The thesis is devoted to the research of electric power converters with modular structures and reduced ripple levels for resistance welding.
Resistance welding is widely used for joining metal parts in the electronics industry, instrumentation, mechanical engineering, automotive and aircraft construction, space technology, medicine, and other industries.
Resistance welding is carried out by clamping two metal parts between two electrodes with the required compression force and passing through them a pulse of electric current of the desired shape, amplitude, and duration. At the point of current flow, the parts are heated to the melting point and connected to each other. The welding process is quite complex because the electrical resistance of the welding zone is complex, nonlinear, and depends on the material, thickness, and surface roughness of parts and electrodes. Such properties of resistance welding complicate the process of designing power supplies for its implementation.
In most cases, the strength of welded joints is the main and only requirement. However, there are industries where precision miniature parts have to be connected, and the absence of splashes of molten metal particles, as well as the high frequency of reproduction of weld point parameters, is critical. The requirement to avoid splashes when welding miniature parts or parts of responsible components is due to the fact that metal particles that have hardened after the welding process can cause short circuits in the functional elements of electronic devices, signal distortion, noise, etc. High repeatability of joints is necessary for the manufacture of complex products while performing a large number of welds, which predominantly affects the quality of the finished product.
A power supply (also called current pulse shaper) is supposed to provide the necessary parameters for welding – the required shape, amplitude, and pulse duration of the weld current. In practice, various shapes of weld current pulses are used, such as DC or AC pulses, pulsating current pulses, or a combination of the above-mentioned current pulse shapes. The use of DC pulses can improve the quality of the connections, especially in the case of welding miniature parts. A number of studies show that the level of weld current ripples plays a significant role when welding parts up to 0.5 mm thick. The small amplitude of the ripples allows obtaining high-strength welded joints without metal splashes. Also, additional filter elements to reduce current ripple bring significant inertia to the feedback loop, which can degrade the quality of the control system and affect the accuracy of current reproduction.
To date, there are many ways to build power supplies for resistance welding equipment. The most promising is the construction of a power supply based on a transistor converter, which is able to provide high accuracy of the current generation in the load, as well as a higher level of energy efficiency compared to other types of power supplies. It is known that transistor converters with continuous control allow obtaining high accuracy of the reproduction of the welding current curve, but provide fairly low energy efficiency. Pulse-controlled transistor converters provide higher efficiency, but the accuracy of the output current is reduced. The use of converters with the joint use of continuous and pulse transistor control modes allows reducing the power loss at the same time while maintaining the high accuracy of the current generation. However, for such solutions, the issue of improving energy efficiency remains relevant. The use of a step-down converter with low ripple, operating in pulse mode, will provide high accuracy of welding current generation at the same time with low power loss, inherent in transistor converters with pulse control.
The use of a modular topology of a power supply with n unified converter modules connected in parallel and working for a common load will improve the accuracy of the current generation, increase the power level in the load without the use of bulky components, as well as increase manufacturability, flexibility, and unification of the converter.
