SMPS Welding Inverter Circuit

If you are looking for an option to replace conventional welding transformer, the welding inverter is the best choice. Welding inverter is handy and runs on DC current. The current control is maintained through potentiometer.

 Written and Submitted By: Dhrubajyoti Biswas

When developing a welding inverter, I applied forward inverter with two switches topology. Here the input line voltage traverses through the EMI filter further smoothing with big capacity. However, as the switch-on current pulse tends to be high there needs the presence of softstart circuit. As the switching is ON and the primary filter capacitors charges via resistors, the power is further zeroed by turning the switching ON the relay. The moment the power is switched, the IGBT transistors gets used and are further applied through TR2 forward gate drive transformer followed by shaping the circuit with the help of BC327. The control circuit used in this scenario is UC3844, which is very much similar to UC3842 with pulse-width limit to 50% and working frequency to 42 kHz. The control circuit draws the power from an auxiliary supply of 17V. Due to high currents, the current feedback uses Tr3 transformer. The voltage of 4R7/2W sensing register is more or less equal to the current output. The output current can be further controlled by P1 potentiometer. Its function is to measure the feedback’s threshold point and the threshold voltage of pin 3 of UC3844 stands at 1V.

One important aspect of power semiconductor is that it needs cooling and most of the heat generated is pushed out in output diodes. The upper diode which consists of 2x DSEI60-06A should have the capacity to handle the current at an average of 50A and loss till 80W. The lower diode i.e. STTH200L06TV1 also should the average current of 100A and loss till 120W. On the other hand, the total max loss of the secondary rectifier is 140W. The L1 output choke is further connected with the negative rail. This is a good scenario since the heat sink is barred from hi-frequency voltage. Another option is to use FES16JT or MUR1560 diodes. However, it is important to consider that the max current flow of the lower diode is twice the current to that of the upper diode. As a matter of fact, calculating IGBT’s loss is a complex procedure since besides conductive losses switching loss is another factor too. Also each transistor loses around 50W. The rectifier bridge also loses power till 30W and it is placed on the same heat sink as IGBT along with UG5JT reset diode. There is also the option to replace UG5JT with FES16JT or MUR1560. The loss of power of the reset diodes is also dependent upon the way Tr1 is constructed, albeit the loss is lesser compared to the loss of power from IGBT. The rectifier bridge also accounts to power loss of around 30W. Furthermore when preparing the system it is important to remember to scale the maximum loading factor of the welding inverter. Based upon the measurement, you can then be ready to select the correct size of the winding gauge, heat sink etc. Another good option is to add a fan as this will keep a check on the heat.

The Tr1 switching transformer is wounded two ferrite EE core and they both have the central column section of 16x20mm. Therefore, the total cross section calculates to 16x40mm. Care should be taken to leave no air gap in the in the core area. A good option would be to use 20 turns primary winding by wounding it with 14 wires of 0.5mm diameter. The secondary winding on the other hand has six copper strip of 36x0.55mm. The forward drive transformer Tr2, which is designed on low stray inductance, follows trifillar winding procedure with three twisted insulated wire of 0.3 mm diameter and the windings of 14 turns. The core section is made of H22 with the middle column diameter of 16mm and leaving no gaps. The current transformer Tr3 is made of EMI suppression chokes. While the primary has only 1 turn, the secondary is wounded with 75 turns of 0.4 mm wire. One important issue is to keep the polarity of the windings. While L1 has ferrite EE core, the middle column has the cross section of 16x20mm having 11 turns of copper strip of 36x0.5mm. Furthermore, the total air gap and the magnetic circuit are set to 10mm and its inductance is 12uH cca.

The voltage feedback does not really hamper the welding, but it surely affects the consumption and the loss of heat when in idle mode. The use of voltage feedback is quite important because of high voltage of around 1000V. Moreover, the PWM controller is operating at max duty cycle, which increases the power consumption rate and also the heating components.

The 310V DC could be extracted from the grid mains 220V after rectification via a bridge network and filtration through a couple of 10uF/400V electrolytic caapcitors.

The 12V supply could be obtained from a ready-made 12V adapter unit or built at home with the help of the info provided here: