Design of emergency power inverter circuit using single chip microcomputer SPWM

The inverter is an important part of the emergency power supply. In order to achieve timely adjustment of the inverter output AC voltage in the emergency power supply, reduce the output voltage harmonics and achieve the purpose of digital control of the inverter circuit, the three-phase inverter circuit adopts the sinusoidal pulse width modulation (SPWM) control method, with C8051F020 microcontroller and SA4828 as the core, to complete the generation of SPWM waves and system control. The peripheral control function is completed by using the unique port connection of the microcontroller, which reduces the processing time of the emergency power supply for waveform generation, ensures that the waveform has high accuracy, and the circuit hardware connection is simple.

introduction

With the development of society, the more information and modernization it has, the more it relies on electricity. Sudden power outages will affect people's normal life order and learning, especially for important loads in production and life. Once the power supply is interrupted, it will cause significant economic losses. Emergency power supply products have become an indispensable and important equipment in many important places. It is also an effective way to most effectively solve problems such as power outages and unstable power quality. The inverter circuit is an important part of the emergency power supply. The role of the inverter circuit in the emergency power supply is to invert the DC voltage provided by the battery into a three-phase AC output when the city power is cut off or an abnormality occurs, so as to ensure the normal operation of important loads or equipment. At present, most inverter power supplies use sinusoidal pulse width modulation (SPWM) technology, and their control circuits are mostly implemented using analog methods. Although analog control technology is very mature, it has many disadvantages, such as: many components of the control circuit, complex circuit, large volume, and insufficient flexibility. This paper designs a fully digital three-phase PWM inverter power supply, which uses a dedicated SPWM waveform generator to connect with a single-chip microcomputer to generate an inverter drive signal SPWM wave. The design uses a single-chip microcomputer C8051F020 control and MITEL's SA4828 chip as a waveform generator.

The structure and working principle of inverter circuit

Figure 1 shows the structure of the inverter circuit. The DC power provided by the battery is converted into AC power through a three-phase inverter circuit. Its fundamental frequency is the output frequency of the inverter power supply. The AC signal is isolated by the output transformer and then filtered out by a low-pass filter to obtain the three-phase sinusoidal AC power required by the load.

Figure 1 Structure of inverter circuit

In the inverter circuit, the inverter and its control are the core of the inverter circuit. The inverter is controlled by SPWM control. This paper uses SPWM wave generator and single-chip microcomputer to realize the control of inverter and output voltage. The SPWM wave generated by the controller controls the on and off of the switching device, thereby controlling the output voltage and its waveform, and stabilizing the output voltage.

Design of main circuit of three-phase inverter

The main circuit of the three-phase inverter is shown in Figure 2, which consists of a three-phase inverter bridge, a transformer, and a filter.

The inverter switch device uses a 6-unit IPM intelligent power module. The role of the inductor L in the LCR low-pass filter is to suppress the passage of high-order harmonics; the capacitor C provides a bypass for the high-order harmonics generated by the inverter; the resistor R acts as a damper to prevent or suppress the generation of harmonics. When the mains power is interrupted or abnormal, the battery voltage is added to the DC bus, through the inverter composed of intelligent power modules, and then through the filter composed of LCR and the three-phase power transformer, forming a three-phase sinusoidal AC with a phase voltage of 220 V to power the load. The switch device of the three-phase inverter uses a six-unit IPM intelligent power module model PM100CVA60 from Fuji Corporation of Japan. Its withstand voltage can reach 600 V, the maximum allowable collector current is 100 A, the safe working area is wide, the driving power is small, the switching frequency is high, and the saturation voltage is reduced. In addition, the module also has the characteristics of overcurrent control, small filter size, low noise, easy heat dissipation, and high reliability. The driving signal of the module is a sinusoidal pulse width modulation (SPWM) signal.

The power element intelligent power module IGBT-IPM is a new hybrid integrated circuit that uses the power device IGBT as the main body and integrates functional circuits such as drive circuits, multiple protection circuits and alarm circuits in the same module. Using intelligent power modules as power devices for power supplies can simplify the design of hardware circuits, reduce the size of power supplies, and more importantly, improve the safety and reliability of the system. When selecting an intelligent power module IPM, choose according to the voltage and current ratings. The current rating of the power element considers a safety margin of (2 to 3) times. When calculating the current, it should be satisfied that the output power can still be met when the input voltage fluctuation is the lowest. According to the given technical parameters, the maximum output power, rated current value, and rated voltage value of the power element are calculated, and finally a 100 A/600 V intelligent power module, model PM100CVA60, is selected.

