum value defined by register OCRnRB, first decreases to zero and then increases to the maximum value. When the value of the PSC counter is equal to the value of register OCRnSB and OCRnSA, the level of PSCn output pins PSCOUTn0 and PSCOUTnl will be changed respectively.
The relationship between its triangle wave carrier period fSPWM and PSC counting frequency fPSC and registers OCRnRB and OCRnRA (n=0 or 2) is:

2.3 Full-bridge inverter circuit
The inverter circuit is the core component of the inverter power supply, completing the process of converting DC voltage into AC voltage. The full-bridge inverter circuit is composed of 4 IGBTs as power tubes. The circuit has the characteristics of convenient control and high power tube utilization. The IR2110 drive circuit can be used. IR2110 adopts highly integrated level conversion technology, which greatly simplifies the control requirements of the logic circuit for power devices and improves the reliability of the drive circuit. In particular, the upper tube uses an external bootstrap capacitor, which greatly reduces the number of driving power supplies compared to other drivers, as shown in Figure 2.

2.4 Output LC filter circuit
Since the output of the full-bridge inverter circuit is a 50Hz fundamental wave and other high and low harmonics, in order to obtain a pure sinusoidal waveform, it is necessary to consider setting up a filtering link. Because the harmonics are mainly concentrated around 1 and 2 times fSPWM, the cutoff frequency fc is set to fSPWM/5. Then:

Where, R is the equivalent load resistance of the DC side, Vout is the output current and voltage, and P is the rated power of the inverter.

3 System Software Design
3.1 Introduction to the MCU
AT90PWM2 is an AVR series MCU. The main features of this MCU are: 1) High-performance 8-bit RSIC structure, 1 MIPS/MHz, supporting a clock of up to 16 MHz; 2) 8 KB system programmable (ISP) Flash memory, supporting bootstrap programming, 512 bytes SRAM, 512 bytes EEPROM; 3) 4 PWM channels provide various PWM signals and can set dead zones; 4) 8-channel 10-bit ADC module, 1 10-bit DAC module, 2 analog comparators; 5) Up to 29 interrupt sources; 6) Support UART, SPI, I2C communication methods. Its pin function distribution diagram is shown in Figure 3.

Among them, the PSCOUT00~PSCOUT21 pins are SPWM wave output pins, which are used to drive the power switch tube, and the SD pin is the enable control of IR2110, which is used to open or lock the conduction of SPWM. The V_MAIN, V_OUT, and I_OUT pins are used to sample the bus voltage, output capacitor current, and inductor voltage respectively. The single-chip microcomputer also includes the use of LED alarm function, the drive fan to start the cooling function when overheating, the ISP online download and update driver, and the reserved serial communication function.
3.2 Control strategy
This digital system adopts inductor current feedback control. The output inductor current is introduced into the control system, and a double closed-loop control is formed together with the output capacitor voltage. By sampling the output inductor current and output capacitor voltage, the control signal of the outer loop voltage error is used to control the inner loop current, and the current is adjusted to make the output voltage track the reference voltage value, thereby improving the dynamic response of the system. Due to the existence of the inner loop, the double closed-loop control system increases the bandwidth of the control system, accelerates the dynamic response of the inverter, reduces the harmonic content of the output voltage, and enhances the adaptability to nonlinear loads. The outer loop regulator can adjust the given value of the inner loop regulator according to the load change, so that the regulation system still has good quality. Therefore, the dual closed-loop control system has a strong adaptability to load changes and can greatly improve the output performance of the inverter power supply. In order to simplify the design of the voltage outer loop, the current inner loop can be regarded as a proportional link and the voltage outer loop can be regarded as a PI link. Figure 4 is a block diagram of the dual-loop control system.

3.3 Software Flow
The system software design adopts modular design, mainly including: hardware initialization module, sine table initialization module, output protection module, dual closed-loop regulation module, interrupt processing module. The flow of the main program and SPWM interrupt processing module are shown in Figure 5(a) and Figure 5(b) respectively.

4 Experimental results
According to the above ideas, a 1 kW prototype was made, using IRF3805 as the push-pull boost power tube, RHRP8120 as the rectifier diode, LRG4-PC50UD as the full-bridge inverter power tube, SG3525A PWM wave frequency set to 80 kHz, SPWM wave frequency set to 10 kHz, output filter inductor L is 2.5 mH, output filter capacitor C is 4.7 μF, and the sinusoidal AC output voltage accuracy is 220 V±1%, frequency accuracy is 50 Hz±0.1%, and THD is less than 1%. The inverter efficiency is greater than 90%, and the test waveforms at no load and full load are shown in Figure 6(a) and Figure 6(b).

5 Conclusion
A design method for a digital inverter power supply is proposed, in which the front stage is controlled by SG3535A for push-pull boost and the back stage is based on AT90PWM2 for full-bridge inverter. The waveform generator of AT90PWM2 is used to generate SPWM waveform, which effectively simplifies the circuit. The front-end negative feedback and the back-end double closed-loop regulation make the inverter power supply have high performance of output voltage characteristics. The reserved serial port communication and online download functions enable the inverter power supply to communicate with the host computer and have expansion functions. The perfect protection measures can make the inverter power supply have perfect indication and automatic processing functions when a fault occurs.