Reef Central Online Community

Go Back   Reef Central Online Community > General Interest Forums > Do It Yourself
Blogs FAQ Calendar Mark Forums Read

Notices

User Tag List

Reply
Thread Tools
Unread 12/18/2009, 08:18 AM   #1
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
DIY LED driver for reef lighting

I've gathered some of the material I've been promising for weeks (months?) in the various LED threads and figured if I make a thread about it, I'll be motivated to get the rest of the material together if there's enough interest.

If you're new to HP LEDs for reef lighting, here's some reading:

The Mother Thread:

http://reefcentral.com/forums/showthread.php?t=1678127

Some others:

http://reefcentral.com/forums/showthread.php?t=1662682

http://reefcentral.com/forums/showthread.php?t=1751598

http://reefcentral.com/forums/showthread.php?t=1672708

I'm starting this thread to detail the DIY driver I've mentioned a few times in those various threads. I decided to DIY drivers for a few reasons:

1) It seemed like an interesting project for someone like me with moderate DIY electronics skills to learn some more and expand my skillset
2) It gave me control over the end product I was getting, vs. living with the limitations of a COTS product.
3) It'll end up being a few bucks per driver cheaper than any of the commercial products that compare, though that's NOT accounting for time involved.

If you want a fun DIY project involving LEDs, read on. I'll be doing this thread in sections as I get time to document this project. Also, for all you "real" EE's out there, if you'd like to rip apart my design, please do so. It'll help me learn.


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 08:30 AM   #2
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Before I get to the actual design, I want to explain how I arrived at it, so if anyone else is interested in DIY'ing drivers they'll understand why I made the decisions I did.

Design goals:

I wanted the following from a driver. These were the goals that led to this design.

1) Highest efficiency reasonable. At least as high as commercial products like the buckpuck drivers, or meanwells.
2) Controllability. Ideally, a 5v PWM signal in the ~400hz range (since that's what an Arduino can generate) would work.
3) 6-12 LEDs per driver. Any less, and you've got a billion drivers for a reasonable sized fixture. Any more, and your resolution for controllability drops off.
4) All through-hole soldering. I have no reflow oven and (at least at the time I started this design) don't/didn't trust my SMT soldering skills enough to go that route.
5) A product that was designed for constant current LED driving. I don't trust myself well enough to work voodoo magic on a fan-driving chip to get it to work, for instance.

I looked at chips from TI, OnSemi, National Semiconductor, ST, Maxim, etc. (hooray for free samples if you have a business-like sounding name/address!) and the above criteria cut the field down to a VERY small number of choices. I chose the NCP3066 from OnSemi in the end, though there are arguably a few other chips that would probably work just as well.

The NCP3066 is a switching regulator (high efficiency possible compare to linear regs) that's flexible and can run in a boost, buck, or SEPIC topology. It can use an external switch for high currents, but I'm keeping my drive current reasonable (700mA is the MAX I've done on this design, though it should be good for more than that if you watch thermal management.) I'm using it in a boost topology, since according to OnSemi, that's a little more efficient. It also lets you use more LEDs per driver on a common and cheap 24v power supply compared to buck topology.

Here's the product page for the NCP3066:

http://www.onsemi.com/PowerSolutions....do?id=NCP3066

There's also the NCP3065 - a pin-compatible version that's identical (and interchangable - I've swapped the two on built drivers) EXCEPT it doesn't have the EN pin used for PWM dimming:

http://www.onsemi.com/PowerSolutions....do?id=NCP3065

Next up, schematics and BOMs. . . .


