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Old 28th August 2008, 02:54 AM   #1
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Default Newby needs SMPS help

I have to admit defeat ( ), and request help from the forum . . .

I require 3 outputs:
+3.3V @ 2A
-15V @ 1A
+15V @ 2.5A

The expected input voltages are:
+45V
-45V

Those +/-15V don't need to be very tightly regulated since the plan calls for linear post-regulation. The 3.3V rail needs to be pretty tight however (+/- 5%).

Looking for the simple approach, I had considered 3 separate controllers - LM5576 from National. Using the WEB-BENCH app, it comes up with all the parts needed. I estimate the cost at around $20 per output in low quantity build - or around $60 total.

Is it better to go with a flyback design, transformer coupled, for example? I'm looking for suggestions and/or pointers in the right direction.
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Old 28th August 2008, 06:08 PM   #2
star882 is offline star882  United States
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45v is well within the working voltage range of 48v DC/DC converters. Such DC/DC converters are common in IT and telecom applications.

Is isolation needed? My suggested approach is to use the +15v as the primary output, -15v as an auxiliary output, and 3.3v is derived from +15v with a separate buck regulator.
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Old 28th August 2008, 07:59 PM   #3
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That sounds expensive. Aren't those DC/DC bricks large too? No isolation required.

Also you are suggesting several layers of regulation:

45
|
+15 -> +12 (from linear regulator)
|
3.3

The efficiency is going to pretty low getting from 45 to 3.3 and +12 as well.
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Old 28th August 2008, 09:17 PM   #4
star882 is offline star882  United States
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Then use two buck regulators to get +15v and 3.3v from +45v and another to get -15v from -45v. That'll get you independent regulation of all three outputs.
A flyback approach can be used if looser regulation is OK. I assume the 3.3v is for logic so connect the feedback to it. The +-15v (for opamps?) are then indirectly regulated. Note that a minimum load would be required on 3.3v to ensure regulation of the +-15v outputs.
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Old 29th August 2008, 01:03 AM   #5
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Well, that gets me back to the original issue of cost - and maybe real estate. 3 regulators is going to cost a bunch, unless there's a lower cost alternative that I don't know about.

Yes, you are correct about the +/- 12V, it's for analog stuff - which is why it's going through linear regulators. The 3.3V rail is for digital logic and needs to be regulated reasonably tight. The 15V rails don't need to be quite that tight.
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Old 29th August 2008, 02:03 AM   #6
star882 is offline star882  United States
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Flyback is probably the best approach, then.
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