Design of power supply control circuit for resistance welding machine based on DSP

Abstract: Based on the current status of medium frequency inverter technology and the principle of medium frequency inverter power supply, a high-power medium frequency inverter resistance welding power supply control circuit based on DSP (Digital Single Process) is proposed and designed. The feasibility and superiority of this design scheme are verified by experiments, namely, the control circuit is simplified, the number of components is small, the size is small, the cost is reduced, the short-circuit protection action is reliable, the requirements of performance indicators are met, and the control accuracy of the system is improved.
Keywords: medium frequency inverter, drive, DSP, IGBT

1 Introduction

Resistance welding is an important welding process with the characteristics of high production efficiency, low cost, material saving and easy automation. It is widely used in aviation, aerospace, energy, electronics, automobile, light industry and other industries. In recent years, with the rapid development of manufacturing industries such as automobiles and canning, special resistance welding machines have also achieved unprecedented development, gradually moving towards automation and robotization. The welding control power supply is an important component of the resistance welding system. Due to the rapid progress of power electronics technology, the medium-frequency DC inverter resistance welding power supply, as a new type of control power supply, has become the development direction of resistance welding power supply with its remarkable high quality and low consumption characteristics [1][2].

The inverter-controlled high-power DC power supply is an energy-saving, efficient, and simple-structured power supply. However, the current power is too small, the welding technology is not very good, and the welding quality cannot be guaranteed [2]. The key is that the switching loss of the power switch tube is large. It not only wastes electric energy, but also affects the reliability of the inverter circuit. Therefore, how to overcome and reduce the switching loss in the high-power resistance spot welding DC power supply has become an important issue [6]. At present, most of the domestic inverter power supply resistance welding machines use analog control, the control circuit is quite complex, maintenance is difficult, and the whole machine is large [1]. The digital resistance welding machines developed by many foreign manufacturers have a high level of welding automation and reliable quality, but the price is very expensive. In response to this problem, this paper applies DSP ( TMS320LF2407 A) control technology to the research of inverter resistance welding machine to ensure the static and dynamic characteristics of the inverter resistance welding machine, and further reflect the characteristics of the inverter resistance welding machine such as lightness, energy saving, safety, and reliable protection. The design of high-power medium-frequency inverter resistance welding power supply based on DSP introduced in this paper is a good solution to the problem.

2 Medium frequency inverter power supply

The power supply of the medium frequency inverter DC resistance welding machine is a three-phase industrial frequency AC power supply that is converted into a DC power supply through a rectifier circuit and a filter capacitor, and then converted into a medium frequency square wave power supply through an inverter circuit composed of power switching devices. The power is then input into a transformer for step-down and rectified into a DC power supply through a high-power diode with a low tube voltage drop, and then supplied to the welding machine's electrodes for welding the workpiece (as shown in Figure 1). The inverter usually uses current feedback to implement pulse width modulation (PWM) to obtain a stable constant current output. Circuit principle and waveform

As shown in Figure 1. In the figure, Upower is the power supply voltage, Uprimary inverter outputs the intermediate frequency voltage, and the transformer secondary working current Isecondary. Controlling the PWM pulse width can control the size of the working current Isecondary.

Figure 1 Overall block diagram of the main circuit of the medium frequency inverter power supply

Fig.1 Medium Frequency Inverter Electrical Source Theory Frame

According to the basic transformer formula U = kfNSBm, where: U - transformer input voltage, f - input voltage frequency, N - transformer turns ratio, S - transformer effective magnetic conductivity area, Bm - core maximum working magnetic flux density. It can be seen that when the transformer input voltage and the maximum value of magnetic induction intensity are constant, increasing the input voltage frequency can reduce the number of winding turns and reduce the core cross-sectional area. The volume of the transformer is mainly determined by N and S. Therefore, increasing the inverter frequency can greatly reduce the volume and weight of the power supply, thereby saving a lot of copper and magnetic materials [1]. At the same time, due to the increase in the inverter frequency, the pulsation frequency of the secondary rectifier output increases, and a smaller filter reactor can be used to achieve better results, thereby reducing the output filter reactor volume and the time constant of the output circuit. In conjunction with the control circuit, the dynamic response speed of the inverter power supply can be greatly improved to meet the requirements of different welding processes. The inverter DC power supply has superior technical and economic indicators, and thus has become the most promising direction for the development of inverter power supplies [1].

3. Design of medium frequency inverter power supply circuit

The inverter power supply circuit consists of DSP and its corresponding peripheral circuits and drive and protection circuits, as shown in Figure 1. The primary and secondary protection signals of the inverter power supply and the protection signals of the IGBT are used to achieve timely protection functions through sensors and detection circuits. DSP generates PWM waves and external detection signals, protection signals and PWM to drive the IGBT inverter through the driver chip. The driver chip uses M57962AL. M57962AL has good driving ability, high power, good protection performance and other characteristics to meet the needs of the design. The drive circuit is shown in Figure 3. The drive circuit contains drive power, detection protection and drive chip. The internal circuit structure diagram of the driver chip is shown in Figure 2.