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 08:45 AM   #3
jonewald
Registered Member
 
jonewald's Avatar
 
Join Date: Sep 2006
Location: South Jersey
Posts: 102
Subscribed! Looking forward to this as I'm in the early stages of making my own DIY driver like I mentioned in the mother thread and this was the chip I decided to go with as well.


jonewald is offline   Reply With Quote
Unread 12/18/2009, 08:46 AM   #4
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
So sadly I don't have a schematic of the EXACT configuration I'm using, but it's very close to some of the schematics in the product docs, so hopefully that's good enough. If there's enough interest I can work on a real schematic of the actual design. Here's a schematic lifted from the app notes of the NCP3065:



This is pretty much my circuit, with a few changes:

1) I'm using a SINGLE resistor in place of the 6 parallel resistors shown for R1-R7 in the upper left corner. As long as the final resistance is correct, you can use any combo of series/parallel resistors you want, of course.
2) I'm NOT using the dimming junk connected to the COMP pin in the lower left corner. Since I used the NCP3066 version of the chip (not the 3065 shown here) I can dim on the EN pin, so I don't need to dim on the COMP pin. What this means is you can leave off everything to the left of the NU (not used) resistor near the bottom left corner of the schematic.
3) Since I am using the 3066 version with the EN pin, the "NC" (no connection) pin shown on the upper left corner of the chip is really an EN (enable) pin. I have a 1k resistor connected to that, and I supply my PWM signal to that 1k resistor.

Now, on to parts lists. Here is the list given for that schematic in the docs:



Of course, these are mostly incorrect. They're all surface mount, and the values are wrong: this is for a 12v input 350mA driver. I want a 24v, 500mA driver. If you want something different, it usually just means one or two component changes, so stay tuned for discussion on that (if there's interest).

Up next, components I actually used. . .


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 09:15 AM   #5
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Parts I used:



I included digikey numbers for the parts I got from them. Besides this list, I used some things I had from other sources:

1) I couldn't find a good resistor for R1 from digikey at a reasonable price. R1 is the current limiting resistor (for the entire circuit, it does NOT independently set the drive current! It basically sets the max spike the circuit can hit when trying to generate the drive current, which is controlled by Rsense.) The circuit wants .15ohm and a half-watt rating. I got a HUGE bag of .22 ohm 1w resistors from eBay (100 pcs I think) for about $6 shipped, which is what digikey wanted for only a handful of comparable parts.

2) I also struggled to find an exact match for Rsense, so I got the .47ohm resistors listed in the above BOM and used one of them in parallel with one of the .22ohm resistors I mentioned above for an Rsense value of .69ohm. This gives a drive current of around 350mA (very tame by today's standards.) You can JUST use the .47ohm resistor from digikey if you want 500mA drive current, or you can use a .34ohm resistor if you want 700mA. The 350mA and 500mA drive currents work totally fine with the rest of the components listed, BUT if you want 700mA or above you'll need to adjust other values to compensate, since the peak current on the chip will be exceeded unless you do.

3) 1k ohm resistors (for R8 and Rpwm) are a dime a dozen. If you don't have them laying around, shame on you.

4) I used an 8-pin DIP socket to build this, rather than soldering the drivers in. I did this to protect the drivers, and so I could switch between 3066 and 3065 chips as I tested (disconnect Rpwm if you use 3065). If you trust your soldering skills, you can just solder the chip in.

5) For layout, I don't have any diagrams. I used this protoboard from ratshack:

http://www.radioshack.com/product/in...ductId=2103799

It's $2 for two of them, and they're designed exactly for little DIP-IC based circuits like this. Probably not ideal (I can hear people screaming about trace widths and ground planes now) but it works which is good enough for me. If you want to save a few bucks and you're buying your DC power supplies from mpja.com, they have this part at 75 cents for two, which brings cost down.

That should be enough to get people started. Lemme know if there are specific questions. In the next installment (maybe this afternoon, I need to get some real work done) I'll post some action photos and info about actually using these drivers.


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 09:44 AM   #6
stugray
Premium Member
 
stugray's Avatar
 
Join Date: May 2004
Location: Longmont, Co
Posts: 4,566
Just curious, did you look at the STCS1 driver chip?

I does not meet one of your requirements - it is only available in a surface mount chip.
However you shouldnt worry about that too much.

The very first time I tried to solder surface mount, I did just fine.
It can be done with a standard soldering iron, you dont need an oven or reflow station.
You can also get "surfboards" that allow a surface mount part to be placed into a DIP socket.

Here is a pic of the schematic:



Notice how there are far fewer components on this one - ONLY 5.
Also - NO Inductors required.

Stu


__________________
Some people think that I have Attention Deficit Disorder. They just dont understand that........ Hey! Look a chicken!

