Dan Mills, asdf44, and BradtheRad, thank you very much for your thoughts!!
I will look into the LT3790. I am still getting my head around the fundamentals of buck and boost in circuitmaker, so it will be awhile before I can evaluate SMPS IC's.
I have also considered just using a big resistor. The problem is, it needs to vary in resistance I think, depending on the welding activity, and would require another battery to be used, to get to 24V. 12V welding is not really possible with SMAW by hand, apparently.
asdf44: yes, this is definitely a crude application. And what I have noticed is that almost universally, SMPS app notes and reference designs are all geared towards low emi, very high efficiency, 100% duty cycle (overall), and very small size, none of which are concerns in this application, which is what makes me think there is room here for it to work out.
In other words, it can be big, get very hot, only work for 20% overall duty cycle, waste 1000W (plenty of electrical energy to go around from lead acid + vehicle charging system, the waste heat is another issue of course)..it can have all those faults and be a success.
There are several vehicle powered welding systems, check out this link for a somewhat disorganized but thorough write up comparing a bunch:
https://www.parksoffroad.com/miscinstalls/welderreview/weldermain.htm
These designs require either lugging around either one (or TWO!!) additional lead acid batteries, besides the one in the vehicle, or adding an additional alternator to the vehicle.
They do not have any real current or voltage control. You even have to manually control the engine speed on the alternator version to get to the right voltage. As battery voltage drops, welding performance drops.
I think these designs are clumsy and very overpriced, and do not take advantage of SMPS technology.
In my opinion, a vehicle powered welder should connect to the existing battery in the vehicle, with no changes to the vehicle. The welder should draw whatever current it needs to output roughly 24V at around 100A. This should allow stick welding (SMAW) of 1/4" steel plate using a simple rod holder, with the chassis acting as return. Electrode positive is actually ideal for this particular welding type.
1/4" steel is about the biggest you would find in an emergency situation and would allow pretty much anything that needed a weld to be repaired.
Winches routinely draw 3kw+ at similar duty cycles, directly from the battery/charging system, at higher duty cycles, so I dont think its a question of the electrical power from the vehicle. If you look at the electrical source you basically have an unlimited amount of current available at 12V, backed up by a charging system that probably puts out another 100A. At 20% duty cycle there should be no issue drawing 200A for 30 to 60s.
Its just a matter of getting that power conditioned to the right voltage and amperage limit.
I am still not sure if buck or boost or buck/boost is more appropriate.
In order to use a buck-only, you will need an additional lead acid battery to get things up around 24V. This is possible. A smaller lead acid battery could provide the current at this duty cycle and not be too cumbersome to carry around as part of the welder. Circuitry could be added to charge it when welding is not taking place. The buck would act on either the second battery voltage of 12V or the entire series voltage of 24V, whatever is easiest.
A boost-only system is the nicest because it requires no additional battery which can lose charge or become damaged from storage. It would likely have the greatest currents though, to boost 12V to 24V.
Buck boost may also be the most appropriate, but Im not sure. It may be the worst option if it requires the most complex SMPS design as well as an additional battery.
I did some stick welding yesterday to see what it was like. I do lots of TIG welding but have never done stick. I set the welder to 100A. I measured the open circuit voltage and it was 65V. The welding went VERY easy. However its extremely common for the electrode to stick to the plate, and then you essentially have a short circuit composed of the cables, ground clamp, rod holder, and rod. This is where you would want welding current limited, otherwise who knows how much current would be drawn from a lead acid battery.
You also need welding current reduced during welding. If its too high you can blow through thinner metals. You can also damage the batteries by drawing too much current, not to mention increasing the design requirement for the SMPS to pass all that extra current that isnt even needed. So I say minimum current needed is the goal here.
Something I have not done yet is weld with 24V being the maximum voltage. I am not sure how easy or hard that is. I cant sent my welder to a fixed output voltage.
BradtheRad, thank you for the example.
Regarding finding components rated at such high currents, arent most of those component ratings assuming air-cooling and 100% duty cycle? If I have some kind of liquid/oil bath possible with a pump, operate at 20% duty cycle, and allow for things to get very hot, would that open up some more doors as far as component availability?
Here are the welds I did, the joint between the two pieces. All the other stuff wasnt part of it. The question here is what voltage the welder was maintaining. Ill have to do it again and get my meter connected to the welder output terminals.
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What I think the right direction here is a real dirty, crude design, but careful design, that just slams electrons to the right voltage and is probably immersed in coolant of some kind. Not pretty, not EMI friendly, gets hot, can only run for a short time, etc...but reliable and wont fail. Pretty much a complete deviation from almost every typical SMPS design.
One more option I didnt mention is possible using a 2kw+ transformer and push-pull the vehicle 12V through it to get 24VAC out. That could then be rectified to 24VDC, and the current limit would come from transformer saturation, just like every buzz-box AC welder. Im not sure if 24VAC could be used directly, eliminating the rectification.
This may be an attractive option because you really could submerge the transformer in oil for cooling. And, I think transformers of that power are realistic for the application. My harbor freight MIG welder weighs about 25 lbs I think and consumes about 2.4kw. Thats about right for this. So instead of a SMPS design it would be a very simple push-pull bridge and a transformer design.