Development of a new type of single-phase inverter power supply

1 Introduction

At present, most variable frequency power supplies use sinusoidal pulse width modulation, the so-called SPWM technology. Most of its control circuits are implemented using analog methods. The circuits are relatively complex and have temperature drift, which affects the accuracy and limits the performance of the system. The control circuit composed of 80C196MC or TMS320F240 as the core can realize full digital control of the power supply, but the system is relatively complex, the software workload is large, and the development cycle is long. The enhanced SPWM wave generator SA4828 produced by MITEL can be connected to the single-chip microcomputer to complete the peripheral control function and make the system intelligent. The single-chip microcomputer only takes a short time to control it, so it is capable of detecting, protecting, controlling, and displaying the entire system. Based on the above reasons, the control circuit uses SA4828 and AT89C52. At the same time, in order to eliminate the noise of the output filter inductor, the transformer and inductor are integrated together, and the leakage inductance of the output transformer and the capacitor are used to form an LC low-pass filter, which not only eliminates the noise generated by the output filter inductor, but also simplifies the main circuit design.

2 Hardware Circuit Design

The system hardware circuit consists of the main circuit, control drive circuit, protection circuit and keyboard display circuit. The system block diagram is shown in Figure 1.

2.1 Main circuit design

As shown in Figure 1, the three-phase AC input voltage is used as the input of the inverter after rectification and filtering. The dotted part in the figure is the output transformer. As can be seen from the figure, no separate filter inductor is used in the circuit. Instead, according to the principle of magnetic circuit integration, the transformer and filter inductor are integrated into a magnetic component, and then a filter network is formed at the secondary of the transformer with appropriate capacitors. A sinusoidal voltage output is obtained from both ends of the capacitor.

2.2 Control and drive circuit design

The control circuit is based on AT89C52 and SA4828, which completes the generation of SPWM waves, system detection, control, update display and query key functions. SA4828 is a three-phase SPWM wave generator designed by MITEL for motor control circuits. It can also be used for static variable frequency power supplies. It is an enhanced version of SA8282, with full digital operation and high output waveform accuracy; wide operating frequency range, output power frequency can reach 4kHz, and frequency control accuracy can reach 16 bits; flexible working mode, equipped with a microprocessor interface, its working parameters: carrier frequency, power frequency, output amplitude, dead zone, etc. can be easily written through the microprocessor, and only need to be refreshed when the working mode is changed. In addition, it also has watchdog timing, three-phase amplitude independent adjustment and other functions. For detailed information about SA4828, please refer to reference [1]. This system uses the two-phase four-way SPWM output of SA4828 as the control signal.

Figure 1 System Block Diagram

When working, the microcontroller 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. When working normally, the control data of SA4828 is modified as needed to realize system feedback and real-time control, as well as voltage and frequency regulation. In order to realize the voltage stabilization function of the system, average value feedback PI regulation is adopted. The output voltage is isolated and sampled, and then AD conversion is performed. The conversion result participates in PI operation, and the operation result is the control word of the amplitude control register of SA4828. When adjusting the voltage, the user changes the given value of the voltage in the PI algorithm through the voltage adjustment button on the control panel, and changes the output voltage through PI regulation. The frequency control does not need to form a closed loop. When modulating the frequency, the microcontroller directly modifies the control word of the frequency control register of SA4828 according to the user setting to change the output frequency of the power supply.

In order to save the user's adjustment results, after the user adjusts, the voltage and frequency given values ​​are stored in the serial E2ROMX25045 with watchdog and undervoltage protection. The next time the machine is turned on, the voltage and frequency given values ​​are called out from it. The system uses LED display to display the output voltage, current and frequency at the same time. The display circuit is composed of MAX7219 as the core.

It must be emphasized that SA4828 is sensitive to clock signals. The clock signal generated by the CPU's internal clock circuit cannot make SA4828 work normally. An independent external oscillation circuit must be used to generate the working clock. In order to make the system work stably, the microcontroller and SA4828 can share the same clock source.

The driving circuit of this system adopts the IGBT driving module M57959L produced by Mitsubishi Corporation of Japan. The module has strong driving capability, good isolation and built-in collector voltage detection circuit.

