This is another part of an ongoing larger project. The final product is a dual 0-80V 0-10A programmable power source for driving electromagnets. The buck converter preregulator has been discussed quite extensively in this thread:
https://www.eevblog.com/forum/projects/buck-converter-pre-regulator/Originally, this was to be powered from an unregulated half-bridge converter. I'm pivoting a little to make this board more useful and more interesting to design. It is now being configured as a regulated current-mode push-pull converter.
Ultimately I decided that a push-pull converter made the most sense from a cost, complexity and size perspective. While it does require an additional winding on the transformer, it eliminates the need for bootstrap gate drivers. A half bridge would require a high-side current sensor, while a full-bridge would require an extra set of switching devices. The EMI filter and bridge rectifier will be located on a separate board.
I'm using more or less the equivalent of a FT-290-77, with a core area of right around 2 cm^2. With an input voltage of 175 V with a 75 kHz switching frequency and B_max = 1400 gauss, I should be okay with 20 turns. I could almost certainly get away with a smaller core (wind more turns, etc) and have acceptable losses, but I happen to have these cores on hand, and it should give me flexibility to use these boards for other purposes.
L2 was calculated to be about 60 uH for a 66% ripple fraction at 75 kHz and 80V output with a 1:1 transformer. Some of these values may change, but at this time I'm considering a T157-2 core wound with about 60 turns. I need to do some more calculations before I feel comfortable with this solution, however.
On the secondary side, I'm using a TL431 and PC817 for the error amplifier and isolator. I've got a few different feedback divider options for increased flexibility. Since it only added three parts, I included an option to use this board alone as a preregulator. Not sure if I'll use it or not, but it could be a useful option to have.
Because this should be somewhat adjustable, and because I already have control power supplies in this system, the primary and secondary side controls are powered externally.