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Old 4th June 2007, 02:16 PM   #1
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Default buck regulator

so i searched around with no avail.

i was thinking of using a mc33025 for a buck ps.

however to get the outputs together i thought about simple diode anodes on pin 14 and 11 then bringing the cathodes together,

any thoughts

i want to take 12-18 volts down to 6 to 8 volts..
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Old 5th June 2007, 05:54 AM   #2
luka is offline luka  Slovenia
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Hi

MC33025 will be great. I wanted too use it for buck too. You will need P-fets and save yourself a lot of trouble. If it is more powerfull you could use 2 fets, each on one output and drive same output inductor, or same it two phase one.

You will need good output diode, that will be with enough current and very fast

Maybe you will do something like this
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Old 5th June 2007, 06:32 AM   #3
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James

I think you should consider a part that supports active rectification (Synchronous buck).

This will give you the benefit that you can use N-channel mosfets and you will always run the converter in continuous conduction mode.

N-channel mosfets generally have lover Rds on and active rectification means that the regulation will be faster than a standard rectifier solution.

Maxim and TI have some nice parts that might support your needs without you having to think about gate boosters.

What output current are we looking at here?

\Jens
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Old 5th June 2007, 03:42 PM   #4
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oh

somewhere around 16 to 18 volts at 50 amps.

i'll have to look into specific chips.

i was thinking about driving an n-ch with a transformer or ir21xx
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Old 5th June 2007, 04:06 PM   #5
Danko is offline Danko  Hungary
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At 50 Amps sync. rectification gives you better performance, but increases circuit complexity.
Using a simple diode (maybe more paralelled) is very simple, but it gives little bit lower efficiency.
you can not use gate driver transformer, becouse the gate driving signal is not symmetric, and the core (the transformer's core) will saturate. As fas as I know ...

IR2113 is good.

50 Amps, oh my God! :-)
You will need as thick wires, as my finger :-D
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Old 5th June 2007, 04:31 PM   #6
luka is offline luka  Slovenia
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Hi

But why even use IR or any kind, if you use P-channels, which aren't that bad, you will save money, number of components, circuit complexity. And efficiency can't be that bad, it won't be 95%, but higher then 85% for sure. Also what freq. will you be going for?

You can't use gate driver transformer as Danko said
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Old 6th June 2007, 06:02 PM   #7
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oh 75 to 200 khz...........

any suggestions on p channels

i was going to feed the buck convertor with dc from batteries, so sync rect is not nesecsary and it is not isolated, simple straight through positive buss with the fet and inductor in serires.........
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Old 6th June 2007, 06:21 PM   #8
luka is offline luka  Slovenia
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Hi

maybe irf9540n, FQA17P10, IRF4905 something like those
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Old 6th June 2007, 09:36 PM   #9
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ok i ordered

samples of uc2573
and some fairchild p-channels samples

thanks
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Old 7th June 2007, 02:52 PM   #10
Danko is offline Danko  Hungary
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Just an idea: You can put the switching devide into the negative rail. Then you can use N-channel MOSFET (better performance, cheaper device), adn you can use very simple and cheap MOSFET driving circuit.

I made a DC-DC buck regulator, like that, and worked well.

But in this case the feedback is a litthe bit complicated. You may need an optocoupler in this case, in the voltage feedback path.
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