Well, We KNOW GOD exists, but for US to exist without a GOD is preposterous….Umm wait a minute…. Sounds a bit circular to me…

Current Tank Info: 125 Gal. display w/80 gal mud/caulerpa sump. Basement sump w/ LED Grow Light,Gravity fed Reeflo200 skimmer w/ ORCA Recirc, DIY calc reactor & kalk stirrer. Inline plumbed 75 Gal frag/settling tank.
stugray is offline   Reply With Quote
Unread 12/18/2009, 10:07 AM   #7
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Stu, I'm thinking about doing some prototypes with that chip right now. At least one or two other forum members are already using that chip. I avoided it in the first round because of the lack of a through-hole package, plus it's (likely) not as efficient since it's a linear reg. However, matched very carefully to a PS so the voltage was close to what the LEDs need I'm sure it would be close.

Those chips have a ground pad for thermal management under the chip, though. I suppose you could use conductive epoxy to glue that pad down, then solder the actual connections?


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 10:33 AM   #8
stugray
Premium Member
 
stugray's Avatar
 
Join Date: May 2004
Location: Longmont, Co
Posts: 4,566
"Those chips have a ground pad for thermal management under the chip, though. I suppose you could use conductive epoxy to glue that pad down, then solder the actual connections?"

Ahh.... I didn't notice that. You're right.

The epoxy might work fine since we would be running close to half the rating.

Otherwise we would need an oven & solder paste.

Stu


__________________
Some people think that I have Attention Deficit Disorder. They just dont understand that........ Hey! Look a chicken!

Well, We KNOW GOD exists, but for US to exist without a GOD is preposterous….Umm wait a minute…. Sounds a bit circular to me…

Current Tank Info: 125 Gal. display w/80 gal mud/caulerpa sump. Basement sump w/ LED Grow Light,Gravity fed Reeflo200 skimmer w/ ORCA Recirc, DIY calc reactor & kalk stirrer. Inline plumbed 75 Gal frag/settling tank.
stugray is offline   Reply With Quote
Unread 12/18/2009, 12:47 PM   #9
james3370
Registered Member
 
james3370's Avatar
 
Join Date: Aug 2008
Location: US ARMY - El Paso TX
Posts: 3,678
great....ANOTHER thread for me to follow for my impending build sometime next year (fingers crossed LOL)


__________________
Yes I hear voices in my head, but they speak spanish
so I can't understand anything they say.

there's no place like 127.0.0.1

It's a shame that stupidity isn't painful....

Current Tank Info: currently tankless....but planning an AIO
james3370 is offline   Reply With Quote
Unread 12/18/2009, 12:53 PM   #10
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Quote:
Originally Posted by james3370 View Post
next year (fingers crossed LOL)
Cool, but I hope someone tries it before then!


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 01:01 PM   #11
troylee
Moved On
 
troylee's Avatar
 
Join Date: Oct 2008
Location: here.
Posts: 2,509
Quote:
Originally Posted by der_wille_zur_macht View Post
Cool, but I hope someone tries it before then!
thats only 13 days away..." james"....get busy!!!!!


troylee is offline   Reply With Quote
Unread 12/18/2009, 01:02 PM   #12
james3370
Registered Member
 
james3370's Avatar
 
Join Date: Aug 2008
Location: US ARMY - El Paso TX
Posts: 3,678
i'm gonna let all you guys work the bugs out while in basic & ait & then when i get settled somewhere, poach all the info & start on mine

Quote:
Originally Posted by troylee View Post
thats only 13 days away..." james"....get busy!!!!!
i said "sometime" next year....not new years


__________________
Yes I hear voices in my head, but they speak spanish
so I can't understand anything they say.

there's no place like 127.0.0.1

It's a shame that stupidity isn't painful....

Current Tank Info: currently tankless....but planning an AIO
james3370 is offline   Reply With Quote
Unread 12/18/2009, 01:10 PM   #13
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Quote:
Originally Posted by james3370 View Post
i'm gonna let all you guys work the bugs out
I've had these running for a few months, I hope there's no more bugs.

I have some photos of these on a tank, but that's another writeup. . .


