Sine wave inverter power supply based on ATmega8 microcontroller control

　　In the wind power industry, it is often necessary to repair wind turbines in the field, and various repair tools and instruments must be powered. Therefore, it is necessary to design a portable, low-power, intelligent sinusoidal inverter power supply to power these devices, which can greatly improve the efficiency of wind turbine repair.

　　This paper designs an intelligent sinusoidal inverter power supply based on a single-chip microcomputer based on this situation.

　　1 Design of sine inverter power supply

　　The inverter designed in this paper is a portable power converter that can convert DC 12 V into 220 V sinusoidal AC voltage and can be used for general electrical appliances. At present, low-voltage and low-power inverters have been widely used in industrial and civilian fields. Especially in transportation, field measurement and control operations, and electromechanical engineering repairs where the mains electricity cannot be directly used, low-voltage and low-power inverters have become one of the necessary tools. It only needs to have a battery with sufficient power connected to it to generate the AC voltage required by general electrical appliances. Since the working environment of low-voltage and low-power inverters is in the wilderness or in harsh environments and places with a lot of interference, the design requirements for it are relatively high, so it must have the characteristics of small size, light weight, low cost, high reliability, strong anti-interference, and good electrical performance.

　　In view of these characteristics and requirements, a simple and practical sine wave inverter power supply is studied, which meets the actual requirements with a circuit composed of low-cost and simple components, which will be widely welcomed by the market. At present, there are many schemes for designing low-power inverter power supplies. The early design scheme is to directly control the DC voltage with a dual switch tube, and generate a 220 V square wave inverter voltage under the action of a 50 Hz square wave.

　　However, as the requirements of power equipment for inverter performance continue to increase, square wave inverters have been eliminated in most situations, and the application of sine wave inverters has become an inevitable trend. At present, there are three main design schemes for low-power sine wave inverters on the market.

　　1.1 Primary Inverter Sine Wave Inverter

　　This solution also adds the DC voltage to be inverted directly to the dual switch tube, and then uses a sinusoidal pulse width modulation pulse train with a frequency of dozens of times higher than 50 Hz to directly drive the switch tube, and then "smooths" the output voltage to obtain a continuously changing waveform similar to a sine wave. The advantage of this method is that the circuit is inverted once, which is efficient and simple. However, the transformer is too bulky and cannot meet the requirements of small size and light weight.

　　1.2 Multiple Inverter Sine Wave Inverter Power Supply

　　This solution is to divide the 50 Hz signal driving the switch tube into several drive signals with different phases but the same frequency, and drive each switch tube separately, so that each output voltage is also staggered by a certain phase, and then superimposed to output a multi-step ladder wave, and then filter it to output the required sine wave voltage. This solution circuit is relatively complex, and once a group of switch tubes fails, the output waveform will be greatly distorted.

　　1.3 Secondary Inverter Sine Wave Inverter Power Supply

　　As high-frequency switching tube technology matures, the circuit design of inverter power supplies tends to convert voltage first and then frequency, that is, first convert the DC voltage into high-frequency AC power, and then convert the high-frequency AC power into a 50 Hz sinusoidal AC power supply. The principle block diagram is shown in Figure 1.

　　Since the switch tube is cheap, the unit circuit constituting FIG1 has a high cost-performance ratio. Currently, most low-power inverters on the market are produced with this design.

　　2 Sine wave inverter power supply based on single chip microcomputer control

Among the three inverter power supply design schemes 　　listed above , the secondary inverter sine wave inverter power supply is the best. According to this idea, the early specific circuit solutions mostly used PWM control chips such as TL494, SG3524, SG3525A, etc. to control the switch tubes of the DC/DC and DC/AC parts at a fixed frequency, and used a correction circuit to correct the output waveform in order to meet the requirements of the sine wave. However, this pure PWM chip controlled circuit cannot automatically correct the aging, heating, interference and other conditions of the components, or the correction ability is poor, which often leads to circuit failures in actual applications. With the development of single-chip microcomputer technology, designers have been trying to introduce single-chip microcomputers into the control of sinusoidal inverter power supplies. However, for the control of the high-frequency part, low-cost single-chip microcomputers cannot complete this function, and high-cost single-chip microcomputers will reduce the cost performance. Therefore, this article proposes another design scheme, which is to use a low-cost ATmega8 single-chip microcomputer, in conjunction with TL494, IR2110 and switching tubes to form a small, low-cost, and highly controllable sinusoidal wave inverter power supply. Its block diagram is shown in Figure 2.

