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AN3200 Application note
2.5 kW MMA welding machine
Introduction
In this application note a power AC-DC converter, suitable to be used as the current supply for the MMA welding process, is described. The MMA welding method is the most common method used for welding iron or steel. Usually this method, also the most simple, is dedicated to small home repairs but also to light professional use. Inrecent years, the availability of low cost and reliable solutions, based on power electronics, has moved the approach to this application from a standard magnetic solution, based on a low frequency iron transformer, to a high frequency switching solution. This gives the possibility of having compact and light solutions at a relatively low price. The electronic control used on modern weldingmachines allows a simple and safe use, so even an unskilled person can use it with good results in terms of weld quality.

September 2010

Doc ID 17388 Rev 1

1/19
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Contents

AN3200

Contents
1 2 3 4 5 MMA welding process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 MMA welding power supply . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 6 State of the art forward converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Current control implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Welding machine hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 5.2 Power board description . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 11
5.1.1 Power board schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Control board schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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Doc ID 17388 Rev 1

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List of figuresList of figures
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. MMA welding process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Output characteristic of an MMA welding power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Schematic diagram of a double switch asymmetricforward converter . . . . . . . . . . . . . . . . . 6 Some significant acquired waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Block diagram of current control circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 MMA welding machine demonstration board STEVAL-ISW001v1 (side view) . . . . . . . . . . 11 MMA weldingmachine demonstration board STEVAL-ISW001v1 (top view). . . . . . . . . . . 12 Power board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Control board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Doc ID 17388 Rev 1

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MMA welding process

AN3200

1MMA welding process
In the following paragraph a general description of the Manual Metal Arc (MMA) welding process is explained. The process is very simple. A plasma arc, able to generate a very high temperature in a restricted area, is used to fuse iron or metal. The plasma arc is created by a high current at low voltage. Two wires, one fixed to the iron or metal pieces to be welded by ametallic clamp, and one with a special tool on its end called an electrode holder, and able to fix a metallic rod, are used to be connected to the power supply’s negative and positive outputs, respectively. A shielded electrode is used at the same time to conduct the current, necessary to sustain the plasma arc and to introduce additional welding material into the welding area. The electrode is...