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 01:43 PM   #14
OTCHU
Premium Member
 
Join Date: Jun 2006
Location: New columbia, PA
Posts: 137
good job !

I am interested about this thread too. If you need me to do the PCB no problem ... I have everything at my job to do it. I am electronic engineer so I can help you if you want.

I will be off line during the holiday (going to France ... I am French) but will come back the 28th december


OTCHU is offline   Reply With Quote
Unread 12/18/2009, 02:30 PM   #15
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Thanks for the offer. Whatever drivers I end up using on my big tank will most likely be on a "real" PCB but IMHO there's value to building these things on the super-cheap protoboards if you don't want the hassle of making your own PCBs or the expense of going to a board house. Plus I think it presents an easier entry-point for people who aren't complete experts at DIY electronics (like me!)


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/18/2009, 02:44 PM   #16
terahz
1x10^12 Hz
 
terahz's Avatar
 
Join Date: Oct 2009
Location: Stamford, CT
Posts: 1,185
Great project. I've already started planning a bigger tank and I'd definitely do the drivers as well this time. For my 10g it wasn't worth the effort but for a bigger tank I'm sure this will work quite nicely.


terahz is offline   Reply With Quote
Unread 12/18/2009, 06:34 PM   #17
va7zt
Registered Member
 
Join Date: Dec 2009
Location: Robson, BC
Posts: 15
Awesome! I'll be ordering my parts from digikey tonight!!! I need to build about 15 of these for my planned hood!
Thanks for posting the digikey #'s .... it will sure make it easier than trying to figure out what's what!

I would be interested in seeing a final schematic and possibly the board layout you used...a couple pics would do that nicely!


va7zt is offline   Reply With Quote
Unread 12/18/2009, 06:52 PM   #18
Altpers0na
ARS Caesar
 
Altpers0na's Avatar
 
Join Date: Feb 2006
Location: Western PA
Posts: 1,235
im going to say im interested in any sort of thing like that. so , yes please.


__________________
We are not descended from fearful men.


The Final Countdown is now stuck in your head.

I use "..." excessively

Current Tank Info: tankless
Altpers0na is offline   Reply With Quote
Unread 12/18/2009, 07:44 PM   #19
va7zt
Registered Member
 
Join Date: Dec 2009
Location: Robson, BC
Posts: 15
In point 2 you mentioned different values for rsense for the drive current. So does this mean that if you went with the .47 ohm for 500mA then the maximum you can drive the LED's is 500mA? Just want to make sure I understand exactly what this means. For my build I would want to be able to go to 700mA in order to ensure I had the ability to get light to the bottom of my tank but still not max out the LED's. What other values would have to be adjusted to drive at 700mA?


va7zt is offline   Reply With Quote
Unread 12/19/2009, 04:45 AM   #20
wismie
Registered Member
 
wismie's Avatar
 
Join Date: Nov 2008
Location: Geneva, Switzerland
Posts: 99
Subscribed ! Thanks for sharing, I have a similar project with the Arduino.


__________________
Click on the Red House for my tank setup

Current Tank Info: 95 gals corner tank, LED + Arduino-based projects (see my website - in French !)
wismie is offline   Reply With Quote
Unread 12/19/2009, 06:53 AM   #21
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
I'm writing out a way too long response to your question about setting current but I need breakfast first, so stay tuned.


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/19/2009, 09:11 AM   #22
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Coffee consumed. Now I can think clearly. As with all my posts in this thread, if any of the more experienced EE types see error in my logic, PLEASE point it out! I'm a self-taught hobbyist and the below is based on experimentation and my interpretation of the supporting docs for this chip.

And while I'm on the subject, a big thanks to the various early LED pioneers that got me hooked on this stuff in the first place (Soudwave, stugray, liveforphysics) and kcress for answering a lot of basic EE questions in threads and via PM when I was first building these drivers.