2.3 Protection Circuit

This system has comprehensive protection functions, including collector overvoltage protection, overcurrent protection, overload protection and overheating protection. The collector overvoltage protection signal is provided by the driver chip M57959L; the overcurrent protection signal is obtained by sampling the bus current by the Hall sensor; the overload protection signal is obtained by sampling the output current; the overheating protection signal is obtained by the temperature relay. In addition, the P2.3 port of the single-chip microcomputer will also output a protection signal. After the logic transformation, each protection signal is connected to the SET-TRIP terminal of SA4828. The SET-TRIP terminal is an automatic emergency protection shutdown port. When this pin appears high level, SA4828 will automatically shut down the PWM output until SA4828 is reset; the other way is connected to the P3.5 port of AT89C52. When the single-chip microcomputer queries this high level, it will perform corresponding protection processing in the software, and output the protection signal through the P2.3 port at the same time, and block the output again. In addition, when the single-chip microcomputer detects an abnormality in the system, it will also output a protection signal through the P2.3 port.

3. Software Design

Software design is a core work of the system, which determines the output characteristics of the inverter power supply, such as voltage, frequency range and stability, the dynamic response speed of the system, the improvement of protection functions, working reliability, etc. The system software adopts modular programming. The main program block diagram is shown in Figure 2. The microcontroller completes the initialization of the microcontroller, SA4828 and other programmable devices in the initialization program, and then performs a self-check on the system. If it is normal, it enters the slow start module. Through SA4828, it is very easy to realize the slow start function. Just add the control word of the SA4828 amplitude control register from 0 to the normal value. Then enter the PI adjustment module, and use the time after the output voltage stabilizes to perform keyboard scanning, fault detection, update display, etc. In order to prevent the program from falling into an infinite loop due to unexpected circumstances, a software watchdog is designed. If the PI adjustment times out, it will cause the software watchdog to reset and the program to exit the PI adjustment module.

The PI adjustment module is the most important module in the system program. It determines the stability, accuracy and dynamic response speed of the inverter output voltage. The program flowchart is shown in Figure 3. The incremental PI algorithm with dead zone is adopted, and its expression is: (1)

Where: Δu(K)=(KP+KI)e(K)-KPe(K-1)[2]

Due to the limitation of the operation word length and the small sampling period, the incremental algorithm is prone to integration insensitivity [2]. To eliminate the integration insensitivity, both sides of equation (1) are divided by 2 when performing PI operation, and double-byte complement fixed-point operation is used. Before outputting u(K), the u(K) value calculated by the PI algorithm is shifted left by one bit as the output value. This method greatly reduces the integration insensitivity and is easy to program. In addition, overflow may occur during complement operation, so overflow judgment must be performed after the complement operation.

To prevent spike interference, the program performs digital filtering after AD sampling, using the median filtering method [2].

Figure 2 Main program flowchart

Figure 3 PI adjustment subroutine diagram

4 Anti-interference measures

For microcomputer systems, the environment in the power system is very harsh. Whether the anti-interference measures of the microcomputer system are appropriate may determine the success or failure of the design. This system adopts strong anti-interference measures in both hardware and software. Practice has proved that the design is successful.

(1) Hardware anti-interference measures are as follows:

——A stable and clean power supply is a prerequisite for the stable operation of the CPU system. The use of high-quality line filters can greatly improve the working environment of the system;

——Reasonable trend and distribution of PCB board routing;

——Use hardware watchdog to prevent the program from running away;

——Reasonable configuration of decoupling capacitors.

(2) Software anti-interference measures are as follows:

——Design multiple software watchdogs to monitor the operation of the entire program and important modules;

——Use instruction redundancy technology to reduce the probability of program failure;

——Design software traps to immediately bring runaway programs back to normal operation track.

5 Research results

After testing, the main technical indicators of this power supply are:

——Output voltage: 100V~130V adjustable, step 1V, stability 1%;

——Frequency: 380～420Hz adjustable, step 0.1Hz, stability 0.1%;

——Total harmonic content: ≯3%;

——Overload protection: When overloaded, the longest delay protection is 10 minutes. The heavier the overload, the shorter the protection time. When the rated load exceeds 20%, protection will be activated immediately.

——System noise: ≯45db.

The experiment shows that after the inverter uses AT89C52 and SA4828, the control circuit is greatly simplified, the cost is reduced, and the reliability is improved. The main circuit uses an integrated inductor transformer, which simplifies the main circuit design and greatly reduces the system noise. It is believed that this power supply has a good application prospect.