MC34063 and transformer question - Page 1
Forum section: electronics / projects

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: November 04, 2013, 04:31:59 pm
content: 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 turnwill be used to power gate drivers for MOSFETs. Instead of having two separate topologies, I thought it would be much morepractical to use one. So, instead of an inductor, I thought that it would be nice to use a transformer, with the primary as thestep up inductor and the secondary for the isolated rail. I need the output transformer output rails to be over 15V afterrectification 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 isolatedside, might and probably will, drop, something that I really don´t want. So I thought that if the non-isolated side has more loadthan 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 stepdown converter, with that being said, it´s pretty much useless, since I´ll be powering my system will 12V and I need more than 15Vat 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 valueand 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 fromthis 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 2Wand 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 aroundThanks for your time and patience, in advance!Best Regards, LefterisGreece

author: Bored@Work
author_rank: Super Contributor
timestamp: November 04, 2013, 08:51:42 pm
content: 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.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: November 05, 2013, 01:18:02 pm
content: 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 thenuse each seperate winding for each supply. But still again I run into the problem that I need a higher power source. In that configurationI 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 willtry 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 averagecurrent 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 awayfrom 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 havemany solutions.This project serves more educational purposes but I could end up with a very good H-bridge design... so yeah :-PIf it is good at the end, I will make it open hardware ;-)Thanks for your post,Best Regards, LefterisGreece