Design and EMC analysis of inverter power supply based on AVR microcontroller

Introduction: This paper introduces a design scheme for an inverter power system. The inverter power system uses a high-performance AVR microcontroller as the core control chip. The hardware circuits in the inverter power system are analyzed and designed. Combining the advantages of fuzzy adaptive control and digital PI control, a dual closed-loop control system model based on fuzzy adaptive setting PI control is given to ensure the reliability of the inverter circuit. The EMC situation of this power supply is tested and analyzed through the three elements of electromagnetic interference: interference source, transmission path and sensitive equipment.

1 Introduction

In recent years, with the development of modern industry and power electronics technology, the application of inverter power supply has become more and more extensive. The stability of its work, the quality of its output performance and the level of its working efficiency directly affect the performance and application field of the inverter system. With the rapid development of power electronics technology and control theory technology, the traditional single SPWM modulation method can no longer meet the high steady-state precision output of high-performance inverter power supply. Therefore, this paper explores and studies the design of a set of inverter power supply system, adopts the high-performance ATmega series AVR microcontroller as the core control chip, designs the software and hardware of the inverter power supply system, and focuses on the control algorithm of fuzzy adaptive tuning PI in the closed-loop feedback system. In order to make the performance of the inverter system more stable, dual closed-loop control is introduced in the system. By comparing the output voltage and current feedback with the reference signal, the control waveform is output stably, thereby improving the steady-state and dynamic response speed of the single-phase inverter power supply system. Finally, EMC test is carried out according to GB17743-2007 "Limitations and Measurement Methods of Radio Disturbance Characteristics of Electrical Lighting and Similar Equipment", and the effectiveness and feasibility of electromagnetic compatibility rectification measures are analyzed.

2 Overall design

2.1 Overall design of inverter main circuit system

In view of the requirements of medium and high power input and electrical isolation in inverter power supply, this paper designs an isolation transformer circuit structure as the main structure of the DC/DC link. Due to the need to reduce interference, the main system adopts an external synchronization circuit. The inverter main circuit system design adopts a two-level structure of DC/DC push-pull boost and DC/AC full-bridge inverter. At the same time, the drive signal of the DC/DC boost circuit is output by the PWM control chip, and the DC/AC inverter circuit is output by a high-performance microprocessor. The purpose is to effectively control the volume and quality of the system, while avoiding the use of power frequency transformers and improving the working efficiency of the inverter system. The main circuit system structure block diagram is shown in Figure 1.

Figure 1 Inverter main circuit system structure diagram

2.2 Introduction to High-Performance Inverter Controller AVR MCU

AVR series MCUs are mainly used in IT fields such as industrial control, modern communication equipment, medical equipment, GPS, etc. They have a high cost-effectiveness, integrating AD, timer/counter, PWM waveform generator, EEROM, flash memory, RAM, etc. They are very powerful and require few peripheral circuits. They are widely used in modern pure sine wave inverter products.

ATmega16L microcontroller adopts enhanced AVRRISC structure, and is a low-power, high-performance 8-bit CMOS microcontroller. It has an advanced instruction set and single-clock cycle instruction execution time, which speeds up the CPU operation speed. The data throughput of ATmega16L can be as high as 1MIPS/MHz, so it can handle the contradiction between power consumption and processing speed of the system very well. At present, ATmega16L has a variety of packaging forms. This design adopts PDIP packaging form, and its pin diagram is shown in Figure 2.

Figure 2 ATmega16L pin diagram

3 Power supply hardware and software design

3.1 DC/AC stage design

The DC/AC inverter stage is an important core part of the entire inverter power system, which converts the high-amplitude DC voltage after input boost rectification into 220V AC voltage. The DC/AC inverter circuit structure is shown in Figure 3.

Figure 3 DC/AC inverter circuit structure diagram

The design of the inverter circuit uses four MOSFET power tubes to form two groups of bridge arm full bridge circuits. In Figure 3, the VCC terminal is connected to the output 350V DC voltage of the DC/DC stage, and the output terminal is connected to the LC filter circuit to remove the high-order harmonics of the output waveform. The drive of the four MOSFET tubes comes from the ATmega16L microcontroller to generate two complementary SPWM signals. Among them, since Q1 and Q4 are connected to one SPWM signal and Q2 and Q3 are connected to another SPWM signal, the on-off of Q1 and Q3 is complementary, and the on-off of Q2 and Q4 is also complementary. When Q1 and Q4 are turned on, VCC is connected to the "+" end of the load; when Q2 and Q3 are turned on, VCC is connected to the "-" end of the load, and finally the alternating voltage signal Uo can be obtained through filtering.

