Galvanic isolation DC/DC

[johnny_cash]

Hello,

I need to have a galvanic isolation between 2 supply lines. My input 'll be 5/40v DC, my output is 5v DC, but GNDs need to be different. The output will draw max 500mA.

My first idea is to use a PCB transformer : http://www.murata-ps.com/data/magnetics/kmp_1605.pdf?sel=magnetics/kmp_1605.pdf then a voltage regulator BUT:
- is such a transforme able to work from DC/DC ? in my thought, transformers were only usable in AC/AC

I plan to use something similar to LTM8048: aka an LDO with isolation, is this a pretty good reference ?

Thank you !

 

[Harald Kapp]

This transformer alone will not work. It requires AC.
>ou want a DC/DC convert module similar to those here. But you'll have to dig a bit to find one suitable for 5V...40V input.
The LTM8048 comes near, but does not quite fit the bill with Vinmax=32V

 

[BobK]

Most DC to DC converters are not isolated. You would need to find one that is, which will indeed use a transformer.

Bob

 

[Harald Kapp]

Traco, Recom, TDK to name only a few manufacturers all have isolated converters in their portfolio.

 

[johnny_cash]

EXCELLENT Harald Kapp ! i found 2 suitables. Kudos !

It is forbidden to chain DC/DCs ? Thank you !!!

Notice 1st regulator is not isolated. Then i could use Vout1+ and Vout1- as VDD and VSS, right ?

 

[(*steve*)]

It is forbidden to chain DC/DCs ?

No, but remember that your losses multiply...

If all the power supplies are 85% efficient, then for each watt out, you need 1/0.85 W in to a single stage DC-DC, but (1/0.85)*(1/0.85) if you have 2 stages. (That's 1.18 vs. 1.38)

 

[johnny_cash]

Hello, i finally choose this 5VDC to 3.3VDC isolated converter: MEE1S0503C (DS: http://www.murata-ps.com/data/power/ncl/kdc_mee1.pdf )

Now i have a trouble:

1) no application notes. Does it mean this DC/DC is not requiring side components (such as Vout 10uF capa)
2) there is a "minimum load" chapter in the DS (top of page 5). Is a 74AHCT126 a correct load or it is totally not suitable ?

Thank you !

 

[(*steve*)]

Well, a single 74AHCT126 is unlikely to provide a load large enough to satisfy the minimum load, so the output voltage could rise to just over 4V.

Considering this device is specced for a power supply between 2V and 5V, it's not a worry. Is there anything else that demands a voltage lower than 4V?

 

[johnny_cash]

no, thank you for the answer. I simply want to display this line logical status (with resistor + led 7mA) without perturbing the input.