Inverter controller design

The function of the control circuit is mainly to generate SPWM drive signal. SPWM is a control method to adjust the inverter output AC voltage and reduce the output voltage harmonics. The inverter constructed by SPWM control has good regulation performance and fast regulation speed. It can make the frequency and voltage match in the regulation process to obtain good dynamic performance, and the output voltage waveform is close to sine. In order to realize this function and the digitization of the inverter circuit, this paper uses a single-chip microcomputer and a dedicated SPWM waveform generator SA4828 integrated circuit to form an inverter controller. This method is simple to program software, the emergency power supply has less processing time for waveform generation, and can ensure that the waveform has high accuracy, and the hardware connection is simple.

1. Three-phase SPWM waveform generator SA4828

1.1. Performance characteristics of SA4828

SA4828 is an integrated circuit chip developed by the British MITEL company, which is dedicated to the generation and control of three-phase SPWM signals. It has the advantages of high precision, strong anti-interference ability, simple peripheral circuit, and no temperature drift. It is mainly used in industrial fields such as inverter power supply, variable frequency speed regulation and emergency power supply. The main features of the chip are: it has an enhanced microprocessor interface and is compatible with more single-chip microcomputers; it can adjust the output of each phase separately and can be used for any unbalanced load; it has a built-in "watchdog" timer for monitoring, and the program runs safely and reliably; it provides a soft reset control function; the modulation wave frequency uses 16 bits, which increases the frequency resolution and improves the accuracy of the inverter output frequency; the on-chip ROM provides three waveforms to choose from, which are suitable for a variety of applications, among which the pure sine wave can be used for static inverter power supply and single-phase AC speed regulation.

1.2 Working principle of SA4828

The principle block diagram of SA4828 is shown in Figure 3. It receives and stores microprocessor initialization commands and control commands, and is mainly composed of bus control, address/data bus, memory, and control register, which are implemented in the form of control words. Each phase output control circuit of the three-phase output control circuit is composed of pulse cancellation and pulse delay circuit. Pulse cancellation removes pulses with a pulse width less than the cancellation time, and the delay circuit ensures the dead zone interval to prevent the bridge switch device from being directly connected at the moment of conversion.

The selection of three different waveforms is set by transmitting commands to the initialization register and the control register to set the three waveform ROMs. When the "watchdog" circuit receives the command issued by the single-chip microcomputer, if there is a problem, the bus control will send a reset "watchdog" signal to delay the "watchdog" to shut down the drive signal. SA4828 has added 8 register units to improve the frequency accuracy and can independently control the amplitude of the three-phase waveform. When the system is initialized, the microprocessor writes data to its initialization register through the bus control and decoding circuit inside SA4828 to complete the setting of the carrier frequency, modulation frequency range, pulse delay time and counter reset. During operation, data is written to the control register in real time to refresh the parameters such as the modulation wave frequency, modulation wave amplitude, forward/reverse, overmodulation, and output prohibition, so that the SPWM signals output by the RPHT, RPHB, YPHT, YPHTB, BPHTB, and BPHT6 pins change.

2. Control circuit hardware design

The controller circuit design with SA4828 and single-chip microcomputer C8051F020 as the core is shown in Figure 4. The control circuit outputs a three-phase SPWM signal to the intelligent power module IP-M according to the given parameters to realize the on-off control of the power transistor. C8051F020 is connected to the address and data pins AD0~AD7 of SA4828 through the 8-bit P0 port. When working, the single-chip microcomputer first initializes SA4828 and defines parameters such as carrier frequency, power frequency range, dead zone, and minimum pulse cancellation time. Then, parameters such as the frequency control word and amplitude control word of the power supply are transmitted to the control register of SA4828. During normal operation, the control data of SA4828 is modified as needed to realize the feedback and real-time control of the system, as well as the voltage regulation control. In order to realize the voltage stabilization function of the system, the average value feedback PI regulation is adopted. The output voltage is sent to the A/D conversion port of the C8051F020 single-chip microcomputer, that is, the P3.0 port, through isolation. The conversion result participates in the PI operation, and the operation result is the control word of the amplitude control register of SA4828. The SPWM control signals of corresponding frequency and voltage are output from the six pins of RPHT~BPHB. After passing through the isolation circuit, the six IGBTs of the intelligent power module IPM are controlled to be turned on and off respectively, and finally a symmetrical three-phase SPWM voltage is generated on the three output terminals. SA4828 is a peripheral of the microcontroller and is connected in parallel with the microcontroller. By programming the microcontroller, it only needs to write the initialization information and control information of SPWM into the relevant registers of SA4828 to generate an accurate fully digital three-phase SPWM waveform.