MC34063 and transformer question

[TrickyNekro]

Hallo again people,

I am trying to build a step up dc/dc converter for my project using the MC34063 part. In my project I need two power rails.
I need a ground referenced 15+ V rail and a floating 15+V rail. The rails will be used to power 12V regulators, which in turn
will be used to power gate drivers for MOSFETs. Instead of having two separate topologies, I thought it would be much more
practical to use one. So, instead of an inductor, I thought that it would be nice to use a transformer, with the primary as the
step up inductor and the secondary for the isolated rail. I need the output transformer output rails to be over 15V after
rectification so that I am sure that the regulators work properly, I don´t really need a precision circuit.

Since the MC34063 works with a comparator, it will be skipping pulses. The problem with that is that the voltage on the isolated
side, might and probably will, drop, something that I really don´t want. So I thought that if the non-isolated side has more load
than the isolated side, it will probably work. The question is... Is my thought correct? If not, want should I do?
From some research, I found that LM25017 would be great for producing both an isolated and non-isolated rail, BUT it´s a step
down converter, with that being said, it´s pretty much useless, since I´ll be powering my system will 12V and I need more than 15V
at the output, and I really don´t want more circuitly (something like a step-up to make my originally 12V rail 24V volts for it to work).
Still, while reading through the datasheet, I understood that this concept can work.

And then my question about the transformer. I got some samples from Coilcraft for the transformers. The provide with the DCR value
and the maximum power rating. I dug a little bit around the site of Coilcraft and found in their F.A.Q. that the Irms can be found from
this formula: Power rating = Irms² x DCR. Am I right about this? I mean that I got some JA4650-BL. They have a power rating of 2W
and a DCR of ~0.122 Ohm. That means that the Irms = ~4A, right? I just want to be sure that the transformer is good for the task.
I don´t think that I will be drawing more than an average of 150mA - 200mA per rail.

Anyhow, these are my questions. I hope that I was able to clarify them as good as I can, but if in need of more details, I´ll be around 

Thanks for your time and patience, in advance!

Best Regards, Lefteris
Greece

[Bored@Work]

Stabilize the unregulated output with a bigger cap, establish a minimum load on the regulated output.

In general I would suggest to use e a symmetric design. I.e. flyback topology with two separate secondary windings, one regulated.

[TrickyNekro]

I can do the later. I have two windings for the primary and two for the secondary. I can use the primary windings in series and then
use each seperate winding for each supply. But still again I run into the problem that I need a higher power source. In that configuration
I can use the MAX5075.

Anyhow, I will draw the two possible schematics and upload them, so that people can understand easier what I want exactly. Plus I will
try the circuit with the MC34063 tonight and I will upload my results. :-)

I hope that it will work, probably it will. The funny thing is that I just calculated the current needs of each rail and if I am correct the average
current shouldn´t be more than 20mA per rail, which gives me also other opportunities but still I want to try this one. I just want to get away
from bootstrap. When you deal with until 15V supplies bootstrap is a nice method but when you want to go over than, you don´t get to have
many solutions.

This project serves more educational purposes but I could end up with a very good H-bridge design... so yeah :-P
If it is good at the end, I will make it open hardware ;-)

Thanks for your post,
Best Regards, Lefteris
Greece