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Want to know why some welders for a given output are much bigger than others. Already found one of the big reasons is needing to run on 120V instead of 220V

How much space does a 220V AC input power supply save compared to a 120V one?

But one thing I could be overlooking is the switching frequency - the faster, the smaller the transformer & capacitors have to be. I know IGBTs have higher switching losses. I did an initial calculation of losses for 400V input (approximate voltage after rectifying 220VAC) @ 15amps (avg) & 100% duty cycle (unrealistic but for sake of maximizing conduction loss). I'm assuming a welding power supply uses the same design as a computer power supply like described here and here.

  1. IGBT (STGP15H60DF):

conduction loss = 15A * 1.6V = 24W

switching loss = 30KHz * (0.14 + 0.21 mJ) = 10.5W

  1. MOSFET (STW70N60DM2):

conduction loss = 42mOhm * 15A^2 = 9.5W

switching loss = Vds * Idrain * freq * (Qgate_drain + Qgate_source) / Igate (don't know what Igate is, but probably much lower than IGBT)

I can't say for sure, but it seems the MOSFET even switching @ 100KHz will be more efficient than the IGBT, and will allow a smaller transformer & capacitors. So why aren't welders using them?

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    \$\begingroup\$ Cost probably, make it as cheap as possible... if manufacturers all built to the highest quality possible at all times, society would be different. \$\endgroup\$ – Solar Mike May 14 '19 at 6:22
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It's silicon carbide mosfets that have made high amperage/high voltage circuits possible. Now they don't have to use (what I think is, I never tore one open ) transformers and capacitors. Carbide is near indestructible. I had a wielding class a few years back and the high amperage welders ranged from the size of the night stand to a large kitchen desk. Now they can fit the same functionality into something the size of a microwave oven, not only can they do that, but the machines have more switching functionality and they have so much room left over, they combine the same funtionality of 3 welders (tig,mig and stick) into one. It is increadible.

Silicon Carbide (SiC) Power MOSFET devices are rapidly being introduced in different applications due their advantages over their silicon (Si) counterparts. Higher switching frequency (ƒsw) coupled with planar magnetics opens new possibilities in terms of increasing the power density of converter, while also improving their efficiency and lowering costs. In this paper, SiC MOSFETS and planar magnetics are employed to demonstrate new possibilities in arc welding machines technology. Design details of high power density power source for a multi-process welding machine are presented. Test results for 3.9kW/150A and 200 kHz switching frequency working at rated load and under over load condition are included.

Source: https://www.researchgate.net/publication/300408761_Next_generation_arc_welding_machines_based_on_Silicon_Carbide_MOSFETS_and_high_frequency_planar_magnetics

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  • \$\begingroup\$ Given that SiC FETs are still rather new, are these welders already commercially available? And if so, are they any cheaper than existing welders? I've done research on SiC power devices, and those switching transients are so fast that it can get hard to deal with the high harmonic content. \$\endgroup\$ – Hearth May 14 '19 at 14:51

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