The main circuit is still the old one. Because of the small capacity, 4 IRF360 (25A/400V) is used as the full-bridge four-arm. The feedback sampling is still the same. The main transformer takes the TDK/EI70 core, and the rectifier takes the IR Schottky tube 400A. /100V, full wave rectification. The working frequency is 110KHz.
The difference is that the output filter inductance is very large, up to 120 microhenries. The secondary inverter adopts full-bridge output, and the switch tube selects IXYS company's FET IXFN75N10, with 6 arms per arm. The full load voltage drop is only 1.3V, so the heat sink is not too large.
The control adopts UC3825, which directly drives the pulse transformer 100A aluminum welder, which is AC/DC square wave output, the voltage is 12V, the current is 200A, and the modulation frequency is 400-1500Hz.
Why use a low-voltage FET full bridge as a secondary inverter? Instead of the usual half-bridge IGBT inverter?
This is because:
1. Calculated from static power consumption, although the low-voltage FET full bridge is connected in series with two tubes, the low-voltage FET has a very low on-resistance, and the voltage drop is still lower than that of the IGBT. In addition, the low-voltage FET full bridge only needs full-wave rectification, and one diode drop is lost. Not only that, but also uses a driving voltage much higher than the normal voltage, up to 18V, under normal circumstances, the FET only needs about 7V. However, it is necessary to know that low-voltage FETs and high-voltage FETs are different, and their on-resistance components are not proportional. The main part of the low voltage is the bulk resistance of the channel resistor. The channel resistance is continuously reduced as the VGS increases.
2. The reliability of the FET is not comparable to that of the IGBT.
3. Since the primary inverter output has no filter capacitor, if the secondary inverter adopts the IGBT half bridge, the inductor current will be nowhere to be released, and the high voltage will be generated during the commutation. This high voltage will break through the rectifier tube to prevent this. When something happens, it has to use a large-capacity RC absorption loop, and the inductance must be limited, and the overall efficiency is significantly reduced. This is not the case with the low-voltage FET full bridge. He can use the four-tube simultaneous turn-on method to provide a current channel (extended drive pulse width, so that the two pulses overlap for a short time), so there will be no High pressure is produced. At the same time, the continuity of the current is maintained. This has no particular benefit for electroplating, but if it is used for welding, the reversing moment will create a high pressure between the workpiece and the torch, which will act as an automatic arc.
4. The FET is simple to drive. Here, four Toshiba TLP250s are used, and only 1.9/pieces are disassembled. Single power supply can be used.
5. Low-voltage FET full bridge can use unlimited filter inductors and infinitely long welding cables. As mentioned above, too small inductance, including inductor saturation, will break, but because of the large inductance and commutation high-voltage arc, this circuit can still be perfectly soldered when the welding current is only 6A.
The 100V withstand voltage low-voltage FET can be used for any size of square wave aluminum welder. In theory, he can withstand 200V input, even if the 630A aluminum welder no-load voltage is only about 70V, there is still a lot of the amount. The full-bridge secondary inverter requires only a single power supply instead of the positive and negative power supplies like the IGBT half-bridge.
You can combine 400A manual welding machine and low-voltage FET full bridge to get 400A square wave aluminum welding machine easily.
Note: The start of the square wave modulation circuit must be automatically triggered and maintained by the welding current. Once the current disappears, it must be stopped immediately. That is to say, the working process should be: output unidirectional voltage - arcing - modulation on - square wave welding - full stop.
This machine also has the disadvantage that too many FETs increase the assembly difficulty (the cost is not high). In addition, the output must use four terminals, he does not have a common terminal. The four terminals can jump off the secondary inverter and directly output, and the efficiency is high.
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