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Old 30th April 2006, 09:50 PM   #1
gmikol is offline gmikol  United States
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Default Discussion of methods for high-current regulated PSU

Hi all--

I've been considering some designs for a high current PSU using a 3 terminal regulator, and I'd like to hear some opinions about the various methods to accomplish it:

1) Parallel regulators..this should work well for LT1083, but maybe less well for LM338
2) Regulator driving base of 1 or more pass transistors
3) Pass transistor in parallel with regulator & very small value resistor to create Vbe drop for pass transistor.

Here are my thoughts:
1) LT1083's are expensive ($13/ea. at DigiKey). Paralleling them gets costly.
2) Don't need a very high-current regulator. Maybe a higher dropout voltage than a 3-terminal device, and this might lead to extra dissipation.
3) The resistor to generate the voltage drop is definitely extra power dissipation, about 5 Watts (for 0.75V drop with 7.5A load)

Anyone else care to chime in?

Thanks--

Greg
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Old 30th April 2006, 10:24 PM   #2
Eva is offline Eva  Spain
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If you have to deal with more than 5A and your application is not too much EMI sensitive, you may consider building a switching regulator. Low voltage switching MOSFETs are cheap and small (TO-220 case) yet they can switch 30A per device in a quite efficient way.

Concerning linear regulators, there are very elegant LM317 and LM337 boosted approaches, where an external boost transistor is connected in such a way that it adds "current gain" to the regulator (I mean the transistor is forced to provide to the load exactly a multiple of the current provided by the own regulator, say two to ten times). Essentially all the features of the regulator are preserved, including the SOA protection that becomes also effective for the boost transistors.
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Old 30th April 2006, 10:50 PM   #3
gmikol is offline gmikol  United States
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Do you have a reference for the boosted LM317/337 approach? I didn't see anything like that in the data sheets.

I've seen a lot of your posts...you seem to be a big proponent of SMPS designs. But a +/- 68V 16A SMPS doesn't seem like a good starting point for me. Wouldn't the output coil need to be absolutely massive for something like that? I can't help but think it would be an expensive proposition.

Thanks for the input, though.

--Greg
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Old 30th April 2006, 11:04 PM   #4
gmikol is offline gmikol  United States
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Just wanted to add that this PSU is for a 41hz.com AMP2 with the high-power mods. I'm looking for 2x500W output.

I need a regulated supply to get maximum power out without tripping the 70V over-voltage protection on the chip.
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Old 30th April 2006, 11:05 PM   #5
Danko is offline Danko  Hungary
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Hello!

It's not a boosted LM317, but I think, boosting an LM317 with a few beefy transistors would be very similar to this:
http://www.mitedu.freeserve.co.uk/Ci...er/1230psu.htm
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Old 30th April 2006, 11:25 PM   #6
gmikol is offline gmikol  United States
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Yes, that's exactly what I was describing as 3) in my first post. But Eva alluded to a precise current "gain", which the link you provided does not have.

I don't think the precision is necessary, though.

Of the 3 styles of design I mentioned initially, is there a "best" and a "worst"?

--Greg
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Old 30th April 2006, 11:32 PM   #7
Eva is offline Eva  Spain
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Hi Danko

Indeed, that schematic clearly shows the same current multiplication principle that I explained for LM317/337.

Concerning the output inductor(s) for a 68V 16A supply, they won't be so big. It depends a lot on design and operating frequency, but you could expect a 40mm diameter iron powder toroid core or an ETD39 or E42/21/15 gapped ferrite transformer to do for each output (considering operating frequencies in the range of 50Khz).

However, note that building a symmetric regulated power supply for a classs D amplifier is not a trivial task, because these circuits don't only draw current from the rails but also return it. A classic linear regulator is going to have lots of trouble handling that, some regulators may even blow due to reverse biasing, thus a specifc output stage with both source and sink capabilities would be required on each rail. On the other hand, switching regulators are more tricky because output inductors may be coupled in order to allow mutual energy transfer between rails and even self balancing.

Do you have a dual trace oscilloscope?
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Old 1st May 2006, 06:21 AM   #8
gmikol is offline gmikol  United States
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I hadn't considered the reactive nature of the Class D output filter. I'll have to try simulating an output stage to see exactly what's going on.

I do have access to a good 'scope at work...what should I be looking for in both the sim (and when I eventually get around to building it, the real thing)?
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Old 1st May 2006, 11:02 AM   #9
Eva is offline Eva  Spain
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There are far too much waveforms and criteria to check to explain in a single post, but the important fact is to have access to one
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