Control of three level NPC DC/DC converter and application of MMA welding machine

Welding is a manufacturing process that is used to make metallurgical bond between the similar compounds. The first evidences of welding were from the bronz age. During the first World War, the need of weapons caused the start of developement of welding technologies. Nowadays, the developments in the field of industry and the discovery of new materials have forced the development of new welding processes. At the same time, the welding power units and their controllers have started to progress day by day due to new developments of welding processes. When considering the development of welding power sources, there are many welding units classified according to their constant current or constant voltage characteristics as well as AC and DC outputs. The simplest welder unit is a transformer. This type of machine alters the input high voltage-low current to low voltage-high current at the secondary side which is needed to weld. The current control can be done by using tap charger to control step-by-step or movable shunt or coils for contious control. If DC output is needed, the current can be converted to DC by using un-controlled rectifier. This types of machines are bulky and their efficiency is low. Moreover, they can not guarantee the output regulation when the mains fluctuates. For this reason, the silicon controlled rectifiers known as SCR can be used to make the output regulatation insensitive against the input fluactations. In addition, there are alternators and generators that use a gasoline or diesel engine to provide welding current in the fields that there is no electricity. Among these types of welding power sources, one of the most advanced technology is the inverter topology. In inverter welding machines, main supply voltage is rectified and filtered to obtain smooth DC voltage. Then they make square wave AC voltage in the primary side by switching the semiconductor switches at very high frequency. In secondary side, it is converted to low voltage high currents, rectified and filtered to get smooth DC welding current. They can keep the output regulated against the input fluctations. When the frequency increases, the transformer sizes become smaller. Therefore, they are light and compact. Their efficiency is high. Half bridge, forward and full bridge topologies are the most popular isolated DC/DC converters used in welding machines. In this types of convertes, when the frequency increases the sizes of the components with magnetic winding such as transformer and inductor as well as capacitors. Thus, volume and cost are reduced, power density is increased. However, frequency increase brings some problems. These are EMI noise and the increase of switching losses. Swithing losses are the losses that occur when the current and voltage crosses during turn-on and turn-off. Due to switching losses, efficiency reduces, the required heat-sink size increases. In addition to this loss, the reverse recovery of diodes and discharge of parasitic capacitances of switches are the supplementary losses. EMI noises are the noises that occur due to rising rate of current and voltage during transition. Since the hard switching causes some big problems, soft switching techniques have been developed. These techniques are zero current switching (ZCS), zero voltage switching (ZVS), zero current transition (ZCT) and zero voltage transition (ZVT). ZCS is the technique that used during turn on times to limit the current rise rate by the help of serial inductance. In ZVS technique, The rise rate of voltage can be limited during turn off bu using parallel capacitor . Both of two techniques can not eliminate the energy loss completely, can just limit the losses. Switching losses are consumed by some snubber circuits while can be recovered by other snubber circuits. ZVT and ZCT are the techniques that switching losses can be eliminated completely by using the quasi-resonant method. Thanks to these soft switching techniques, power density increases, switching losses and EMI noises are reduced, switching frequency can be increased, volume of the circuit is reduced, efficiency increases and heatsink size shrinks. Forward topologies used in welding machines are the basic and easy to control. The magnetizing current must be reset at each cycle in this converter, otherwise core goes into saturation. In single switch forward converters, there is third winding in transformer to reset the core, however two switch forward can be preferred to get rid of the third winding and reduce the voltage peak on switch. Half bridge are also basic and easy to control converters that used in welding machines commonly. In general, hard switching is used in this type converters. The voltage on primary is half of the input voltage in conduction mode while the primary voltage is stray during turn off times. Because of this reason, oscillations can occur and voltage quality can degraded. In additon, the current flows through the leakage capacitance during turn off times can be interrupted. The same situation can be observed in full bridge DC/DC converters which are frequently used in welding machines.The current that continues to flow due to leakage inductance is interrupted at turn off times, the oscillations between leakage inductance and parasitic capacitors occur. So, phase shifting method that provides the soft switching is used instead of hard switching technique. Thus, parasitic oscillations occured during turn off can be prevented. Also, the primary voltage can be kept at real zero voltage during free-wheeling intervals in full bridge converters. Thus, not only soft switching can be provided but also voltage quality becomes excellent. Among these converters mentioned above the usage of the three level npc DC/DC converters in different fields has been increased. At this method that the application of soft switching technique in this type of converter is brand new, during conduction the voltage of the primary is the half of input dc voltage while it is short circuit during freewheeling intervals. Thus, the oscillations on primary voltage can be prevented.There are some advantages of three level npc converters in comparison with 2 level converters. The aim of the this thesis is the application of soft switching three level npc DC/DC converter in coated electrode welding machine and realizing the control of welding machine. In this thesis phase shifting method is applied by using the transformer leakage inductance and realizing the quasi-resonant. Thus ZVT is provided during turn-on times of switches. During turn off times, ZVS technique can be realized bu using the parallel capacitors of switches. In addition, the forward, phase shifting full bridge topologies that frequently used in welding machines are designed and application of them are realized. The simulations of these topologies were realized in PSIM software, PCBs of them were designed in Altium Designer and experimental results were obtained. In addition of these topologies, the half bridge converter that frequently used in welding machines was also simulated, the pcb of it was designed in Altium and experimental results were obtained. At this last stage of thesis the arc control parameters of MMA welding machine was examinated and implemented in these four converters.