Linear Regulator using FETs for pass device

[BrianG]

OK, let me start by saying that I know this is NOT the forum for this type of question, but I hope there is enough brain power here to help me out.

Here is an excerpt from a thread I started a while back about a couple of transformers I got:

Originally Posted by BrianG

The second transformer (1,000w) was a little larger with a little bigger windings. The output of this one was 15.9v AC unloaded with no center tap. With a 0.75 ohm load, the output dropped to 15.1v AC, which equates to 20.1A. Calculated output impedance is 0.04 ohms. With a 0.46 ohm load, the output was 14.6v (calculated 31.8A). Even though this one is capable of higher power and has lower output Z, the output V starts out too low (unloaded) to be useable for a PS at heavy load unless I make it a strict 12v PS. Then, I could use this to its full potential up to 65-70A.

Since then, I got another one of these transformers. The really low output impedance is good for a fairly stable voltage output. I want to build a regulator out of these, but I have two issues:

1: Loaded to ~30A, the AC output drops to 14.6v AC. So, if the regulator is built to supply ~13vDC, I think that the output will drop too far for the pass transistors to work, even after rectification and a lot of filtering. So, to minimize the drop-out effect, I need to set the PS to regulate down to ~12v. So, my choices are: be happy with ~12v, or find a solution with a lower drop-out value. If I can find a low drop-out solution, I can increase the load to get more current out of it.

2: Coming up with a pass transistor solution to pass at least 40A of current AND have the power dissipation value is problematic. Paralleling bipolar transistors is a no-no unless you do all kinds of magic to prevent individual thermal runaway.

So, I'd like to use MOSFETs. Their low rdson value would make for a low drop-out regulator. And they are easily paralleled (not to mention paralleling would reduce the total rdson). However, I have no idea on how to bias them for use in linear mode. Can anyone help get me pointed in the right direction? Artur maybe?