smps design using 12Vdc to CCS for LED lighting - diyAudio
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Old 20th August 2012, 05:22 PM   #1
AndrewT is offline AndrewT  Scotland
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Default smps design using 12Vdc to CCS for LED lighting

Hi, tried looking up Zetex AN & DN. Tried searching here for a 12Vdc to a generate a constant current source for LED lighting. Need to get upto about 20V to 30V to drive a string of 7 to 12 white leds.

Lots of DC to DC converters, but they all have regulated voltage as output.
Zetex show some CCS outputs into 1, or 2 or 3 LEDs from 1, 2, or 3 cells, but I could not find any for higher voltages to suit car battery and such like.

What ICs and circuits, very simple if possible, could give a DC to DC but with a controlled Current output?
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Old 20th August 2012, 05:58 PM   #2
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Im not sure if the smps control ic knows or cares what you actually are controlling with the feedback - the current or the voltage...
Why doesnt any control IC do the job for you?
Just put a series resistor to your load and take the feedback from it, and if necessary amplify feedback to the control IC with opamp???
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Old 20th August 2012, 06:22 PM   #3
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This get you closer? :

LT3465 - 1.2MHz White LED Step-Up Converters with Built-In Schottky in ThinSOT - Linear Technology
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Old 20th August 2012, 10:51 PM   #4
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AndrewT, most IC manufactures will have an inventory of parts that will do this for you and in a simple and efficient way, they are usually listed under their sections on LED lighting and in your case you'd be after a boost converter. I am sure if you look again using those as search/selection parameters you will find what you are after. Buck converters are step down only, boost converters are used when the output voltage needs to be greater then the input voltage and buck-boost (if configured correctly) converters will automatically switch between the mode of operation that is required.

As for boost converters I have used, the LT3517/8 work well but require an external FET. The zetex ZXLD1370 is similar to the LT3517 and also comes with a spreadsheet that designs the circuit for you - I picked it mainly because it's only 2.21 vs the 5.66 of the linear tech part over a RS. These drivers will work at lower output currents but are mainly for driving something more then a few mA.

If you need something for a lower output current, then the LT3519 works quite well, I've used it to drive 80mA into a string of 12 leds, it does not need an external switch if used without PWM dimming.

Obviously all of these parts (I think) are SMD.
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Old 21st August 2012, 09:39 AM   #5
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As 5th Element says, you need a Boost part.

Of the Zetex lineup (the only one I am familiar with), the 137x family can do what you want. The 1374 has an integrated switch and does not require external MOSFET, so probably is closest to what you want.
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Old 21st August 2012, 10:43 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi all,
thanks for the pointers.
I'll do some more looking and see what seems to suit, then probably come back with build questions.
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Old 22nd August 2012, 04:45 PM   #7
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It really depends on what you're doing.

LEDs are surprisingly tolerant. They strike at voltages below the specified Vf.

You'll see guys over on candlepowerforums.com powering 3.6V nominal white LEDs from a rechargeable LiPo with a fresh charged output of 4.2V with no dropper resistor.

This runs the LED pretty hot (may go purple) while the cell is hot, which reduces its life a bit, but the cell discharges and so what?

If you want to run a string of LEDs from say 12V, you can put 3 whites in series for 10.8V and drop the 1.2V using a couple of rectifier diodes. If it's a car battery use a 9V LDO regulator and put 3 diodes in series with the grounded leg. If the battery gets a bit flat and the supply drops out of regulation, so what?

The longer the string the better in some ways.

This is not necessarily what you want to do if you're designing for high reliability or a guaranteed lumen output for a client, but it's simple, cheap and it'll work within limits.
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Old 22nd August 2012, 05:29 PM   #8
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So what? You are either purposefully damaging the LED or settling for a light that will have less then optimal output characteristics. Why would you actively do this if you can easily avoid it? LED drivers aren't that expensive, nor are they difficult to implement, they are also highly efficient and in the case of the LiPo batteries you'd probably get light for longer, with the driver, then without.
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Old 22nd August 2012, 07:00 PM   #9
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Quote:
Originally Posted by 5th element View Post
You are either purposefully damaging the LED or settling for a light that will have less then optimal output characteristics.
Depending on the exact source.

Damaging things is permitted when they belong to you. Optimal is not always the goal. Never heard of good enough?

Quote:
Originally Posted by 5th element View Post
Why would you ... do this?
Because of the low design effort, low parts count, low space occupancy and low cost? Taken individually or all together?

It all depends on the exact application. I've got a UV lightbox here built with LED strings on the basis outlined above, 12V supply, diode dropper. The LEDs will run at a milliamp or so up to 20 milliamps. It's turned on 5 minutes at a time, discovered empirically. The LEDs cost a few dollars. I could have built constant current supplies for each string, for each LED, but I chose to save the money and effort.

So what? My choice.

I'm offering an alternative that may be acceptable and may not occur to someone with limited experience of LEDs. It cuts across conventional wisdom, but it's based on experience, experience you evidently never took the risk to acquire.

Not everybody has the same concerns as you. I shouldn't have to point this out.
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Old 22nd August 2012, 08:32 PM   #10
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Quote:
Originally Posted by counter culture View Post
Not everybody has the same concerns as you. I shouldn't have to point this out.
It has nothing to do with concerns of my own, what you're basically advocating is ignoring all conventional wisdom and guidelines/max ratings, throwing caution to the wind and abusing things just because it's easier/cheaper.

If you took the same mind set to other areas of DIY here, as in ignoring SOA curves, xmax ratings and maximum part ratings we'd have lots of projects which spontaneously combust or are prone to premature failure. Which ever way you look at it, this is BAD design, if you're okay with the things you build potentially going up in smoke, then hey good for you.

Quote:
Originally Posted by counter culture View Post
I'm offering an alternative that may be acceptable and may not occur to someone with limited experience of LEDs. It cuts across conventional wisdom, but it's based on experience, experience you evidently never took the risk to acquire.
No I've taken the experience, I've abused LEDs and I know how quickly they can fail if you exceed their recommended operating conditions, either through over current or inadequate heat sinking. It isn't worth it most of the time and if you're going to the trouble of building something using LED lighting, then you're probably doing it for the right reasons, ie efficient lighting that lasts almost forever. If you overdrive the LEDs then you lose out on both of those advantages and using LEDs then turns into being a liability rather then something that should be a perk/bonus.
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