Design of DSP three-phase SPWM inverter power supply with perfect protection function

1 System Introduction

According to different structures, variable frequency power supplies can be divided into two categories: direct variable frequency power supplies and indirect variable frequency power supplies. The variable frequency power supply studied in this paper adopts an indirect inverter structure, that is, an AC-DC-AC conversion process. First, the AC-DC conversion is completed through a single-phase full-bridge rectifier circuit, and then the DC power supply is converted into a three-phase SPWM waveform under the control of the DSP to supply the post-stage filter circuit to form a standard sine wave. The variable frequency system controller uses the industry's first floating-point digital signal controller TMS320F28335 launched by TI. It has a 150MHz high-speed processing capability, a 32-bit floating-point processing unit, and a single instruction cycle 32-bit accumulation operation, which can meet the application's requirements for faster code development and integrated advanced controller floating-point processor performance. Compared with the previous generation of leading digital signal processors, the latest F2833x floating-point controller can not only improve performance by an average of 50%, but also has the characteristics of higher precision, simplified software development, and compatibility with fixed-point C28xTM controller software. The overall system block diagram is shown in Figure 1.

Figure 1 System overall block diagram

2 System Hardware Design

The hardware circuit of the variable frequency power supply mainly includes 6 modules: rectifier circuit module, IPM circuit module, IPM isolation drive module, output filter module, voltage detection module and TMS320F28335 digital signal processing module.

2.1 Rectifier circuit module

The diode uncontrolled rectifier circuit is used to improve the grid-side voltage power factor. The rectified DC voltage is stabilized by a large capacitor to provide a DC voltage for the inverter. The circuit consists of 6 rectifier diodes and a DC stabilizing capacitor that absorbs the inductive reactive power of the load. The rectifier circuit schematic is shown in Figure 2.

Figure 2 Schematic diagram of the rectifier circuit 2.2 Voltage detection module

Voltage detection is an important part of completing closed-loop control. In order to accurately measure the line voltage, the SPI bus and GPIO port of TMS320F28335 are used to control the attenuation/amplification ratio of the input line voltage to meet the A/D module's requirements for the input signal level (0-3V). The voltage detection module uses a 256-tap digital potentiometer AD5290 and a high-speed operational amplifier AD8202 to form a programmable signal amplifier/attenuator. The input characteristics of each input channel are 1MΩ input impedance + 30pF. The circuit schematic of the voltage detection module is shown in Figure 3.

Figure 3 Schematic diagram of voltage detection circuit

3. System software design

After the system is powered on, it bootstraps the loader according to the selected mode, jumps to the main program entry, and performs related variable and control register initialization settings and sine table initialization. Then, it enables the required interrupts, starts the timer, and then loops through fault detection and protection, and waits for interrupts. It mainly includes three parts: timer period interrupt subroutine, A/D sampling subroutine and data processing algorithm. The main program flow chart is shown in Figure 5.

Figure 4 Main program flow chart Figure 4 Experimental results

4.1 Measurement waveform

On the basis of completing the above hardware design, this paper adopts a specific PWM control strategy to make the inverter drive the induction motor to run, and conducts short circuit and motor stall experiments, proving that the inverter has stable performance and can reliably achieve overcurrent and short circuit protection. Figure 6 is the steady-state voltage waveform recorded by a digital oscilloscope under no-load conditions. The amplitude is 35V and the frequency is 60Hz.

Figure 6 Output line voltage waveform

4.2 Results Analysis

It can be seen from the line voltage waveform observed by the oscilloscope that the waveform is close to a sine wave and is basically distortion-free; it can be seen from the data in the table that at different frequencies, the maximum absolute error of the output line voltage is only 0.6V, and the relative error is 1.7%.

5 Conclusion

The three-phase sine wave inverter designed in this paper adopts asymmetric regular sampling algorithm and PID algorithm to make the output line voltage waveform basically a sine wave, and its absolute error is less than 1.7%; it also has a fault protection function, which can automatically cut off the input AC power supply . Therefore, this system has the advantages of simple circuit, good anti-interference performance, good control effect, etc. It is convenient for engineering application and has great practical application value.