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Old 13th June 2006, 11:34 AM   #1
TroelsM is offline TroelsM  Denmark
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Default DC-DC converters

Hi.

I need advice on a powersupply-issue.

I'm designing a piece of electronic equipment and I need a lot (16-24!) isolated DC-voltages. Output will be approx 2W. Each output needs to be galvanic isolated from the others.

One option is the good old transformer. With 4 secondaries that limits the number of transformers, but it's still a "clumsy" solution.

I could also go with one big transformer (SMPS?) that gives ~ 28V and then convert this to 5-12V with a number of small DC-DC converters. Problem is that these are not cheap and that they (can) have a pretty large capacitance from input to output. This is not acceptable as I need "AC" isolation as well.

Third solution is to develop a custom SMPS with the necessary number of outputs, but I don't need more work right now :-) ...

Any hints, ideas whatever?

Regards TroelsM
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Old 13th June 2006, 11:41 AM   #2
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you can use multiple switching chips on one line -- i did something like this with On-Semi's MC34166 which were quite inexpensive at the time. each chip has its own reference and the output voltage can be set independently. there are myriad chips to choose from and a dozen manufacturers, i just happened to have several hundred of the On Semi devices on hand. if you need "quiet" the simplest way of reducing the switching noise is to follow it with a linear regulator. you burn some watts in the design but you save a lot of time in design.
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Old 13th June 2006, 03:19 PM   #3
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Default Isolation

Jack,

He needs them to be galvanically isolated from each other as well as the line. MC34166s work fine for non-isolated setups.

Troels,

Expanding on Jack's idea, though gives me an idea. Try an LM2577 SimpleSwitcher for each output, connected as an isolated-output flyback. Here is the link for the dadtasheet:

http://cache.national.com/ds/LM/LM1577.pdf

And here is the link for the Application Note on designing isolated output SimpleSwticher circuits:

http://www.national.com/an/AN/AN-1095.pdf

Just the chip, a simple off-the-shelf flyback transformer and a few other support components will get you going in the right direction. The transformers, as mentioned in the LM2577's Datasheet notes are available as samples (in small qtys) or for sale (in larger qtys) from Coilcraft.

The design note shows for the LM2587, but it is also applicable for the '2577 as well.

Jack, thoughts?

Cheers,

Steve
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Old 13th June 2006, 04:00 PM   #4
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Steve -- I was just in '8-land over the weekend.

Charge pump? Isolated CUK Converter?
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Old 13th June 2006, 05:41 PM   #5
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Jack-

Sorry, I was just on 0-land over the weekend. Normally, I am on the 147.180 USECA Repeater (100Hz PL). HF, I play 40M. Dunno if it makes into the Garden State. Perhaps on a band-opening.....

Isolated Cuk inverter sounds interesting. Gives zero ripple and galvanic isolation, too. I have never done one of these, so I'm not sure on the compensation scheme or even the design procedure, but it's worth a try. Either the '2577 (52kHz) or the '2587 (100kHz) would work for this. I think National may have had an App note on this.

Troels, try either LM2577s or '2587s as I previously described, only configure them for the isolated Cuk topology. Jack always comes up with good ideas..................

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Old 13th June 2006, 05:54 PM   #6
poobah is offline poobah  United States
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TRoelsM,

What is your source voltage? Is line AC available?
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Old 14th June 2006, 12:16 AM   #7
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Hello TroelsM,

how high is and which max. frequency has the isolation voltage?

My solution for a similar task was to use simply small and cheap common mode inductors as a 1:1 transformer :

Example with a 2x 47mH 0.25 A inductor I measured a coupling C of 11pF!

The "primary" You can feed either with a rectangle or (for much less common mode noise) with a sinus voltage of about 5 to 20 kHz.

Many "generators" are possible :
EG the secondary of a small SMPS!
Or You can use simply a audio poweramp, add a LC circuit in the feedback - and You have a sinus-power generator!

Because the secondary rectifier`s take only current near the top of the sinus the amp will not get hot but he must can deliver the peak current and should work on the lowest possible rail voltage.

If the output voltage is not stiff enough : use small 78...

I am curious : Can You explain Your application a little bit nearer?

Regards
Heinz!
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Old 14th June 2006, 05:55 AM   #8
TroelsM is offline TroelsM  Denmark
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Hi all.

The application is a multi-channel electronic load. The source is 230Vac.

Right now I'm investigation if its possible to have a toroid custom-made with 24 or 48 separate secondaries. (everything is possible, but the cost could be high..)

I can afford to spend some serious cash on this, so the custom-made transformer might be the solution.

Thanks for all the help.

Regards TroelsM
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Old 27th June 2006, 09:01 AM   #9
frank1 is offline frank1  United Kingdom
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Quote:
Originally posted by TroelsM
Hi all.

The application is a multi-channel electronic load. The source is 230Vac.

Right now I'm investigation if its possible to have a toroid custom-made with 24 or 48 separate secondaries. (everything is possible, but the cost could be high..)

I can afford to spend some serious cash on this, so the custom-made transformer might be the solution.

Thanks for all the help.

Regards TroelsM
Personally, I would buy a toroidal transformer, remove the secondary windings, then wind the required number of new secondaries. Having just modified a transformer, it is not difficult once you calulate the "turns per volt".

Frank
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Old 28th June 2006, 06:58 PM   #10
Bakmeel is offline Bakmeel  Netherlands
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Well, 16 to 24 isolated voltages will not come very cheap in any way, but you could try building 16 - 24 small flyback transformers.

Power Integration Systems (Website) ) sells nifty little Off-line SMPS controllers that will give you just that amount of power.

You might have to design your own flyback transformer, but since they will be only small for 2W, they wo'nt cost you as much as a 50Hz tranny with 24 secondaries. Try Wurth Elektronic, RS components or Farnell for some standard-issue SMPS trannies for Flyback.

Advantage is: since you let all TOPswitch controllers run off a single 325VDC bus, you will have a good AC seperation between the channels.

Good luck!
Bouke
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