An inverter welding machine is a constant-current power supply that must strike an arc, hold it through a short circuit, and survive being dropped on a workshop floor. The transformer and output choke carry the full welding current, so their design sets the duty cycle, the arc stability and the weight of the machine.
1. Electrical envelope of a typical inverter welder
| Parameter | MMA / stick (120–200 A) | MIG/MAG (200–350 A) |
|---|---|---|
| Input | 230 V single phase or 400 V three phase | 400 V three phase |
| Inverter frequency | 40–100 kHz | 20–60 kHz |
| Output current | 120–200 A | 200–350 A |
| Duty cycle at rated current | 35–60% | 60–100% |
| Open-circuit voltage | 60–80 V | 50–70 V |
At 100 kHz and 200 A output the transformer handles roughly 5–6 kW in a package that must weigh under 200 g, which is why ferrite with a low loss density at high frequency — not laminated steel — is the standard choice.
2. Transformer design sequence
- Turns ratio from the worst case: minimum input voltage, maximum duty cycle, and the output rectifier drop. A common mistake is computing the ratio at nominal mains; the machine then loses regulation at −15% line.
- Core selection from area product: the ferrite cross-section and window must carry the flux swing at the inverter frequency, not the 50 Hz flux density. Use a flux swing well below saturation because the short-circuit transient pushes it further.
- Winding layout: primary often foil or multiple parallel strands; secondary is a small number of turns at very high current, so copper section and termination dominate the design. Half-turn structures allow a one-turn secondary.
- Leakage inductance as a feature: a controlled leakage inductance limits the short-circuit current rise and stabilises the arc. Specify a range, not just a maximum — too low and the di/dt stresses the rectifier; too high and the arc sputters.
- Thermal: duty cycle is not continuous. Design for the RMS current over a 10-minute duty cycle, with the hot spot limited to the insulation class.
3. Output inductor: powder core territory
The output choke carries 100–350 A DC with a large ripple. A gapped ferrite core would saturate or need an impractically large gap; distributed-gap powder cores (iron powder, sendust, high flux) are the practical answer because they hold inductance under heavy bias and fail softly rather than abruptly.
Practical selection points: choose the permeability that reaches the target inductance with a workable turn count, verify the inductance at the actual DC bias rather than the zero-bias value, and check the AC loss at the ripple frequency — at 350 A and 5 kHz ripple with a 60 µ core, the flux swing is small but the core volume is large.
4. Arc stability and dynamic response
Welding is a fast transient load: a short circuit is a step from 70 V to under 10 V. The output inductor’s value and the current-loop bandwidth together decide whether the machine stutters. Too much inductance gives a sluggish arc; too little gives a harsh one. Most designs land at 50–150 µH for 200 A machines, tuned on the bench rather than calculated alone.
5. Input side: PFC and EMI
Single-phase 230 V machines increasingly include active PFC to meet IEC 61000-3-2 harmonic limits, which adds a boost inductor to the BOM. The EMI filter needs a common mode choke rated for the input current plus the inrush of the bulk capacitor. Both are powder or nanocrystalline core applications.
6. Ruggedisation
Workshop environments mean vibration, dust, humidity and occasional welding spatter through the vents. Varnish impregnation is the minimum; potting is common for the transformer. Terminations must survive repeated thermal cycling, and the mounting must not rely on a single screw.
Shaanxi SiRui Technology supplies Magnetic Core series ferrite cores for the welding transformer and iron powder, sendust and high flux cores for the output choke, together with wound assemblies built to the machine’s duty cycle.
Send your output current, duty cycle and enclosure size to sales@sirui-tech.com.