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Old 28th April 2003, 09:39 AM   #1
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Default Switching Regulator design Questions

I've come to a stand still in my power supply
I've been looking at nationals line of switching regulators but all of them (a few exept) have internal fet's so you can't add any extra current capabiliy to them.
Data sheet for LM2679
My idea is to put a resister on the pin of the output (which is a switched version of the input voltage) and let it drive a bigger fet(s). Would this work?
I'd need a bigger inductor (one per fet?) and some higher rated diodes.
Is there any other simple solution to this besides a linear regulator?
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Old 28th April 2003, 12:22 PM   #2
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Some of the older "Simple Switcher" chips from National can just be stacked -- it's not something which the company endorses, but their technical people told me that it is done.

You can also design a stacking circuit which pulls in a second switcher chip when demanded -- Elektor had the circuit in their recent subwoofer power supply design. Note, however, these Single Ended designs are noiser than push-pull designs.
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Old 28th April 2003, 03:18 PM   #3
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If you're going the National SimpleSwitcher route and want more current, try the LM2677. It puts out up to 5 Amps, and is synchronizable. Simple put a 300kHz clock (like a 555) on its sync pin, and you can papallel as many serctions as you want. I recently did a 24-12V DC-DC Converter paralleling 3 chips and clocking them from a 555, and I get uot nearly 12 Amps. Works great. I am using the surface-mount 12V version, LM2677S-12. Samples are available from National.

Best of Luck.

'73,

de N8XO
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Old 29th April 2003, 03:07 AM   #4
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When I meant higher currents I was aiming at really high currents >40amps
Stacking is an option but a rather expensive one since you can only get sample orders of 4 or less from memory.
Was my original idea feasable?
The block diagram shows that its only switching the input voltage so if I put a bipolar stage in there (if its needed) then it should drive added fets? and woulds N channel power mosfets be ok for the task?
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Old 29th April 2003, 09:58 AM   #5
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Oh, OK. In any case, you really don't want to stack regulators that aren't synchronized, because of what National engineers call the generationa of a "beat" frequency. This is where similar, but not matching, frequencies are in close proximity to each other. Any differences between the two (or three or four, etc.) regulator chips will generate a beat frequency, which is not good.

Couple o' questions: What are the input and output voltages desired? Do you need isolated output(s) from the input? AC input? Hi-voltage DC input? These parameters will GREATLY affect the design of the power supply. I have many good circuits that I could flood this thread with, but in the interest of getting to the solution, asking yourself (and us in the thread) these questions will greatly help in meeting with success. Pls advise.
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Old 29th April 2003, 10:58 AM   #6
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I need a variable 40-50 amps supply from about 6-16v (probly lower than 6v if i can for more flexibility).
Input can be pretty much anything since I havn't purchased any transformers yet. I was expecting to use a 15v AC through a bridge so around 21v dc.
I'd rather an efficient design so i need less transformers and spend less on heatsinks.
Was my original idea workable?
If a fet only requires voltage to switch on (correct where wrong) it should be able to operate off the Vout of the reg with the addition of a resistor much like a pass resistor would
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Old 29th April 2003, 12:58 PM   #7
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Default if you want to pay the shipping to OZ

I am getting a few of these in surplus, weigh about 70 pounds -- they'll do 55 amps, all you have to do is pay the shipping.
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Old 29th April 2003, 01:23 PM   #8
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Looks like a very nice PSU

/Henrik
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Old 29th April 2003, 04:48 PM   #9
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They (3 of them) just arrived by freight -- and they are very heavy -- and they will handle more power than I thought -- 16 volts at 55 amps AND 19 volts at 15 amps. The power transformer is larger than that of my Hallicrafters HT45 Linear. I am happy to get rid of them to anyone who wants to pay the shipping.
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Old 29th April 2003, 10:33 PM   #10
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fr0st:

I could drown you out in a litany of good stuff, but jackinnj's PSU looks like it will more than do the job. I would take him up on his most generous offer.

jackinnj: generous offer! I would take one, but I already have WAY too much stuff around the 'shack already.

N8XO
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