Design of three-phase inverter power supply based on SA8282

　　1. Introduction

　　The inverter is the core part of the AC uninterruptible power supply (UPS) and AC variable frequency speed regulation system. It can independently form a system to become a variable frequency power supply device. Inverter power supplies are widely used in industry, but most inverter power supply circuit structures and control software are complex. The inverter power supply designed with SPWM integrated circuits and shutdown switches has many advantages. The new fully digital three-phase SPWM generator SA8282 launched by the British MITEL company has a wide output frequency and high precision. It can interface with a microprocessor, complete peripheral control functions and protection functions, and realize the intelligent design of the system [3].

　　This article introduces a simple three-phase inverter power design, which is composed of AT89C51 single-chip microcomputer, SA8282 waveform generator and AC-DC-AC main circuit. SA8282 only requires a small amount of peripheral hardware, without complex software programming, making the circuit structure of this system simple, easy to control, and stable in performance. At the same time, a certain protection circuit is designed, and the system works reliably.

　　2. System composition and main circuit design of inverter power supply[2]

　　The voltage-type inverter power supply consists of two parts: the main circuit and the controller, as shown in Figure 1. The main circuit adopts an AC/DC/AC power supply type inverter structure, which consists of a rectifier, an intermediate filter, an inverter and an isolation transformer. The input power stage adopts a simple and reliable three-phase bridge-type uncontrolled rectifier to rectify the AC power of the power grid into DC, and obtains a smooth DC voltage through filtering by the intermediate filter. The inverter switch uses three groups (six) of Fuji's 40KHz two-unit IGBT modules to form a three-phase H-bridge circuit. In order to achieve electrical isolation between the input and output of the inverter power supply and meet the requirements of the output voltage amplitude, a transformer must be included in the inverter power supply. For the three-phase variable frequency power supply, a transformer △/Y is connected at the output end for isolation and voltage transformation to reduce the size and weight of the power supply.

　　3. SA8282 features, functions and structural principles

　　SA8282 is a fully digital three-phase PWM generator with a wide frequency range and high precision. It can interface with a microprocessor and complete peripheral control functions, thus achieving intelligence.

　　3.1 Features

　　SA8282 adopts 28-pin DIP package, and its external pins and internal structure are shown in Figure 2. The functions of each pin are as follows: AD0~AD7 is an eight-bit address and data multiplexing bus, which is used to receive address and data information from the microprocessor. The three pins ˉˉWˉR(R/ˉW), ˉˉRˉD(DS), and ALE(AS) are Intel(MOTOROLA) control mode. SA8282 can automatically adapt to Intel or MOTOROLA control mode when working. When the ALE(AS) pin becomes high level, the internal detection circuit of SA8282 will automatically latch the state on the ˉRˉD(DS) line. If the detection result is low level, the MOTOROLA control mode is adopted; if the detection result is high level, the Intel control mode is adopted. ˉRˉSˉT is the reset terminal, which is valid at low level; ˉCˉS is the chip select input. This control line allows SA8282 to share the same bus with other peripheral interface chips. RPHT, RPHB, YPHT, YPHB, BPHT, and BPHB are standard TTL level output terminals (i.e., PWM drive signals) that can drive the six power switch devices of the three-phase inverter respectively. ˉTˉRˉIP is an output blocking status indicator, which is used to indicate whether the output is sealed, and the low level is valid. SET TRIP is the shutdown trigger signal input terminal. When the input is high, ˉTˉRˉIP and the six PWM output terminals will quickly lock in a low level state and can only be released when ˉRˉSˉT is reset. WSS is the waveform sampling synchronization port; ZPPB, ZPPY, and ZPPR are the zero-phase pulse output terminals of the three-phase signal respectively. CLX is the clock signal input terminal. VDD is the +5V bias power supply. VSS is the ground terminal. In addition, the SA8282 chip also has the following features:

　　⑴Fully digital

　　When SA8282 is connected to a microprocessor, it can automatically adapt to both Intel and MOROTOLA bus interfaces, and programming is simple and convenient. Its fully digital pulse output has high accuracy and stability.

