Isolated 3.3V to 5V 100mA design

[hrc]

Making a boost circuit is easy....making it 1500V isolated is a little more tricky. Anyone have a favorite circuit they have used?

 

[VEBill]

Motor, insulated shaft, generator. ;-)

 

[OperaHouse]

Slightly on this subject. I have used PCB mount common mode line voltage chokes for transformers that operate well in the 40-60K frequency. They have great UL documented isolation. Most of the transformers designed for 3.3V won't have that isolation.

 

[sdmays]

You could always design an inverter using only enough secondary turns on the output for 5V. Zetex has some App notes showing how to design an inverter using their medium power BJT's.

Have you considered using PCB mounted isolated DC-DC power supplies? The isolation of such an animal (if you can find one for that DC voltage in and DC voltage out) is pretty high.

Taking a quick second look at OperaHouse's post reminded me of a technical paper I downloaded the other day: "fetdrvr-t.pdf" You can do a quick Google search for it and you will find "AN ISOLATED MOSFET GATE DRIVER". The paper shows several similar topologies for creating an isolated power source at a high frequency. For the high voltage isolation, you will probably need to separate the primary from the secondary with dielectric tape.

 

[IRstuff]

Why not buy one off the shelf? Pico and others make this type of power supply

TTFN

 

[sdmays]

Yeah, like my second paragraph...

Martek Power,
C&D Technologies,
International Power Sources,

Unfortunately, my experience with one of these isolated DC-DC converter solutions resulted in about 100mV of noise riding on my 5V rail.

 

[hrc]

I'd go for an off the shelf solution right now, but the SIZE is an issue. I'm really tight on board space. Dropping a design down on a board I can usually make it very compact.

But...maybe I will look around. Most DC/DC are not boost but bucks as well

 

[itsmoked]

I'm not even clear on what you want... You say 3.3V to 5V to 1,500V what the dickens are you discribing?

 

[IRstuff]

And how much room are you claiming to have?

TTFN

 

[sdmays]

ItSmoked,

"Perhaps the TV was up too high and you miss interpreted" what hrc said the circuit wants/needs .

hrc wants/needs a circuit that will give 5V @ 100mA out from a 3.3V input source and that will have up to 1500V isolation between input and output.

 

[hrc]

Correct. I actually found something, C&D Technologies NTE0305M, but....not sure if it will pass UL tests. Its rated at 1000VDC isolation. Doesn't UL have to be 1500V? Or is it 1500VAC?

 

[sdmays]

1000V, 1500V, 1500VAC,...really depends on the UL classification you are shooting for. I've seen UL hipot voltages from 500VAC to over 2000VAC. For voltages greater than 50V, the hipot voltage is typically 1000VAC + 2 X Rated Voltage (again, depending on the Standard to which you are trying to comply).

 

[itsmoked]

Thanks sdmays.

hrc; Find a DC-DC controller chip Maxim et. al. that shows you how to use a transformer on the output. Generally this scheme is used to provide multiple output voltages using only one controller.

Then you use one of the small enclosed torriodal transformer blanks made by Amadon. Essentially they are a fully enclosed bobben, high frequency transformer, that is held together with a single nylon screw thru the center. When the screw is removed the housing splits in half down the center and out comes a little white bobben. The bobben has a divider in the center to keep the windings separate. When you wind one side of the bobben with the primary and the second side of the bobben, (on the other side of the little divider), with your secondary, they have a very high isolation, easily 1500 V.

You can you use this in production or you can take the functioning prototype transformer that you created to many different transformer companies. They'll measure its characteristics and can quote you production on a transformer that will perform identically, but may be an entirely different shape, if you would prefer.

http://www.amidoncorp.com/aai_specifications.htm

This is a complex fedback isolated method generally you wouldn't need isolated feedback you just generate two outputs out of the transformer and monitor the one on the regulator side.

And another option:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/175

or see the top of figure 2 in here:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/661

 

[itsmoked]

My post picture didn't make it but on that Amidon link you want to look at the "Ferrites for Switch Mode Power Supplies" Pot Cores and E-Cores.

 

[hrc]

well..i stay away from Maxim at all costs...delivery issues. The EAU is like 1000 so not sure a custom transformer makes sense. But I will look into it..thanks!

 

[itsmoked]

I can totally understand your Maxim bias I have been burnt by them many, many, times. However they used to have no fab of their own... Which ment if business in general started jumpin they would only get the left over capacity. I believe they got their own fab a while ago and have been MUCH better with deliveries.

 

[Comcokid]

Look at the Linear Technology LT1424-5. Example circuit shows a 5V to 5V, 400 mA isolated supply, but the chip will operate from 3V. The LT1425 and LT3439 data sheet has similar examples. You will need to build your transformer for the isolation voltage you need. Type "isolated" into their search and you will get over 100 hits.

 

[hrc]

Well, I've decided on a solution using the C&D Tech NKE0305D. Small packege, maybe a lil pricy but does what I need. I can do a discreet solution for about $5 and this is about $7 so the tradeoff is in the board layout is simple, no issues of isolation for NRTL cert and if it fails...its one part not one of 5....

all for all...not a bad trade off I think

Once again, thanks very much for all the responses.