LED constant current driver - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: July 02, 2013, 02:03:24 am
content: I'm building an LED device, and need to make a controller for it. These are pretty big LEDs though: 2-3.5 V, 13.5 A, 25% duty cycle. I have six of them. They're redPT-54 Phatlights, meant for projectors.I've never designed a power supply beyond a simple linear chip, so I need some help starting. I'm thinking about using aMAX16818, and driving them all in series from a big 12 V 18 A power supply. I'm thinking about using beefy overkill FETs, too.The device will be AC line powered, and efficiency isn't very important. I have a fairly big budget for a one-off, too. I'm most concerned about simplicity and longevity.The desired mode of operation is fairly simple. Normally I want it on but very dim, then ramp up to full power in a second or two on command. I'm guessing this is fairly trivial to do with a microcontroller and PWM.Is this a good driver chip to use? Which SMPS configuration should I use?Thanks!

author: Unknown
author_rank: Unknown
timestamp: July 02, 2013, 02:41:53 am
content: Wow, that is one honking big LED.

author: _Steven_
author_rank: Contributor
timestamp: July 02, 2013, 12:43:24 pm
content: please post your results. We can learn of this. Normally I drive smaller leds with a current source..but this is over my head. That many amps for 1 single led? 13,5 Amps? I cannot think about any linear device that can handle thisBut 13,5Amps??? are you sure??What kind of rectifier are you using than...and what kind of transformer...13amps is really alot.Can you please post some pics or some extra info.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2013, 01:11:23 pm
content: PC power supply, connect LEDs in strings of 3 in series with a resistor to allow operation from the 12V rail, then add a MOSFET to PWM the LEDs.

author: mariush
author_rank: Super Contributor
timestamp: July 02, 2013, 01:52:46 pm
content: Sounds like a normal computer power supply would be a good start ... those can do 3.3v and 5v at about 20a each (but about 130w in total on both 3.3v and 5v so about 15a on each is reliable) + 12A at about 30-50 amps. For 50-60$ it's very hard to do better.There are linear regulators like LT1083 and LT1084 and LT1085 that can do 7.5A and 5A and 3A respectively.LT1083/LT1084/LT1085 datasheet :http://cds.linear.com/docs/en/datasheet/108345fg.pdfLooking at the datasheets it seems thatminimumforward voltages are 2v and 3.5v, but you probably need to look at higher voltages:8.1A driven continuously :min VFmin 2 3.5 3.5 V (R G B )typ VF2.7 4.5 4.6 V(R G B )max VFmax 3.5 5.6 5.7 V (R G B)13.5A with 25% 50% 25% duty cyclemin VFmin 2.2 3.8 4.0 V (red green blue)typ VF3.0 5.0 5.1 V(red green blue)max VFmax 3.8 5.9 6.0 VLT1084 has a current limit set at about 6.5A so you could probably just pair two of them and set them to output 3.7-3.5v with a 5v input and rely on the built in current limitingjust the same you could use 3xlt1085 with its 4A internal current limit to get the 4.6-6v for green and blue from 5v or 12v (3 lt1085 to dissipate the heat easier), with 25% and 50% pwm you're basically going to run these 3 3/4 of the time.

author: Rachie5272
author_rank: Regular Contributor
timestamp: July 02, 2013, 06:16:57 pm
content: They're huge LEDs alright. Internally I think they use a dozen or so dice, but externally it acts like a single 2-3.5 V, 13.5 A LED. I'm trying to drive six of them in series.I'd rather not use a linear power supply, because of the huge power involved. I know the losses aren't as high as they would be with higher voltage, but it seems so inelegant.I have a nice big 12 V 18 A switching power brick meant for some sort of computer workstation, which will be a good starting point. I'm hoping someone can tell me if the Maxim chip is any good for this application, and which SMPS topology I should use (buck-boost, SEPIC, or ?uk).The LEDs are still in the mail, but I'll post pictures of my setup when they come. I'll be using three big CPU heatsinks just to keep them cool.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 03, 2013, 12:06:24 am
content: The buck topology is the easiest to stabilize and lends itself well to constant current applications. Very interesting LEDs you are using, btw.

