With the continuous development of components, how much do you know about the basic knowledge of welding machines?

“Welding machine is a relatively traditional industry, but any industry cannot be stagnant. As the saying goes: to stand still is to step back. Therefore, with the development of Electronic components, various industries are constantly iterating and updating, launching new equipment and winning industry competition. Today we mainly talk about some basic knowledge of welding machine.

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01

Typical topology of a welder

Welding machine is a relatively traditional industry, but any industry cannot be stagnant. As the saying goes: to stand still is to step back. Therefore, with the development of electronic components, various industries are constantly iterating and updating, launching new equipment and winning industry competition. Today we mainly talk about some basic knowledge of welding machine.

First, let’s talk about the overall structure of the welding machine, as shown below:

Today we mainly discuss the inverter part and the selection of IGBT. The development of inverter technology from the initial hard switching technology to the phase-shift soft switching technology to the double zero soft switching technology has different requirements for IGBTs. Because of this, the correct selection and use of IGBTs is very important. important.

?Hard switching topology

The circuit topology is as follows:

UDC is the DC bus voltage, the series combination of R1~R4 and C1~C4 forms the RC absorption network of IGBT1~IGBT4, TX1 is the inverter intermediate frequency transformer, VD1 and VD2 are the output full-wave rectifier diodes, R5, R6 and C5, C6 Correspondingly, the absorption network of VD1 and VD2 is formed in series, L is the output filter inductance, and R is the output equivalent load.

A hard-switched circuit works as follows:

IGBT1 and IGBT4 are turned on and off at the same time, and IGBT2 and IGBT3 are turned on and off at the same time. Among the switching losses, the turn-off loss accounts for a large proportion. Therefore, when selecting IGBTs, it is better to choose fast IGBTs, especially those with shorter turn-off times. Because of the time-hard switching, the waveform we turn on to the switching part has serious oscillations.

Phase-shifted soft-switching topology

The circuit topology is as follows:

Different from the hard switching circuit, the resonant inductance is added to the primary side of the inverter main transformer, and the output voltage waveform will not be distorted when the load characteristic is used.

The phase shift soft switch works as follows:

IGBT1 and IGBT2 are turned on in turn, and each is turned on for 180°, as are IGBT3 and IGBT4. However, IGBT1 and IGBT4 are not turned on at the same time. If IGBT1 is turned on first, then IGBT4 is turned on later. There is a certain phase angle between the two, and there is a certain phase between the turn-on and turn-off of the same group of IGBTs. Therefore, the switching loss is small, but the conduction loss will be large, and the working time of the freewheeling diode is long. When selecting an IGBT, a low-on-state IGBT should be selected, and the freewheeling diode has a strong current capacity.

Double Zero Soft Switching Topology

The circuit topology is as follows:

A saturated inductance Ls and a blocking capacitor C are connected in series in the AB segment, IGBT1 and IGBT4 are turned on at the same time, and they are not turned off at the same time, and the same is true for IGBT2 and IGBT3. According to the circuit characteristics of double-zero soft switching, zero-voltage turn-on and zero-current turn-off make the switching loss of the IGBT very small. Therefore, in the loss, the on-state loss accounts for a large proportion, and it is better to choose a low-on-state IGBT.

It works like this:

02

Several suggestions for IGBT selection

Let’s talk about the selection of IGBT in the welding machine. Because the load of the welding machine is special, the specific selection suggestions are as follows:

①Fast thermal response

This characteristic reflects the ability of the IGBT to transfer the junction temperature to the case under the action of a single pulse, which is of great significance to the size of the heat sink;

②IGBT parasitic capacitance parameters

Input capacitance Ciss, Miller capacitance Cres, and output capacitance Coss are of great significance to driving power, Miller effect, etc. Therefore, it is more advantageous to choose a smaller Miller capacitance and an IGBT with input capacitance;

③Choose the one with smaller conduction voltage drop as much as possible;

Voltage and current level

The input voltage of three-phase 380V, the bus is about √2*380=537V. In the case of hard switching, the rated voltage of IGBT is generally required to be higher than twice the DC bus voltage, which is the reason for the high peak voltage of hard switching. ; And the soft switch is generally 1200V;

Current: If the output current is 400A, the transformation ratio of the main transformer is K=37/5, and the average current on the primary side of the transformer is 400A/K=54A. When selecting IGBT, the rated current Ic=100A, considering the enhanced arc striking and thrust functions, the maximum output current is 500A. The primary side of the transformer is 500A/K=67.6A, which is also the maximum current ICM flowing in the IGBT, and the maintenance time Less than 1ms, consider twice the margin when using, and select the rated ICM>150A.

The working situation of the welding machine may be special. It works in the change of “no-load-short-circuit-load”, so it pays more attention to the current parameters.

As for other ones, such as drive resistors, protection circuits, etc., I won’t expand them here today. All changes are inseparable from the original, combined with the specific analysis of the situation is the right reason.

At present, it seems that some manufacturers are developing towards high frequency. Some friends told me that they are considering a three-level solution. The high frequency mainly reduces the size of the main transformer considerably:

According to the calculation formula of the transformer, U=4.44f*N*B*S

Among them, f is the working frequency of the transformer; N is the number of turns of the primary coil; B is the magnetic flux density, and S is the cross-sectional area of ​​the iron core.

It can be seen that the higher the switching frequency, the fewer turns of the primary coil and the more intimate cross-sectional area, thereby reducing the size of the transformer and improving the overall power density of the welding machine.