Quote:
Originally Posted by va7zt View Post
In point 2 you mentioned different values for rsense for the drive current. So does this mean that if you went with the .47 ohm for 500mA then the maximum you can drive the LED's is 500mA? Just want to make sure I understand exactly what this means. For my build I would want to be able to go to 700mA in order to ensure I had the ability to get light to the bottom of my tank but still not max out the LED's. What other values would have to be adjusted to drive at 700mA?
Bear with me for a moment. I feel a quick overview of the chip's function might help those (like me) who are interested in knowing how things work. You don't need to understand all this to do this project though. I don't want to scare anyone away - if you just want the answer, skip to the bottom.

So, this chip is essentially a switching voltage regulator that operates in a constant current mode. The Rsense resistor is exposed to the current flowing through the LEDs, so the chip can determine that current by reading the voltage drop across Rsense. It switches current to the LED array on and off to keep this voltage at a preset number (.235v). That feedback voltage is preset in the chip, so to adjust the current it equates to (and hence the current through the LEDs) you just change values for Rsense. To find out what value a particular Rsense will result in, V = I/R: .235v = Iled/Rsense.

However, that's only part of the story. Since this is a switching reg, the internal switch is basically turning voltage on and off. We don't want the LEDs to see that on/off waveform - we want them to see a constant voltage at the correct value to maintain the desired current.

Regulators for LED use like this chip can generally operate in three ways. The difference in operation is in the relationship between input voltage and output voltage. The difference in implementation is where the inductor is in the circuit in relation to Vin and the switch.

1) Buck: The regulator chops down the input voltage to keep the LEDs at the desired current. In a buck regulator, the output voltage must always be LOWER than the input voltage. Think of it this way: The input voltage is trying to spike the LED array too high, so the regulator chops it down to keep it correct.

2) Boost: The regulator boosts up the input voltage. The desired output voltage must always be LOWER than the input voltage. Think of it this way: The input voltage is not high enough, so the regulator switches it higher to keep the LEDs happy.

3) SEPIC: It doesn't matter. SEPIC topology uses two inductors, one in each position, so the voltage can be anything (in range of the chips min/max) compared to the desired number.

Buck and boost are much more efficient, but SEPIC is much more flexible. In a situation where input voltage might be all over the map (think: LED lighting in automobiles, where system voltage is pretty wild) SEPIC is good. But in our builds, we generally have DC power supplies that are nearly dead-stable, so we don't need the flexibility. Hence that leaves buck and boost. Boost is a little more efficient and allows for higher LED count on a given input voltage, but buck allows for more resolution when dimming. I picked boost for this design, mostly due to the efficiency.

Ok, moving on. In either buck or boost topology, the chip's switch basically creates a square wave that we don't want to expose the LEDs to. That's where many of the other components in the design come in to play - the inductor stores and releases energy in it's magnetic field as the switch goes on and off, allowing constant conduction instead of harsh on/off. The big cap on the output side further smooths the ripple, protecting the LEDs, and the big cap on the input side smooths the ripple the power supply is exposed to.

Reviewing the rest of the components: The little tiny cap on input and output just cancel high frequency noise, and the timing cap (on TCAP pin) sets the frequency of operation for the internal switch. Out of the major components, that leaves R1, which is a current limiting resistor for the chip - to basically limit the max peak current that can flow through it.

Now that we know what the different components do, we can have a thorough discussion of setting current. There are two things we can change related to current:

1) Drive current From above, we can adjust Rsense to adjust the drive current. What we're really doing here is adjusting the average drive current, since there will always be some ripple. Luckily, we can also change. . .

2) Current ripple We know that there's going to be some ripple on the output, and we can change the intensity and frequency of that ripple. To a certain extent, average drive current (#1) is the most important adjustment, but to be sure you're not frying either the chip or the LEDs you need to understand the ripple, too. There are lots of components that affect the ripple:

1) R1, the current limiting resistor. In normal operation, this resistor doesn't come into play, but if your design has a lot of ripple and/or a very high drive current, the chip will read that through this resistor and chop the current down. Basically the ripple will be a sawtooth wave, and this resistor will hack the tops off the sawteeth. This protects the chip from overcurrent (the chip's max is 1.5A). But if your driver is ever operating with this occuring, you probably won't be reaching your desired average drive current. IMHO it's better to design with an average current and ripple that don't allow this to happen.

2) C1, the big cap on the output side. This cap smooths the output so the LEDs see a less-harsh wave.