　　⑵Flexible working methods

　　SA8282 has six standard TIL level output terminals, which can drive the six power switch devices of the inverter. The circuit's carrier frequency, modulation frequency, modulation ratio, minimum pulse width, dead time and other operating parameters can be set directly through software without any external circuit, thus reducing hardware costs.

　　⑶Wide operating frequency range and high precision

　　The triangular carrier frequency of SA8282 is adjustable. When the clock frequency is 12.5MHz, the carrier frequency can reach up to 24kHz, the output modulation frequency can reach up to 4kHz, and the resolution of the output frequency is 12 bits.

　　3.2 Structural principle

　　The internal structure and external pins of SA8282 are shown in Figure 2. It mainly includes three functional parts: initialization command and control command register part, reading and generating PWM modulation waveform part from ROM, and three-phase output control circuit.

　　⑴ Command register initialization and control

　　It consists of bus control, address/data bus, temporary registers R0~R2, virtual registers R3~R4, 24-bit initialization register and 24-bit control register. Initialization should be performed first when working. The control word is input from the microprocessor to the initialization register and control register to set the system parameters. Then the microprocessor inputs the command word to the two 24-bit registers, which are called the initialization register and the control register. Since the data width of the bus is limited to 8-bit word length, to send data to a 24-bit register, it should be sent to the three temporary registers R0, R1, and R2 three times. The data is sent from the temporary registers R0, R1, and R2 to the initialization register or control register through the virtual registers R3 and R4. R3 and R4 do not actually exist and only appear in the instruction. The write instruction to R3 is used to transfer data from R0, R1, and R2 to the control register, while the write instruction to R4 can transfer data from R0, R1, and R2 to the initialization register. Parameter setting is realized in the form of control words.

　　⑵ Read ROM and generate PWM modulation waveform

　　It consists of an address generator, a waveform ROM, and phase and control logic. Since the modulation waveform is symmetrical about 90°, 180°, and 270°, only the instantaneous value of the waveform from 0° to 90° is saved in the waveform ROM. When working, SA8282 can read the waveform data directly from the waveform ROM according to the signal of the address generator, and then compose it into a complete waveform and a three-phase waveform from 0° to 360° through the phase control logic, and can realize real-time waveform control without the need for processor processing.

　　⑶Three-phase output control circuit

　　Each phase output control circuit in SA8282 is composed of pulse cancellation and pulse delay circuit. The pulse cancellation circuit is used to remove the pulse whose pulse width is less than the cancellation time to ensure that the minimum output pulse width is greater than the switching cycle of the device. The delay circuit can ensure the dead zone interval. Its function is to provide a shorter delay time when changing the state of the two switching devices in any phase, so that both switches are in the off state during this period, thereby preventing the bridge arm switch elements from being common at the moment of conversion (the two switches cause a direct short circuit during the state conversion).

　　4. Controller design

　　The inverter power controller is shown in Figure 1. It consists of a single-chip microcomputer AT89C51, an SPWM generator SA8282, a driver HL402B, a detection data acquisition circuit ADC0809, a protection circuit, a display circuit, etc., and completes the two major functions of control and drive output [4].

　　4.1 Control Circuit

　　The control circuit is composed of the single-chip microcomputer AT89C51, a small number of peripheral expansion interfaces and the SA8282 three-phase SPWM generator. The single-chip microcomputer initializes the SA8282 and performs output pulse width control and frequency control, and completes the calculation and data processing of the open-loop and closed-loop control algorithms. The logic judgment of analog signals and digital signals and protection functions, etc. Since SA8282 and AT89C51 share a quartz crystal oscillator, the synchronization performance is stable and the drift is small. The closed-loop control principle of the inverter output voltage by the single-chip microcomputer is shown in Figure 3. The single-chip microcomputer uses the PI algorithm to adjust the SA8282 parameters to control the inverter output voltage.

