The core circuit remains largely unchanged from previous models. Due to the smaller capacity, four IRF360 transistors (rated at 25A/400V each) are utilized as a full-bridge four-arm configuration. Feedback sampling continues to follow the same approach. The main transformer employs a TDK/EI70 core, while the rectifier uses IR Schottky diodes rated at 400A/100V for full-wave rectification. The operating frequency is set at 110kHz.
A notable difference lies in the increased output filter inductance, reaching up to 120 microhenries. The secondary inverter employs a full-bridge topology, utilizing IXYS FETs (IXFN75N10) for each arm, with six arms per side. Under full load conditions, the voltage drop remains minimal at just 1.3V, allowing for a relatively compact heatsink design.
Control is achieved using the UC3825 chip, which directly drives the pulse transformer for a 100A aluminum welder. The output is an AC/DC square wave with a voltage of 12V and a current of 200A, modulated within a frequency range of 400-1500Hz.
Why opt for a low-voltage FET full bridge as the secondary inverter instead of the conventional IGBT half-bridge? This choice stems from several factors:
First, when considering static power loss, despite requiring two tubes in series, the low-voltage FET boasts a very low on-resistance, resulting in lower voltage drops compared to IGBTs. Additionally, the low-voltage FET full bridge only requires full-wave rectification, with only a single diode drop being lost. Furthermore, a higher-than-normal driving voltage of up to 18V is employed here, whereas FETs typically require around 7V. It’s important to note that low-voltage FETs differ significantly from high-voltage ones, with their on-resistance components not being proportional. The primary component of the low-voltage FET is the bulk resistance of the channel resistor, which decreases continuously as the VGS increases.
Second, the reliability of FETs surpasses that of IGBTs.
Third, since the primary inverter output lacks a filter capacitor, if the secondary inverter were to use an IGBT half-bridge, the inductor current would have nowhere to discharge, leading to the generation of high voltages during commutation. These high voltages could potentially damage the rectifier. To mitigate this, a large-capacity RC absorption loop would need to be incorporated, and the inductance would need to be limited, significantly reducing overall efficiency. In contrast, the low-voltage FET full bridge can employ a four-tube simultaneous turn-on method, providing a current path (by extending the drive pulse width to allow the two pulses to overlap briefly), thereby preventing high voltages from forming. Moreover, the continuous flow of current is maintained. While this may not offer specific advantages for electroplating, it proves beneficial for welding applications, as the reverse moment generates a high voltage between the workpiece and the torch, acting as an automatic arc.
Fourth, FETs are simpler to drive. Here, four Toshiba TLP250 drivers are used, costing approximately $1.9 each. A single power supply suffices.
Fifth, the low-voltage FET full bridge allows for unlimited filter inductors and infinitely long welding cables. As mentioned earlier, insufficient inductance, including inductor saturation, can lead to failure, but due to the large inductance and commutation-induced high-voltage arcs, this circuit can still perform excellent welding even at a mere 6A of welding current.
The 100V-rated low-voltage FET is suitable for any size of square wave aluminum welder. In theory, it can handle 200V input, meaning that even with a 630A aluminum welder having a no-load voltage of only about 70V, there is still considerable leeway. The full-bridge secondary inverter requires only a single power supply, unlike the IGBT half-bridge, which necessitates both positive and negative power supplies. Combining a 400A manual welding machine with a low-voltage FET full bridge can yield a straightforward 400A square wave aluminum welding machine.
Note: The startup of the square wave modulation circuit must be automatically triggered and maintained by the welding current. If the current ceases, it must stop immediately. Thus, the working process should follow this sequence: outputting a unidirectional voltage - initiating an arc - activating modulation - performing square wave welding - halting entirely.
This machine does have some drawbacks, such as the increased assembly complexity due to the numerous FETs (though costs remain low). Additionally, the output requires four terminals, without a common terminal. These four terminals can bypass the secondary inverter and directly output, enhancing efficiency.
Beyond these technical details, the machine demonstrates impressive versatility and adaptability, making it a valuable tool for various industrial applications. Its design reflects a careful balance between performance, cost, and practicality, ensuring its place as a reliable solution in the welding industry.
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