　　As shown in Figure 2, the whole system is mainly controlled by the ATmega8 microcontroller. Whether TL494 and IR2110 work or not is adjusted by the microcontroller according to the feedback signal. The high-frequency switch tube and the drive output part are composed of a single-phase full-bridge inverter circuit. The specific working principle is to use the ATmega8 microcontroller as the core of the system control, and use the function of TL494 to generate high-frequency PWM signals. The pulse width is controlled and output by the microcontroller to control the full-phase inverter circuit composed of high-frequency switch tubes, and the low DC voltage is inverted into a high-voltage square wave. After rectification and filtering, it is sent to the drive output full-bridge inverter circuit. The microcontroller controls IR2110 to output the industrial frequency drive signal, and controls the output drive circuit to output a 50 Hz, 220 V sinusoidal AC voltage.

　　3. Specific design of main circuits

　　The core of the whole inverter system is mainly composed of single chip control circuit and detection circuit, DC/DC conversion circuit and DC/AC output circuit.

　　3.1 DC/DC conversion circuit

　　As shown in Figure 3, a high-frequency pulse output circuit is formed by TL494. The circuit adopts the TL494 integrated block with excellent pulse width modulation controller. The integrated block contains a +5 V reference power supply, an error amplifier, a frequency-variable sawtooth oscillator, a PWM comparator, a trigger, an output control circuit, an output transistor, and a dead time control circuit. The 5th and 6th pins of the integrated block are respectively connected with C1 and R6 to form an RC oscillation circuit, which can make the TL494 output frequency 100 pins to control the DCDC end in the figure. By controlling the dead time control end of the 4th pin, the duty cycle of the output signal can be adjusted between 0 and 49%, thereby controlling the output of the output end Q1PWM~Q2PWM, while the P end, VCC end and VFB end receive the feedback signal from the load, the high-frequency inverter output voltage, and the input voltage, and the circuit inside the TL494 form an overvoltage and overload protection circuit, forming the first-level safety protection network of the inverter.

　　As shown in Figure 4, it is a high-frequency voltage inverter circuit. The full-bridge inverter circuit is composed of four IRF3205 tubes. The IRF3205 is manufactured using advanced process technology and has extremely low on-resistance. In addition, it has a fast conversion rate and a HEXFET design known for its ruggedness and durability, making the IRF3205 an extremely efficient and reliable inverter tube. The high-frequency pulse string input from the input terminals Q1PWM and Q2PWM controls the four tubes to conduct in pairs, chopping the DC low voltage input by VIN, and then inverting it into a high-frequency AC square wave after passing through a step-up transformer. The current flowing at this time is the magnetizing current, so the BYV26C ultra-fast soft recovery diode produced by Philips is selected to form a full-bridge rectifier circuit. The tube has a repetitive peak voltage of 600 V, a forward conduction current of 1 A, and a reverse recovery time of 30 ns, which can meet the parameter requirements of the circuit. The rectified voltage outputs a DC voltage of 260 V after passing through a filter circuit and is sent to the DC/AC inverter circuit. In addition, the 260 VDC is stepped down as a feedback signal and input to the VFB terminal in Figure 3 as a feedback signal for the high-frequency inverter voltage.

　　3.2 Design of DC/AC output circuit

　　The DC/AC conversion output circuit adopts full-bridge inverter single-phase output. Its driving input waveform is driven by the output signal of the single-chip microcomputer to drive the half-bridge driver IR2110 to output the industrial frequency driving signal. The output driving waveform can be adjusted to D < 50% through the single-chip microcomputer programming to ensure that the inverter driving square wave has a common dead time. As shown in Figure 5, the QA1~QA4 terminals are respectively connected to the PB1~PB4 pins of the single-chip microcomputer. The output signal of these pins drives two IR2110s, and the 50 Hz industrial frequency signal is output from PWM1~PWM4 to drive the bridge inverter circuit to generate a sine waveform.

