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Very wide range isolated DC-DC converter

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forrestc:
A while back I asked a similar question (same project), and I don't think I explained what I was hoping to find very well.   I don't seem to be able to find the old topic to re-open, so I'm starting a new one...

I have a design which is used in an environement where there are multiple, redundant power sources, all ground referenced, and each can be either + or - in respect to ground.  I don't really have control over the power sources - I have to live with whatever is available.   There might be a +12VDC source and a -48VDC source as an example.   I need to pull power from whatever happens to be available and handle situations where one power supply dissapears during operation.  In most cases I'm needing 3.3V at about 500ma.

Currently my power supply architecture looks like this:



This turns out to be rather expensive, as the negative to positive supply costs about $10 in parts.   I'd like to figure out some way to make this less expensive.

In an ideal world, I'd find something inexpensive which would allow an architecture like:



Unfortunately that wide of power supply range doesn't commonly exist.   I have found exactly one part which might do this, but it seems to be unobtainable at least through distribution.

I've also tried to come up with some sort of low cost switching method which would allow me to just use a single 10-60Vin 3.3V out isolated supply which switches between the positive and negative rails, but so far I haven't been able to come up with something I felt was reliable and was relatively low cost.    A schematic of what I'm trying to describe is shown:



I know I could probably rig some sort of relay up to do the rail selection, but I hate electromechanical devices (they fail).  I haven't seem to be able to come up with an electronic circuit that I'm happy with - specifically because of apparent failure modes.

Right now, I'm primarily looking for options that I might not have considered because I didn't know about them..   A couple additional notes:

1) I show 2 voltage inputs, I can have up to 3 or 4 at times.

2) I'm ok with an output of like +12V (or other similar intermediate voltage), as regulators from 12V down to 3.3V are cheap and plentiful.  In fact, this might be preferable to 3.3V out directly.

aheid:
Now, I'm just a noob so this might be dumb, in which case I apologize.

From what I can see the NCP1031[1] can handle 8V up to 200V. Now I didn't dig into it to see if you could realistically make it work over such a large range, so there's that...


--- Quote ---The NCP103x can be configured in any single-ended topology such as forward or flyback. [...] The internal error amplifier allows the NCP103x family to be easily configured for secondary or primary side regulation operation in isolated and non-isolated configurations.
--- End quote ---

Maybe it could work for your preferred solution?

edit: forgot to add that at least Farnell stocks it, for about $1.5 a pop.

[1]: https://www.onsemi.com/PowerSolutions/product.do?id=NCP1031

SiliconWizard:
LTC3894: https://www.analog.com/en/products/ltc3894.html
Available at Digikey, Mouser, Arrow (and maybe others).

Not sure I really got your "isolation" requirements here. There is a common ground...? Unless the ground on the right side is not the same as the ground on the left side of your schematics, in which case you should definitely use different ground symbols in order to avoid confusion.

That said, if you need isolation, you can then add a DC-DC isolated converter after the LTC3894 which would down-convert to say 5V. A 5V->3.3V @500mA is easy to find and relatively low cost, such as the Murata MEJ2S0503SC: https://power.murata.com/en/mej2s0503sc.html

If your min input voltage is 10V and it goes through diodes, you obviously can't get a 12V output with a buck converter design only.
But you can always use a 5V-input, 12V-output isolated DC-DC converter for instance after the LTC3894. The RECOM RKZE-0512S would be a good option, it's relatively cheap (around $5). It's a 2W output max; since you first required 3.3V @500mA max, which is 1.65W, this should be ok. Should you still want 3.3V, you can always add a 3.3V buck converter which should you get at least 500mA max output current. Limiting the isolated DC-DC converter to 2W looks like a good idea to keep costs down. Anything higher than that will cost significantly more (often > $10).

Of course you could always design a flyback converter from scratch instead of using the above topology. Would probably lead to a much lower unit cost in the end, but I would advise against that option unless you have already designed flyback converters. Specifying the transformer itself requires skills, and then you'd have to have it custom-made. Eek. You'd have to sell a lot of devices in the end to make that a profitable option compared to the above IMO.

T3sl4co1l:
A load dump protection circuit would probably be alright.  You're only drawing a couple watts output, so you can afford to drop a few more watts at the highest input voltage.  That greatly reduces the burden on the converter, so an 80V, or even 60 or 40V rated part can be used.

Also consider a controller instead of a regulator: you add an external transistor of whatever rating, and you supply a little power to the controller (say 5-40V of a few mA, easily done with a linear reg).  You can get very high ratings this way, probably limited more by switching loss and control stability than by voltage as such.

You could maybe also arrange some kind of mux to pick one good supply, in such a way that it stays redundant, cutting over seamlessly.  You can't use diodes to, for example, disable the -48V supply while +48 is active, but an active circuit could.  This would limit the converter's liability to only 60V, instead of 120V.

It's not immediately obvious what solution(s) would work here -- it's a nonlinear function, so transitioning from one state to another can be tricky.  If a digital solution is used (comparators and switches), it will be prone to oscillation under some conditions (even with hysteresis), and secondary problems like break-before-make and dead time and propagation delay.  A continuous (analog) solution would be nice, and is probably possible, it would just require some thought to synthesize.

Tim

Gribo:
If you can raise the lower limit to 18V, there are 4:1 Input range DC to DC converters from many sources (Traco, Meanwell, etc).I have managed to find an 8:1 Input range converter, however, it is 300$  :scared:

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