IGBT high-power tubes can usually only withstand 10us short-circuit current, desaturation or overcurrent, so fast protection is necessary. M57962AL performs soft shutdown protection when these phenomena occur in the driven IGBT. Circuit M57962AL driver is equipped with a current protection circuit. As shown in Figure 2, a typical application circuit realizes isolation and protection functions. The first foot of M57962AL is connected to the collector C of the IGBT, and an external Zener tube (DZ5 in Figure 2) is used to replace the Zener tube inside M57962AL. In order to prevent high-voltage spikes in the gate drive circuit and damage to the IGBT, two Zener diodes (DZ16 and DZ17 in Figure 2) are connected in reverse series between the gate and the emitter.

From Figures 1, 2 and 3, we can know that the working principle of IGBT drive is as follows: the PWM generated by the DSP in Figure 1 is transmitted to B1 in Figure 2, amplified and inverted by the three-stage tube, and enters the 13th pin of the M57962AL chip (the signal input pin of the driver chip in Figure 2), generating +15V open gate and -10V closed gate voltages at the 5th pin of the M57959AL, driving the IGBT on and off [6].

Similarly, the protection principle of IGBT is: when overcurrent occurs, the Uce of IGBT will be significantly higher than that of normal conduction, and the saturation voltage drop is generally above 7V, which will cause the so-called device/de-saturation phenomenon. The first foot of M57962AL plays a protective role. The built-in timer of M57959AL starts, and the short-circuit current is clamped at a lower value through the gate-off circuit and the step-down circuit. At the same time, the detection circuit pulls the eighth foot of M57962AL to a low level, and the optocoupler (as shown in Figure 2) responds, generating a short-circuit protection signal short1 (as shown in Figure 2) at a low level. Short1 is sent to DSP, and the PWM output is immediately turned off. The drive signal is turned off, thereby playing the role of a protection circuit to protect the IGBT.

Figure 2 IGBT drive circuit diagram

Fig.2 IGBT Driver Circuit Frame

Figure 3 Internal structure of M57962AL

Fig.3 M57962AL Inside Construct Frame

4 Control program flow chart

The software program is mainly a program for DSP to generate PWM. The entire software is mainly programmed in C language and assembly language. After debugging in the CCS2000 environment, it is downloaded to the DSP peripheral FLASH and runs well, achieving the expected goal. The main program flow chart and interrupt program flow chart are shown in Figure 4. In Figure 4, the system initialization includes the initial settings of the A/D port, PWM port, etc. The A/D conversion is completed by the timer generating interrupt. The software timer T4 is used to start the A/D conversion regularly, generate an interrupt signal, and then the DSP performs the corresponding operation.

Figure 4 Main program flow chart and interrupt flow chart

Fig.4 Main and Interrupt Program Flow Chart

5 Experimental waveform and conclusion

After burning the program into the peripheral Flash of the DSP chip, add voltage to the control circuit and the drive module. Here, only the drive waveform of one IGBT is analyzed. Figure 5 is the drive signal waveform of the IGBT working state, the PWM waveform between the E pole (gate) and the G pole (gate) of the IGBT, that is, +15V open gate and -10V closed gate voltage, and the PWM frequency of 5Khz is used here. Figure 6 shows that when there is an overcurrent or the detected signal is incorrect, the drive signal is turned off, making the IGBT non-working. The analysis found that the peak wave of the G pole signal has a maximum value of about 20V and a turn-off time Tc<10us, which ensures the safety of the IGBT.

Figure 5 Normal operation waveform Figure 6 Short circuit waveform

Fig.5 The Waves of Work Situation Fig.6 The Waves of Short Circuit Situation

As shown in the experimental waveforms of Figures 5 and 6, the designed circuit has a switching frequency of 5Khz controlled by the intermediate frequency, has good reliability, and has a timely protection function for the IGBT. The drive circuit controlled by DSP can achieve controllable intermediate frequency inversion, adjustable frequency, adjustable PWM pulse width, and fast and timely protection action. The M57962AL chip and peripheral circuits have good protection performance and can protect and isolate the IGBT in time.

6 Conclusion

The experimental results show that the short-circuit protection action of the control circuit can immediately shut down PWM for emergency protection, meeting the requirements of performance indicators. The control circuit designed by DSP and the drive circuit of M57962AL greatly simplifies the control circuit. Fewer components and smaller size reduce the cost. Intermediate frequency control improves the control accuracy of the system. Excellent performance, microsecond response time, fast control speed, and convenient design of appropriate external characteristics according to needs. The development of medium frequency inverter power supply is an important development direction in the field of modern welding. Its advantages such as high efficiency and energy saving, higher energy input, precise parameter adjustment, and ability to reduce heat and mechanical pressure of the electrode are favored by everyone [1][2][5]. The design idea of ​​this power supply circuit provides a reliable and feasible design scheme for digital medium frequency inverter DC resistance welding.

The author's innovation: A new medium frequency inverter power supply overall design based on digital signal processor is proposed, and the hardware and software implementation of circuit design are specifically analyzed. The economic benefits in 5 years are 50 million yuan, which is estimated based on the current domestic market demand and the current price of foreign resistance welding machines.