　　IR2110 is a high-power MOSFET and IGBT dedicated driver integrated circuit produced by IR. It can achieve the optimal drive of MOSFET and IGBT, and also has fast and complete protection functions. Therefore, it can improve the reliability of the control system and reduce the complexity of the circuit. As shown in Figure 6, HIN and LIN are the drive pulse signal input terminals of the upper and lower power MOS in the same bridge arm of the inverter bridge. SD is the protection signal input terminal. When this pin is connected to a high level, the output signal of IR2110 is completely blocked, and its corresponding output terminal is always low level; when this pin is connected to a low level, the output signal of IR2110 changes with HIN and LIN. Therefore, in this system, the SD terminals of the two IR2110 chips are connected to the PB0 pin of the microcontroller to control whether IR2110 is in a protection state in real time. The bootstrap capacitor between VB and VS of IR2110 is difficult to select, so a 15 V constant voltage is directly provided to enable it to work normally.

　　There are two modulation modes for the inverter sinusoidal voltage output circuit. One is unipolar modulation mode, which is characterized by two power tubes switching at a higher switching frequency in one switching cycle to ensure that an ideal sinusoidal output voltage can be obtained, and the other two power tubes work at a lower output voltage fundamental frequency, thereby greatly reducing the switching loss. However, it is not fixed that one of the bridge arms is always at a low frequency (output fundamental frequency) and the other bridge arm is always at a high frequency (carrier frequency), but it switches every half output voltage cycle, that is, the same bridge arm works at a low frequency in the first half of the cycle and at a high frequency in the second half of the cycle. In this way, the working state of the power tubes of the two bridge arms can be balanced. When the same power tube is selected, its service life is balanced, which is beneficial to increase reliability. The other is bipolar modulation mode, which is characterized by four power tubes working at a higher frequency (carrier frequency). Although a sinusoidal output voltage waveform can be obtained, the cost is a large switching loss. As shown in Figure 6, the inverter output circuit of this article adopts a unipolar modulation mode, which can improve the smoothness of the waveform and increase the reliability of the circuit. PWM1~PWM2 in Figure 6 receive the power tubes from Figure 5 respectively. The output drive signal drives the bridge inverter circuit composed of four IRF840 switch tubes with a withstand voltage of 500 V, converting 260 VDC into 220 V, 50 Hz AC power, which is supplied to the load after LC filtering. The IFB terminal and ACV terminal in Figure 6 are respectively the current and voltage samples, which are sent to the PC4 and PC5 pins of the microcontroller for A/D conversion, and then the microcontroller uses the conversion results for power calculation and circuit protection.

　　3.3 MCU Circuit and Programming

　　This article uses the ATmega8 microcontroller produced by Atmel for control. It has a wide operating voltage range, strong anti-interference ability, and pre-fetch instruction function. This makes it fast, with large pin output current and strong driving ability. The output pulse signal can directly drive the stepper motor drive module without amplification. The ports all have built-in pull-up resistors and can be used as input or output. The specific situation can be flexibly configured through programming. Based on the above advantages, the ATmega8L microcontroller is selected as the controller, which can not only improve the overall performance of the system, but also simplify the peripheral circuit.

　　This article mainly applies it to the signal drive, temperature detection, fan control, safety protection, data display, etc. of the entire system. The ATmega8 microcontroller collects the temperature, current, and voltage from the system circuit respectively, and controls the start of the fan cooling, controls whether to output the alarm signal, and controls whether the SD end and the DCDC end put the system in a protection state according to the conditions of these three parameters. QA1~QA4 outputs a 50 Hz drive signal. The specific programming control is shown in Figure 7. When the system is started, the microcontroller first checks whether the system temperature environment is normal. If it is not normal, it starts the alarm and prompts the error code. If it is normal, it starts the high-frequency inverter circuit to work and detects whether the 260 VDC is normal. If it is not normal, it alarms. If it is normal, it starts the sinusoidal inverter circuit to work and keeps detecting whether the output voltage and current are normal. If it is normal, it outputs, otherwise it alarms.

　　4 Conclusion

　　In summary, the overall design of the sine wave inverter power supply based on ATmega8 microcontroller control can efficiently and conveniently provide the required AC power supply for field operations. The circuit has been successfully tested and put into practical use. Practice has proved that the inverter power supply designed in this paper has small size, light weight, stable and reliable performance.