The stock wiring of the F&P produces a voltage much to high to be used with a buck converter to convert down to 12 volts for battery charging.
One thing you may want to consider here, is what's called a "forward converter". Essentially, it's a high frequency transformer in a push-pull configuration on the primary. There are advantages and disadvantages to going this route, but you might gain some efficiency at the power levels you're working with by going this route. One of the harder things to overcome (at least in my experience messing with them) is the kick-back from the transformer into the MOSFETs. It can be difficult to tame, and results in a lot of magic smoke if this little detail's requirements aren't met.
I was getting better results using lower switching frequencies. I will redesign the circuit to operate at around 10kHz or lower. After reviewing component cost and availability I am pursuing 5 amp power module configuration with single timing controller board, so for a 15 amp output would require one controller board and three power modules. I am not sure if it will work.
You can parallel buck converters, just as you can any other kind of power supply, given that the load is balanced between them. This can be tricky to do from a DIY perspective, simply because tolerances are generally a little bit "sloppier". Just an inherent issue in the "manufacturing" process (doing it by hand).
There are a few methods used to counteract this. One, is to synchronize the converters in a master/slave configuration, where there is one controlling chip that does all the monitoring/feedback/duty cycle operations, and the slaves just do the dirty work. I have a power supply that is set up in such a way here at the house, it has 5 of 6 modules installed, and is capable of 100A @ 13.8V. Each module carries 20A, and the outputs are all tied together on a buss. There is a single conductor that jumps from module to module to provide the switching control for all of them.
Another method used where the DC waveform needs to be smoother but increasing switching frequency isn't practical, is to drive the modules in a "ring". Each module switches n degrees out of phase with the next, evenly spaced out with each other. The result is a higher effective switching frequency, with a lower actual frequency, each converter carrying it's portion of the load. This is probably the most difficult to implement, as you also need the special chip that controls the phases. They are also typically synchronous in nature (MOSFET on both the high and low side, rather than a MOSFET and a Schottky diode). Haven't built one myself, too complex for my blood.
What I need to find out is as more power modules are connected in parallel can the switching frequency remain constant, or does the switching frequency have to increase because each power module will have its own inductor and connecting the modules in parallel will reduce the inductance and cause the inductors to saturate. I am not sure and comments are welcome.
See the above. It really depends on the type of converter you intend to use. Directly applied to your current design, there's no need to increase or decrease the frequency "just because". There are pros and cons to going higher or lower, all depends on what your needs are. But remember that at 10KHz, you're going to be hearing a lot of whining coming from this thing, and if it's going to be where you can hear it, it will drive you nuts.
A typical tradeoff between performance/efficiency/noise is 25KHz. That's fast enough to keep it out of audible range, but slow enough that the losses in the transistors/diodes don't get excessive.
All in all, when you first posted about this, I didn't think that your input voltages would be so high, so a buck seemed pretty much on target. It would have been fine for example if you had a 48V turbine, and wanted to charge a 12V bank. Several hundred volts in (or even a couple) is an entirely different animal. At this point, even given your skill set (which is above many), I'd look into the forward converter from here. A properly built module can handle several hundred watts on it's own, and it would be trivial to parallel two or more modules together to handle multiples of that.
As boB loosely alluded to, I too would stay away from the "cap divider" - it's begging for instability, and will bring it's own set of challenges to the table.
Good luck.
Steve
