Welding inverter is an alternative to conventional welding transformer. Modern semiconductors allow to replace the traditional mains transformer with a
switching supply, which is much lighter, smaller and allows easy current control via potentiometer. The advantege is
also that the output current is DC. Direct current is less dangerous than AC and prevents arc extinction.
For this inverter i chose topology, which is the most common in inverters - forward inverter with two switches. In my article about switchning supplies it is a topology II.D. Input line voltage passes through the EMI filter and is smoothed with large capacity. Since the switch-on current pulse would be too high, there is a softstart circuit. After switching ON the primary filter capacitors are charging through the resistors, which are subsequently eliminated by switchning ON the relay. As power switches the IGBT transistors are used. They are driven through a forward gate drive transformer TR2 and shaping circuits with BC327. Control circuit is UC3844. It's similar to UC3842, but it has pulse-width limit to 50%. Working frequency is 42kHz. Control circuit is powered by an auxiliary supply of 17V. Current feedback, due to high currents, is using current transformer Tr3. Voltage accros the sensing resistor 4R7/2W is approximately proportional to the output current. Output current can be controlled by potentiometer P1, which determines the threshold of feedback. Threshold voltage of the pin 3 of UC3844 (current sensor) is 1V.
Power semiconductors require cooling. Most of the heat is dissipated in output diodes. Upper diode, consisting of 2x DSEI60-06A, must in worst case handle the average current of 50A and the loss of 80W (both diodes). Lower diode STTH200L06TV1 (doube diode with both internal diodes in parallel) must in worst case handle the average current of 100A and the loss of nearly 120W. Maximum total loss of the secondary rectifier is 140W. The heatsink must be able to handle it. To the thermal resistance you must include the junction-case Rth, case-sink Rth and sink-ambient Rth. Diodes don't have insulation, the cathode is connected to the heatsink. Output choke L1 is therefore connected in the negative rail. It is advantageous because in this case there's no high-frequency voltage on the heatsink. You can use another type of diodes, for example the parallel combination of a sufficient number of the most accessible diodes, such as MUR1560 or FES16JT. Note that the maximum current of the lower diode is twice the current of the upper diode. Calculation of the loss of IGBTs is more complicated because in addition to conductive losses there are switching losses. Loss of each transistor is up to about 50W. It is necessary to cool also the reset diodes UG5JT and a mains rectifier bridge. Power loss of the reset diodes depends on the construction of Tr1 (inductance, stray inductance), but is much smaller than the loss of IGBTs. Rectifier bridge has a power loss of up to about 30W. UG5JT and rectifying bridge and placed on the same heatsink as IGBTs. UG5JT diodes also can be replaced with MUR1560 or FES16JT. During construction it is also necessary to decide the maximum loading factor of the welding inverter, and accordingly select size of heatsinks, winding gauges and so on. It is also good to add a fan.
Switching transformer Tr1 is wound on two ferrite EE cores, each with a central column section 16x20mm. The total cross section is therefore 16x40mm, the core must have no air gap. 20 turns primary winding is wound using 14 wires of a 0.5 mm diamater. It would be better to use 20 wires, but they didn't fit to my core. Secondary winding has 6 turns of copper strip 36x0.5mm. Forward drive transformer Tr2 is made with an emphasis on low stray inductance. It is trifillary wound, using three twisted insulated wires of 0.3 mm diameter, and all the windings have 14 turns. Core is made of material H22, middle column has a diameter of 16mm, with no gaps. Current transformer Tr3 is made from EMI suppression chokes on the toroidal core. The original winding with 75 turns of 0.4 mm wire works as a secondary. Primary has 1 turn. Polarity of all the transformer windings must be kept (see dots in schematic)! L1 has ferrite EE core, middle column has cross section 16x20mm. It has 11 turns of a copper strip 36x0.5mm and the total air gap in the magnetic circuit is 10mm. Its inductance is cca 12uH.
Auxiliary switched supply, including Tr4, is described in more detail here. Simplest welding inverter on Pic 1 has no voltage feedback. Voltage feedback does not affect the welding, but affects the consumption and heat losses in idle state. Without the output voltage feedback there is quite high output voltage (approximately 100V) and PWM controller ia running at max duty cycle, thereby increasing power consumption and heating of components. Therefore, it is better to implement the voltage feedback. You can inspire on Pic 2. The feedback can be connected directly because the controll circuit is isolated from mains. The reference voltage is 2.5V. Select the R2 to set the open circuit voltage. You can find useful info in datasheet of UC3842, 3843, 3844, 3845 or in another datasheet. Inspiration for modifications you can also find in 3-60V 40A supply.
Interesting links from which I drew:
http://nexor.electrik.org/svarka/barmaley/kosoy/shema.gif and a little modified: http://nexor.electrik.org/svarka/barmaley/kosoy1/shema.gif