author: Rufus
author_rank: Super Contributor
timestamp: July 03, 2013, 12:56:28 am
content: Seems a waste to switch mode from mains to DC then switch mode again for current control.Manson do some bench power supplies (re-badged by various companies) with digital and analog voltage and current remote control.http://www.manson.com.hk/products/detail/156I see the 32v 15A version listed at 179 euro.

author: lgbeno
author_rank: Frequent Contributor
timestamp: July 03, 2013, 03:36:50 am
content: Looks like a fun project, I would think that you could use some buck topology regulator and low side current sense with an opamp feeding the buck fb to trick the buck into regulating the voltage across the current sense resistor and hence LED current. Another amp could be used to scale the current up with a uC pwm. Doesn't Dave have a video on this from awhile back?13A is a lot though so there will certainly be some challenges.

author: Rachie5272
author_rank: Regular Contributor
timestamp: July 03, 2013, 03:40:40 am
content: It seems like a bigger waste to use a lovely bench supply in a dedicated application Besides, I won't learn anything.Buck alone won't work here, as my input source is right around the series forward voltage of the diodes. It needs to be able to produce a slightly higher or lower voltage.

author: IanB
author_rank: Super Contributor
timestamp: July 03, 2013, 04:00:45 am
content: Quote from: Rachie5272 on July 02, 2013, 06:16:57 pmIt's a minor detail, but read the data sheet carefully. Vf is 2-3.5 V when operated at 8.1 A continuous. Vf is 2.2-3.8 V when operated at 13.5 A 25% duty cycle. It also recommends the pulse frequency should be > 240 Hz when operated in pulsed current mode. It may perhaps be easier to design for the 8 A continuous case?You will need a big heat sink of course, possibly with a fan to assist it. A re-purposed CPU heat sink could be an option. What did you have in mind?Also I see there is a thermistor on board. Ideally you should monitor the thermistor in the driver circuit and reduce power in the event of the temperature being too high.I can't help with the choice of driver, but ignore suggestions of a simple series resistor. That's not sensible for high power LEDs. Look for dedicated LED driver modules. You can typically put these in parallel to get the current you need, and often they have a PWM input for brightness control.

author: Rufus
author_rank: Super Contributor
timestamp: July 03, 2013, 04:11:47 am
content: Quote from: Rachie5272 on July 03, 2013, 03:40:40 amQuote from: Rachie5272Make your mind up then.

author: lgbeno
author_rank: Frequent Contributor
timestamp: July 03, 2013, 12:41:15 pm
content: Quote from: Rachie5272 on July 03, 2013, 03:40:40 amIf simplicity is the goal, I'd suggest making the circuit buck mode only not buck/boost... Maybe a 24 V vin supply?

author: Rachie5272
author_rank: Regular Contributor
timestamp: July 03, 2013, 06:06:30 pm
content: I can do 24 V input. How does the efficiency compare to a 12 V buck/boost?It looks like I'll be making two of these now.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2013, 09:25:37 am
content: Why not just pick one of the hundreds buck current supply ics specifically designed for this?

author: Rachie5272
author_rank: Regular Contributor
timestamp: July 04, 2013, 05:53:01 pm
content: I thought that's what the Maxim chip was. Do you have a better suggestion?

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2013, 07:37:22 pm
content: There are specific ICs for non isolated LED drivers, although hundreds might be hyperbole.http://www.fairchildsemi.com/pf/FL/FL7701.htmlfor instance ... dunno if it can drive the great big MOSFET you will need directly though.

author: IanB
author_rank: Super Contributor
timestamp: July 04, 2013, 07:39:06 pm
content: Quote from: Rachie5272 on July 04, 2013, 05:53:01 pmThe MAX16818 does indeed look like it is designed for driving high current LEDs.