　　4.2 Driving Circuit

　　The HL402B IGBT thick film driver integrated circuit with self-protection function is used, and is equipped with peripheral interface devices. It can be applied to direct gate drive of IGBT power devices with rated capacity of 200A/1.2kV and 400A/600V. The external wiring of HL402B driving IGBT is shown in Figure 4.

　　HL402B的优点是①自身具有降栅压和软关断的双重保护功能，其降栅压延迟时间，软关断斜率均可通过外接电容器进行整定;②能适应不同饱和压降的IGBT驱动和保护，在软关断和降栅压的同时能输出报警信号;③其内部有带静电屏蔽的光电藕合器，可用来实现与输入部分的隔离，显著提高了其抗共模干扰的能力;④可对信号进行脉冲功率放大，因逆变电路使用了6只IGBT，故需6只HL402B来驱动。

　　5. System software design

　　Since the system uses the SA8282 SPWM generator, the programming workload is greatly reduced. The system program is shown in Figure 5, including initialization program, display program, key monitoring program, single-cycle work test, voltage regulation U, frequency regulation F, continuous work control and other modules. In the initialization program, the single-chip microcomputer selects and sends control words to SA8282 to determine the frequency, dead time, output voltage control, etc. The adjustment of voltage and frequency is to control the output voltage and frequency of SA8282 through PI operation according to the difference between the set value and the actual value after the A/D conversion data is processed by the single-chip microcomputer. The display program displays the values ​​of voltage, current and frequency through LEDs respectively. Among them, the continuous work control program module includes data acquisition, PI voltage control operation, frequency control and other subroutines. After the voltage and frequency setting and the actual voltage value are collected and processed by A/D conversion data, the output voltage SPWM pulse width and frequency of SA8282 are controlled, thereby controlling the output voltage and output frequency of the inverter.

　　6. System protection and anti-interference measures

　　The main circuit and H-bridge protection of the inverter power system monitor the inverter output through current and voltage transformers, and HL402B monitors the overcurrent and overvoltage of IGBT. Once the main circuit and H-bridge are abnormal, the shutdown signal directly shuts down the PWM output through the SETTRIP terminal of SA8282. The microcontroller software protection program collects the protection signal sent by the hardware circuit, issues a shutdown command, and displays an alarm signal at the same time. The software protection program can also judge overcurrent and overvoltage based on the test data and perform protection control. A Watchdog Timer is set in the control program to prevent the microcontroller from crashing. In particular, the use of high-quality inlet filters for the power supply of the microcontroller control circuit can effectively remove power supply interference. The reasonable trend and distribution of PCB board routing can reduce harmful coupling. Kelvin grounding is adopted for the gate drive circuit of the IGBT to protect the H-bridge.

　　7. Conclusion

　　Experiments show that the combination of a single-chip computer and a SA8282 three-phase pulse width modulation wave generator greatly simplifies the control circuit, reduces the number of components, reduces the volume, reduces the cost, increases the carrier frequency, and makes the output waveform pure sine. The test results show that the voltage stability is less than 1%, the frequency stability is 0.05%, and the total harmonic content is 1%. The experimental waveform is shown in Figure 6. If the overload reaches 200%, the short-circuit protection immediately shuts down the power supply and implements emergency protection, thereby meeting the requirements of the performance indicators. The thick film drive circuit has a self-protection function, and the use of hardware and software dual protection makes the system main circuit and IGBT inverter work more reliably. The controller adopts closed-loop control to improve the output accuracy of the system.

　　If the inverter is used as a variable frequency power supply for the variable frequency speed regulation system of the AC motor, the frequency and working voltage of the output AC power can be easily changed by simply changing the setting of the SA8282 initialization control word, which saves a lot of programming work and can achieve real-time control. Since the waveform is a pure sine wave, the harmonic impact is reduced and the work efficiency is improved.