3) The inductor. A larger inductor means lower peak current through the driver.

4) The timing capacitor. This sets the frequency of operation for the chip's switch, which basically means it sets the frequency of the ripple in the output. A higher frequency means less ripple, but the chip's switch has a pretty narrow window of operation.

5) Potentially, using an external switch. You don't HAVE to use the built in switch - you can use it as a trigger for an external switch. This gives you much more freedom in the design, because you can use a larger frequency range, and not worry about heat dissipation in the IC itself. IMHO though this isn't really required unless you want very high drive currents, which aren't typical for our reef lighting applications.

6) The ratio of input voltage to output voltage. If they are close together, the ripple is lower, because there's less switching to do. IMHO this is the single most important factor in these designs because it ALSO effects overall efficiency - the lower the difference, the higher the efficiency of the IC itself, AND of the entire design, because there will be less losses through the external components, AND they can be smaller (less ripple!) So, when doing these drivers, figure out what your voltage drop across the LEDs will be, and use a power supply close to that value (above or below depending on buck or boost.)

You want low output ripple for a few reasons. The frequency of the ripple will always be above what we can see, so luckily there's no visual impact. Mainly it's because we want to keep from frying anything with peak currents being too high. The chip itself has a max of 1.5A, but the LEDs we're using typically have a max of 1A or so. Hence, if we raise drive current close to those limits, we NEED to limit ripple to prevent our LEDs and ICs from being exposed to peak currents beyond their limits.

Generally, it might seem like you want to just max all the above variables to get the least output ripple, but they alter the design in other ways (efficiency, etc) so you need to choose carefully. As I noted above, keeping input voltage close to desired output voltage is like free money, so do that first. All the other variables (increasing frequency, increasing inductor and output cap size) drop efficiency, so use them sparingly.

Circling back to the heart of the matter - I chose the component values above to give reasonably efficient performance at 500mA drive current. OnSemi actually has a spreadsheet on their website that will let you play with different component values and see what the resulting drive current and ripple is. If you're going for a higher drive current, you'll probably want to make some changes, to keep from spiking the LEDs too high. I'd probably start by increasing size of the inductor. The size I'm using (100uH) allows for 30% ripple at 700mA, which brings you pretty close to 1000mA in the spikes. I'd probably jump up to 150uH for 700mA, which drops ripple to 20%. These values are for input voltage of 24v and output of 28v (about what 8 LEDs will want). Like I mentioned above, it's also worth keeping the input voltage close to the output voltage. Many good DC power supplies have a trimpot that let you adjust the voltage. If you had a 24v supply and turned it up to 26v, you'd get 20% ripple with the 100uH inductor, which means more efficiency (the 100uH will have less resistance) and you'll have an overall increase of efficiency in general because less switching will be required.

The only downside to having a supply voltage very close to the output voltage is that for those of us that want dimming, you lose resolution. The chip cannot cut current below the input voltage, so the input voltage defines the "floor" of your dimming capabilities. What this means is that if you're driving 8 LEDs on 24v at 500mA and they require 28v to hit that target, you'll only be able to dim them down to 24v, which probably equates to 100mA drive current or so. It's pretty dim, but definitely not "off". For most of us this is probably fine.

Another note on circuit design. The cap I'm using on the input side is very big. I did this because I plan on having several of these per DC power supply, and I didn't want to hammer the PSs too hard by all the on/off switching. I've tested several of these running on the same PS and it works totally fine, so I probably have a too-large safety margin. You could probably use a lower value cap if you really wanted to.

The cliff notes:

1) If you want 350mA or 500mA drive current, do my design above. On a 24v supply, it'll run 8 LEDs at the selected current.

2) If you want 700mA, bump the size of the inductor up to 150uH.

3) In general, keep the power supply voltage close to (BUT NEVER OVER) the desired output voltage - unless you need really fine control over low-range dimmability.


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Unread 12/19/2009, 10:24 AM   #23
va7zt
Registered Member
 
Join Date: Dec 2009
Location: Robson, BC
Posts: 15
Right on Wille!

