Old 01-01-2012, 07:25 PM #17
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Default Re: Open-Source Adjustable CC, CV Power Supply

i may be onboard for a group buy, depending on prices. i dont have much money for it . anyways, +1.


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Old 01-01-2012, 07:37 PM #18
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Default Re: Open-Source Adjustable CC, CV Power Supply

Well, one of the boards (from DorkBot) would probably run... I think $19? In bulk I could probably get it down. Parts... I haven't even sourced most of them yet (not planning on it until I know for sure that I can afford some of these parts, etc.).
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Old 01-03-2012, 03:54 AM #19
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Default Re: Open-Source Adjustable CC, CV Power Supply

Alright, so I found another great IC which may not involve boosting and then bucking... and it will do it all in one package! This one is a SEPIC converter, so it means buck-boost ^_^ I don't know MUCH about board layout specifics for SEPIC converters, but I do know how they work, and this one has been simulated to work and it should I will update this post later tomorrow or something when I stop feeling like a lemon wrapped around a brick is beating my brains out with the .sch and the .brd file.
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Old 01-03-2012, 11:48 AM #20
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Default Re: Open-Source Adjustable CC, CV Power Supply

Why don't you do what we do when designing a new project...

1) design the circuit on paper (or simulator)
2) build a prototype on a Breadboard to test the circuit design...
3) build a prototype on a perforated Proto PBC (Vero Board) to
test the device parameters....
4) Then and only then when the design has been tested and
proven to be within design specs... design the PCB...

BTW... I can't seem to find the data sheet for your LT3786...


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Old 01-03-2012, 04:04 PM #21
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Default Re: Open-Source Adjustable CC, CV Power Supply

Due to the nature of the high switching frequency and the high currents flowing, a normal protoboard would be no good for such a situation. Also, they only come in SMD parts, unfortunately. I realize I could use a breakout board, but then I would have to custom make THAT board in order to make sure that it fits for the application. I'm not sure how much knowledge you have about designing high-power switching PSUs, but they are a nasty bugger get right, from what I can tell.

Also - the IC is actually LTC3786 - not LT3876. That was my bad. Typo.
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Old 01-09-2012, 04:10 PM #22
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Default Re: Open-Source Adjustable CC, CV Power Supply

To fix any additional output ripple problems, you could add a small filter in the form of an inductor and additional capacitor on the output....

I could probably give you a hand at designing this. I've made a buck converter before using an LT11070T IC from linear, and it worked perfectly. It was fully adjustable too! The chip itself is pretty awesome, with the correct heatsinking it can handle up to 5 amps! Plus, circuit design is also greatly simplified. It can do SEPIC, Cuk, boost, buck, buck boost, forward converter, and flyback converter mode.

It's a pretty versatile chip.

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Old 01-09-2012, 04:30 PM #23
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Default Re: Open-Source Adjustable CC, CV Power Supply

Hmm. I never knew that inductors are often used for filters o.o What kind of specs for that inductor are we looking at? I assume it would require full saturation current of 5A, but how much inductance?

And I would love your help on this! And actually - I have a new version to throw up there - it's quite a bit simpler. Lemme update the main post.

EDIT: So the first post is updated. What do you think about the layout of that SEPIC design? I've never done a SEPIC before, so I am not sure if I did it right and all that.

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Old 01-09-2012, 07:10 PM #24
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Default Re: Open-Source Adjustable CC, CV Power Supply

I can't look at the schematic just yet, I'm still at school so I apologize.


So anyway, yes, inductors are used very often for filtering. It doesn't need to be very high inductance; Maybe like 15-20 uH, on just a ferrite rod or a toroid. It gets rid of the higher frequency ripple. Higher capacitance doesn't always fix everything

Also, you might want to try perhaps a forward converter design. It's how most PSU supplies work (except for nicer ones...they use a half bridge inside)

The only problem with forward converters is that they require a transformer. BUT, this allows you to use rectified mains!

SEPIC converters allow for boosting and bucking, depending on which needs to be done. (the voltage is always constant, regardless of the input voltage) The only problem is the amount of components required to get it working. And, since you'd only be stepping down voltage, a SEPIC converter really doesn't make sense. There'd be no need to ever step up the voltage.

To help with the "large resistance" values for the feedback (or, shunt resistor) you just need some sort of error amplifier, just an op amp. It basically multiplies the voltage a certain amount so that 30 mV becomes 3 volts, or whatever you need. That way you can use much much much smaller resistance values. Then to adjust it, you just use a potentiometer to change it.

A buck converter would probably be the best way to go, or forward converter if you can afford to add a transformer to the board. Either way, the feedback methods are still roughly the same. Or if you wanna get really fancy, a resonant converter.

If you want, I can shoot you a PM and give you my email address so we can talk a little quicker than on the forums.


EDIT;

I just looked at the schematic... I can't tell how you've got it hooked up. xD

May I try and see if I can find something else that may work a little better?

Last edited by Hiemal; 01-09-2012 at 08:44 PM.
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Old 01-09-2012, 10:29 PM #25
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Default Re: Open-Source Adjustable CC, CV Power Supply

Ah. See, the intent behind this initiative is that I want to provide a good PSU that can be powered off of a 12V line, because I know a lot of us have old computer PSUs which can easily supply 15+A on their 12V line, which is what this is based off of.

Anyway, the schematic is a little messy - I had to make all the symbols which didn't make for a nice, efficient schematic, but it's all there. I can throw up the LTSpice simulation file too if you want. But, the schematic I used for this design is nearly the same as the SEPIC schematic found in the LT3756 datasheet - I just calculated values, simulated it, and added some features for switches and what not.

The reason I went SEPIC though is because I couldn't find a nice, high-powered buck-boost that could provide 5A at 24V from 12Vin :\

Anyway, later tonight I will probably throw a coil for reducing ripple ^_^
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Old 01-09-2012, 11:18 PM #26
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Default Re: Open-Source Adjustable CC, CV Power Supply

I sent you a reply in your PM's.

I apologize; I forgot about the computer power supply based thing.

SEPIC is definitely the topology of choice then.

And, for an output filter, it usually goes main filter cap, inductor, then another smaller cap.

Also, you may need a minimum load to maintain regulation. Something like 20-100 mA draw no matter what, otherwise you might get bad spikes upon hooking something up.

Last edited by Hiemal; 01-09-2012 at 11:18 PM.
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Old 01-10-2012, 02:26 AM #27
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Default Re: Open-Source Adjustable CC, CV Power Supply

Hmm. Interesting. I'll look into the minimum load for regulation.
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Old 01-10-2012, 02:37 AM #28
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Default Re: Open-Source Adjustable CC, CV Power Supply

Yup, the load should be a 10ohm resistor rated up to around 15-20watts.

It isn't really a problem since its so easy to just solder a chassis resistor to the 5v line on your computer PSU.

Its to do with the nature of switch-mode PSU's
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