I understand that getting more out than what goes in is not going to happen. I am not looking for magic here. But if I have a 100 amp 12 volt battery, I think I should be able to get some serious current for the boosted DC in the end, without going thru AC in the process.
This really depends on what
your definition of AC is.
For most non-techie people, they use the term 'AC' to refer to mains voltage, as that is always AC, a sine wave, with a frequency of 50/60Hz, and RMS voltage of 240/115V depending on where you are in the world.
Technically speaking, if you were to turn say a flashlight on an off, you're producing AC from DC. Even if you turn it on and off once a week - that just means the frequency is very very low.
When we're talking about 'chopping DC', what we are doing is just that, turning a DC current on and off. You end up with a square wave which goes from 0v to your supply voltage,12v in your case. That square wave has a DC component (becuse its mean voltage isn't 0), AND an AC component (because it changes).
So, ALL forms of power converters 'go through AC' (note, this doesn't include linear regulators). - even many of those 'magic over unity' devices alternate current (even if they are just plain wrong).
This does not mean to say you must convert your DC to 'mains AC' at 60Hz. On the contrary, many switch mode power supplies work at a much higher frequency because of efficiency and component size. Exactly 'how' you use a transformer is dependant on the topology used, be it flyback, forward, push pull, boost etc.. - all of which will switch the power supply current into the transformer at a certain frquency/duty cycle, but vary in the way they use magnetic current inside the transformer to transfer energy from its primary winding, to its secondary.
Flybacks and boost converters 'store' small packets of energy in the transformer core itself, as a magnetic field, then collect the energy when it colapses. This means the size of that packet of energy you can transfer is down to the core material and its size. More power through put = bigger transformer.
Forward and push pull transformers however, maintain an average magnetic flux of 0. They can use smaller cores but require more switches (MOSFET's generally) to control the flux in the transformer.
From what I gather, you are just after a 12V to 100V DC to DC converter at reasonably high power. 1-2kW. The principles are pretty much the same as making the same device but with 1W power, so I suggest you start with that. The differences are of course: current. Another tihng to tihnk about is, say you somehow manage to make a boost converter for 1kW.....as nothing is 100% efficienct there will be losses, and for most flyback/boost converters this is arond 80%. At 1kW, you're losing 200W of power in your circuit....effectively making it a 200W heater. At low power,this heat generation is small, and may require a heatsink, albeit a very small one. At 200W its going to be massive.
So, boost and flyback for this power level is a bad idea. I would look at forward and push-pull converters. Or simply google for 'power inverters', there are many >1kW schematics on the web that range in efficiency and complexity. Some use bipolar power transistors (not always a good choice) but many use power MOSFET's.
Regardless of whether you use an existing design, design your own, or re-invent the wheel whth something new and wacky...you're going to have to use magnetics (transformers) to accomplish the task. By far the easiest way to do what you want is to buy a ready made DC-AC inverter, that outputs 115V AC, rectify it with a full bridge, add output capacitors, and possibly drop the output voltage by 15V (at 20A thats not going to be easy). Or modify said power inverter to lower its output voltage (may not be that difficult depending on whether the inverter uses feedback controlled PWM or not).
As from scratch, look up PWM controllers such as the TL494 or SG3524. Their example circuits will give you an idea of what you're up against, plus, they're quite easy to use and will take the sting out of desinging from scratch.
Blueteeth