Since the input of the full-bridge inverter circuit is about 350V high-amplitude DC voltage, the field effect transistor FDP18N50 is selected here. Its drain-source breakdown limit voltage is 500V, the typical on-off time is 405ns, and the maximum time is 1040ns. Therefore, it is suitable for this full-bridge circuit and effectively ensures the safety of the system.

The main function of the drive circuit in the DC/AC inverter circuit is to amplify the low-power drive signal generated by the microcontroller, thereby promoting the normal operation of the MOSFET tube, and at the same time electrically isolating the microcontroller from the high-voltage inverter circuit. This design uses IR2110 as the driver chip of the drive circuit. Since IR2110 uses highly integrated level conversion technology, it greatly reduces the requirements and restrictions of the inverter circuit for switching devices. The upper tube device of the IR2110 combined drive circuit has an external bootstrap capacitor, which greatly simplifies the design structure of the drive circuit. The drive circuit is shown in Figure 4.

Figure 4 IR2110 driver circuit diagram

3.2 Design of push-pull circuit

The DC/DC boost stage adopts a push-pull circuit, which has high working efficiency, simple and reliable structure, and convenient and flexible use. The push-pull circuit structure diagram is shown in Figure 5.

Figure 5 Push-pull circuit structure diagram

The working principle of the push-pull circuit is: when the PWM control signal is input to the gate of Q1, Q1 starts to work in the on state, so Vin is added to the 1 end of the primary coil of the isolation transformer; similarly, when the PWM control signal is input to the gate of Q2, Q2 starts to work in the on state, so Vin is added to the 2 ends of the primary coil of the isolation transformer, where two 33μH inductors are required to be connected to the 1 and 2 ends. Since the two PWM signals are complementary in phase, Q1 and Q2 are turned on alternately, so that an AC voltage signal is generated at the primary of the isolation transformer.

3.3 Design of PWM control circuit

The design of the PWM control circuit is shown in Figure 6. The PWM drive signal of the switch tube is generated by the SG3525 control circuit. The output pins 11 and 14 of the SG3525 generate two phase-complementary PWM square wave signals with a duty cycle of 45% to control the two IXFH50N80s to conduct alternately. The output signal frequency can be changed by adjusting the capacitor of the external circuit pin 5 and the resistor value of the pins 6 and 7.

Figure 6 PWM control circuit design diagram

3.4 Design of fuzzy adaptive PI controller

The control system of the inverter power supply can usually be divided into an open-loop control system and a closed-loop control system. In order to make the performance of the inverter system more stable, a dual closed-loop control is introduced into the system. By comparing the output voltage and current feedback with the reference signal, the waveform is controlled to be output stably, thereby improving the steady-state and dynamic response speed of the single-phase inverter power supply system. By using the basic theory of fuzzy, the conditions of the rules and the methods of operation are expressed one by one with fuzzy sets, and by applying fuzzy reasoning, the optimal control of PI parameters can be automatically achieved, that is, fuzzy adaptive tuning PI control can be achieved.

The fuzzy adaptive PI controller takes the error e and the error change rate ec as input, and can meet the requirements of e and ec for online self-tuning of PI parameters at each moment. The fuzzy logic control rules are used to modify the PI parameters online, which constitutes the basic structure of the adaptive fuzzy PI controller. The key to the fuzzy adaptive tuning of PI parameters is to find the fuzzy relationship between the two PI parameters and the error e and the error change rate ec, continuously detect the error e and the error change rate ec during the tuning operation, and modify the two P and I parameters online using the fuzzy control principle to meet the different requirements for the parameters of the controlled object when different e and ec are input, thereby improving the dynamic and static response characteristics of the system. The design of its inverter equivalent circuit is shown in Figure 7.

Figure 7 Inverter equivalent circuit design block diagram

Among them, the voltage outer loop control adopts PI control with fuzzy adaptive adjustment. The task of the voltage outer loop control is to maintain the stability of the output voltage. As long as the output voltage does not produce large fluctuations and can be stabilized in a small range near the given value signal, the purpose can be achieved.