Thanks for the explanation....that's exactly what I was looking for. You explained things perfectly. I think I'll end up buying both the 100uH and the 150uH and then if I find I'm not getting the light deep enough I can put the larger one in..That's the beauty of doing the drivers yourself...one can play.

So would it also not be an idea to increase the size of the filter cap on the output to smooth the ripple a little more if one bumps up the current...or possibly add another 100mF? Most powersupplies have multiple filter caps on the output to provide a cleaner DC. Would doing this just be overkill given the amount of extra smoothing that would be gained?

Sorry if my questions seem basic! I've been out of electronics for many years! My understanding of the principals is pretty sound but my experience is a distant memory!!

Don


va7zt is offline   Reply With Quote
Unread 12/19/2009, 10:32 AM   #24
Axlerod21
Registered Member
 
Join Date: Nov 2009
Location: Albany, NY
Posts: 11
cutting cost

Seeing how DIY drivers will cut costs, I was wondering if anyone has found a different material to mount these lights to. Ive got a 150g tank and i would need 3 8.5x22 aluminum heat sinks costing around 175 dollars. I plan on using 72 cree lamp 32 white and 32 royal blue with some version of a DIY driver. Any ideas on cutting costs on these large builds would be great.


Axlerod21 is offline   Reply With Quote
Unread 12/19/2009, 10:42 AM   #25
der_wille_zur_macht
Team RC Member
 
der_wille_zur_macht's Avatar
 
Join Date: Sep 2003
Location: NY
Posts: 17,749
Quote:
Originally Posted by va7zt View Post
Right on Wille!

Thanks for the explanation....that's exactly what I was looking for. You explained things perfectly. I think I'll end up buying both the 100uH and the 150uH and then if I find I'm not getting the light deep enough I can put the larger one in..That's the beauty of doing the drivers yourself...one can play.

So would it also not be an idea to increase the size of the filter cap on the output to smooth the ripple a little more if one bumps up the current...or possibly add another 100mF? Most powersupplies have multiple filter caps on the output to provide a cleaner DC. Would doing this just be overkill given the amount of extra smoothing that would be gained?

Sorry if my questions seem basic! I've been out of electronics for many years! My understanding of the principals is pretty sound but my experience is a distant memory!!

Don
Keep in mind that the size of the inductor just makes the ripple a different shape - it's not what actually sets the current (Rsense does). In theory you could run a high current on a small inductor, but you'd have more ripple. If you think you might switch, I'd just build a proto of the driver both ways and see if there's any perceptible difference, and/or just go with the bigger one.

Regarding your cap question, you're starting to get beyond my range of knowledge - I don't think the cap size matters TOO much as long as it's about in the right range for the frequency of the noise you're trying to cancel. Too big and it'll react too slowly for the frequency of the ripple. I'm sure there are calculators out there that can help size caps for certain applications, I basically followed guidelines in the docs for this chip, staying a bit on the big size. You can technically run this without caps on the output side according to the app notes, but that would strike me as a noisy solution.

Also, bigger caps = more resistance = less efficiency!

Quote:
Originally Posted by Axlerod21 View Post
Seeing how DIY drivers will cut costs, I was wondering if anyone has found a different material to mount these lights to. Ive got a 150g tank and i would need 3 8.5x22 aluminum heat sinks costing around 175 dollars. I plan on using 72 cree lamp 32 white and 32 royal blue with some version of a DIY driver. Any ideas on cutting costs on these large builds would be great.
Look around metalworking supply shops in your area. I bet you could find some thick angle aluminum (U channel maybe?) that would work fine. IMHO most people are fairly overkill on heatsinking these things, especially if you stick with reasonable drive currents and use fans. Though overkill is better than not enough cooling!


__________________
Inconveniencing marine life since 1992

"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
der_wille_zur_macht is offline   Reply With Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On



All times are GMT -6. The time now is 09:31 PM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2024 Axivo Inc.
Use of this web site is subject to the terms and conditions described in the user agreement.
Reef CentralTM Reef Central, LLC. Copyright ©1999-2022
User Alert System provided by Advanced User Tagging v3.3.0 (Pro) - vBulletin Mods & Addons Copyright © 2024 DragonByte Technologies Ltd.