DC to DC isolated converter DIYable ?

[savril]

I was planning to do a USB isolator with onboard DC/DC isolated converter to power prototypes.
Using a ready made DC/DC module seem straightforward but I would prefer to make it because I could not find one that was not oversized and using several in parallel is ugly.

For the DC/DC converted, it would have these specs :

• isolated with at 2,5kV isolation
• 5V to 5V conversion
• a little less than 500 mA load (USB specs)
• current limit (but that's something to add separately I think)

I flew over some docs and I came to the conclusion that it won't be easy (maths and LTSpice stuff). The math stuff don't scare me, so it's still doable (it will take time tough but lots of things to learn).
But what I can't decide on the type of isolated design to do in this list : http://en.wikipedia.org/wiki/DC-to-DC_converter#Magnetic

Flyback seem to be the more used design for low power applications. But I'm concerned by the current that may consumed by this type of solution. The current limit for USB is 500 mA in the specs and I would like to be as efficient as possible to get as closer as possible to the USB spec at the output.

What is the more efficient design ?

Another problem I get is that I have a hard time finding an applicable transformer. I've not made any calculation so far, but it seem that I would need these spec for the transformer :

• 1:1.x ratio
• 2,5kV isolation
• between 500mA and 1A current

I was unable to find a single one at RS.
At Farnell, I found some in the Pulse Transformer section ( http://uk.farnell.com/pulse-transformers ), is it the correct category to look at ? Some are named "gate driver transformer" are they applicable ?

As I have read, inductance play a role in DC/DC conversion. There don't seem to be much choice.

Is is it so difficult to find a correct DC/DC transformer and could that lead to a dead end ?

Last question : is a DC/DC converter doable for a hobbyist ? (Well should have asked this first :-)

[PSR B1257]

What about this one http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=mee3s0505sc

Or this one, with 3kV insulation voltage http://www.digikey.com/product-search/en?KeyWords=mev3s0505sc&WT.z_header=search_go

Building such a converter is not that simple. Especially in terms of size and efficiency you will have a hard time to even getting close to the specs of these units.

[ez24]

What about this one http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=mee3s0505sc

could 2 of these be used (and one output reversed) to get an isolated + and -  5 volts ??

[PSR B1257]

Sure, the outputs are floating, hence you can connect them in series as you like.

But that's not necessary since there are also converters with a bipolar output.

[savril]

My plan at first was to make a USB 2.0 Isolator with a ADUM4160 as its wildly used and I wanted to match its 5kV isolation.
So, when I searched first for DC/DC converter the first to meat the spec (5kV and 500 mA) was at 40€. So I though I would go DIY. But when I did not found any matching transformer, I decided to lower the isolation voltage requirement... but I didn't check back on DC/DC modules.

When I took a look at what you proposed, it was much cheaper than what I found previously.
That made me change my mind. 5kV voltage between the 2 parts on the circuit aren't likely to be present on normal abuse. These voltages are more likely to be caused by ESD. And there's other methods to protect from ESD like TVS diodes which I could use.
I've spent the last days to reads things on ESD protection and I'm now ready to do my design.

Thanks for your help.

[BurtyB]

If you're dropping your requirements down to 2.5kV you also might want to look at the ADUM3160 if you haven't already to save a $ or so.

[poorchava]

Off the shelf transformers are very hard to find other than for very well defined applications (eg. Ethernet).

Transformer design and construction is in almost every case the result of desired circuit parameters,  not the other way around.

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