Overview of high power high frequency soft switching inverter welding machine-EEWORLD

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1 Introduction

Inverter welding machine technology has experienced nearly ten years of development, gradually replacing the backward power frequency thyristor rectification technology and entering the era of high-frequency conversion. In the process of high-frequency conversion technology, it has gone through its primary stage, the hard-switching PWM stage, and has entered its second stage, the soft-switching PWM stage in recent years.

The topology of the hard-switching PWM converter is simple, the technology is mature, and it is suitable for mass production. Its main chips such as TL494 and UC3525 are relatively stable and reliable, which are the main reasons why the hard-switching PWM power converter of the inverter welding machine is still widely used.

The so-called hard-switching PWM (pulse width modulation) refers to the working conditions of large current or high voltage at the moment of opening and closing of the electronic switch in the power conversion process, so its workpiece reliability is poor, the efficiency is low, and the electromagnetic interference is extremely serious. Therefore, the hard-switching PWM technology has been unable to meet the requirements of the electronic age. The requirements of social development have forced power supply technology to enter its second stage, the soft-switching PWM stage in recent years.

The so-called soft-switching technology refers to the technology in power conversion technology that realizes the voltage or current at both ends of the main switch device to be zero at the moment of turning off and on. That is the ZVS (zero voltage switch) and ZCS (zero current switch) switching technology often referred to in the terminology. Soft-switching PWM power converter technology is a revolutionary development relative to hard-switching PWM technology. It has indeed improved the three basic performances of power supply products, namely reliability, efficiency, and electromagnetic interference (EMI) to a considerable extent. Most of the high-power switching power supplies developed by domestic peers now use hard-switching PWM control, and only a small number use soft-switching PWM. Most of their soft-switching PWMs use phase-shifting control, using chips such as UC3875, UC3879, and UCC3895. The phase-shifting control technology reduces the switching stress and switching loss of power devices, thereby improving the efficiency of the whole machine. However, this soft switch also has many shortcomings and regrets, such as: ① This medium and high power phase-shifting control soft switching method does not realize soft switching in the full range. ② Due to the existence of circulating current, the conduction loss of the switch tube is large, and the efficiency is low when the load is light, especially when the duty ratio is small, the loss is more serious. ③ There is parasitic oscillation in the output rectifier diode. ④ In order to achieve ZVS of the lagging bridge arm, an inductor must be connected in series in the circuit, which leads to the loss of duty cycle, reduces the output capacity, and increases the primary current rating. In addition, the phase-shift control itself has a disadvantage that is difficult to overcome, that is, the dead time is not easy to adjust. When the load is heavy, due to the large circulating current, the capacitor connected in parallel on the leading bridge arm power tube discharges faster, so it is easier to achieve zero voltage conduction. However, when the load is light, the capacitor connected in parallel on the leading bridge arm power tube discharges very slowly, and the switch tube of the leading bridge arm must be turned on after a long delay to achieve ZVS conduction.

Where there are defects, there is development. For this reason, we absorb the advantages of the traditional hard-switching PWM power converter, such as simple topology, few adjustable points, and stable and reliable. At the same time, we absorb the advantages of the phase-shift control soft-switching PWM power converter, which is easy to achieve ZVS and ZCS under heavy load. A new high-power full-bridge soft switch (FB-ZVZCS) technology is introduced, so that the leading arm is constant frequency and width control to achieve ZVS, and the lagging arm is constant frequency and constant width control to achieve ZCS. Thus, the full range of soft switching (FB-ZVZCS) of the leading arm and the lagging arm is realized, which greatly improves the reliability, efficiency and electromagnetic interference (EMI) of high-power switching power supply products. The realization proves that this control method is very excellent. It can be said to be a revolution of the traditional hard-switching high-power switching power supply.

2 Full-bridge soft switching (FB-ZVZCS) inverter welding machine control board appearance diagram

The appearance of the pulse control board of the high-power inverter welding machine is shown in Figure 1. The circuit board has a total of 18 pins, and the circuit board is a combination of analog and digital circuits independently developed. This circuit can form the driving pulse required for soft switching.

Figure 1 Appearance of the pulse control board of a high-power inverter welding machine

3 Full-bridge soft switch (FB-ZVZCS) inverter welding machine control board internal schematic diagram

The full-bridge soft switch (FB-ZVZCS) control board internal schematic diagram is shown in Figure 2. The functions of each pin are introduced as follows:

Figure 2: Full-bridge soft switch (FB-ZVZCS) control board internal diagram
Pin 1 is connected to the working power supply (UDD=12V or 15V);
Pin 2 is connected to the ground of the working power supply;
Pin 3 is the reference power supply (UREF=5V);
Pin 4 is the reverse input terminal of the voltage error amplifier;
Pin 5 is the non-inverting input terminal of the voltage error amplifier;
Pins 6 and 11 are connected to the timing capacitor (CT1 = CT2);
Pins 7 and 12 are connected to the timing resistor (RT1< RT2);
Pins 8 and 13 are the collectors of the oscillator discharge tube respectively, and a resistor is connected from pin 8 to pin 6, and a resistor is connected from pin 13 to pin 11.
The values of the two resistors should be equal. Changing the size of the resistor can adjust the dead zone of the leading arm and the dead zone of the lagging arm;
Pin 9 is connected to a capacitor to the ground, which acts as a soft start;
Pin 10 is the output terminal of the error amplifier;
Pin 14 is the shutdown pin. Inputting a level of about 0.7V from this pin can turn off all four outputs;
Pin 15 and Pin 16 are the output terminals of the pulse width modulation, which control the leading bridge arm of the full-bridge soft switch (FB-ZVZCS
); Pin 17 and Pin 18 are the output terminals of the fixed pulse width pulse, which control the lagging bridge arm of the full-bridge

soft switch (FB-ZVZCS); 4.1 The main circuit diagram of

the full-bridge soft switch inverter welding machine is shown in Figure 3. The input power of the welding machine is three-phase industrial frequency AC 380V±20%.

Figure 3. Schematic diagram of the main circuit of the full-bridge soft-switch inverter welding machine

4.2 Working principle and waveform diagram of full-bridge soft switch (FB-ZVZCS) The

waveform diagram is shown in Figure 4. The left arm is the leading bridge arm, and the excitation signals of the upper and lower switch tubes are constant frequency and width pulses. The right arm is the lagging bridge arm, and the excitation signals of the upper and lower switch tubes are constant frequency and constant width pulses. Below we briefly analyze the process of realizing soft switching:

4.2.1 Primary state T1:

V1 and V4 are turned on. At this time, the converter outputs energy to the secondary load. The working state at this time is the same as our usual hard switch PWM working mode.

4.2.2 State T2:

V1 is turned off and V4 remains on. Since capacitors C1 and C3 are connected to V1 and V3, when V1 is turned off, the current in the loop is not cut off at the same time, but C1 is charged and C3 is discharged through V4, L and T. At this time, the converter continues to output energy to the secondary load. After the potential of point A passes through T, if the energy of L has not been fully released, the current will continue to flow through the body diode of V3, that is, the voltage across V3 is zero, providing the condition for V3 to turn on at zero voltage. When V4 is turned off, the current on V4 is approximately zero, so V4 is turned off at zero current at this time.

4.2.3 State T3:

At this time, V1, V2, and V4 are all in the cut-off state. Due to the leakage inductance L of the transformer (the leakage inductance is very small), there is still a certain amount of energy in the loop, causing damped oscillation. Its frequency (F=1/T) has nothing to do with the load, but only with L and CV 2DS, CV4DS, C1, and C3. Since C1 and C3 are much larger than CV 2DS and CV4DS, this oscillation is only observed on the DS of V2 and V4, and there is no oscillation on the DS of V1 and V3. This oscillation will increase the loss of V2 and V4, and has no effect on V1 and V3. In order to reduce the loss on V2 and V4 and satisfy V2 and V4 to be turned on in the quasi-zero voltage state, only the following conditions need to be met: T3=T/2 T is the oscillation period. If T is too small, the inductor L can be increased. In order to ensure that V2 and V4 work safely and not be turned on by mistake, T3 should be appropriately increased. At this time, L can be increased according to different situations. C1 and C3 should be smaller when T2 ≥ RC is satisfied. When the power tube uses MOSFET, C1 and C3 are generally several thousand PF (1000~4700PF). When the power tube uses IGBT, C1 and C3 are generally larger (10~20nF).

After T1, T2 and T3, the converter completes half a cycle, and the second half cycle is the same.

4.2.4 Setting of T2 and T3

Reasonable design of T2 and T3 is the key to whether soft switching can be achieved and the maximum duty cycle can be met. From the analysis of the previous working process, it can be seen that if T2 is set too large, the duty cycle will decrease, the peak current of the power tube will increase, and the reverse withstand voltage of the secondary rectifier diode will increase, which will increase the loss of the power tube and the diode, and the high-frequency noise will also increase. Therefore, the duty cycle should be increased as much as possible, but if T2 is designed to be small, C1 and C3 cannot be fully charged and discharged, V1 and V3 cannot achieve zero voltage switching, and their losses will increase, which is not allowed. The optimal value of T3 is relatively critical. If T3 is large, the high-frequency oscillation will increase the loss of V2 and V4. If T3 is small, it is easy to cause V2 and V4 to short-circuit instantly. When the power tube uses MOSFET, T3 is generally around 300nS. When the power tube uses IGBT, it is generally larger, 300nS ~ 600nS.

Figure 4 Full-bridge soft-switching inverter welding machine waveform diagram

5 Full-bridge soft switch (FB-ZVZCS) inverter welding machine drive waveform dead zone and front and rear edge settings

The dead zone setting of V1V3 and V2V4 waveforms, the relative position setting of the front and rear edges of V1V4 or V3V2 waveforms are shown in Figure 5:

Figure 5 Full-bridge soft switch (FB-ZVZCS) inverter welding machine drive waveform dead zone and leading and trailing edge settings

6 Conclusions

The experimental results show that the designed high-power soft-switching arc welding inverter is not only small in size, light in weight and low in production cost, but also has high efficiency and reliability, and the switching loss of IGBT is greatly reduced. The processability, manufacturability and maintainability of the welding machine have reached a very high level.

References:

[1] Zhang Zhansong, Cai Xuansan Principle and Design of Switching Power Supply Electronic Industry Press 1998
[2] Ding Daohong Power Electronics Technology Aviation Press 1992
[3] Zhang Li, Huang Liangyi Power Electronic Field Control Devices and Their Applications Machinery Industry Press 1996
[4] Wang Cong Soft Switching Power Converter and Its Applications [M] Beijing: Science Press 2000

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