Power considerations for switching LED drivers - Page 1
Forum section: electronics / beginners

author: amaschas
author_rank: Regular Contributor
timestamp: June 10, 2020, 10:07:26 pm
content: Hi all, so I've recently been working on a project that involves driving LEDs, and I've hit a point where I'm confused about the best way to proceed re: my power requirements and how best to manage them.My specific implementation uses the Luxeon color 2835 series of LEDs, specifically the deep red, green and royal blue colors, and the datasheet recommends an optimal current of 120mA for all colors. As you would expect, each LED color has a different voltage drop range at 120mA:deep red: 1.8-2.5V, 2.15V nominalgreen: 2.8-3.3V, 3.15V nominalroyal blue: 2.8-3.2V, 3V nominalMy initial plan was to run the LEDs in series strips of 8 with a constant current driver. For my first prototypes I used a 24V power supply and a constant current buck regulator, and while everything worked the 24V power supply is insufficient to guarantee that the green and blue LED strings run at 120mA—though just barely.I was advised in an earlier thread I posted here to stay well away from designing my own AC/DC power supply, so I'm stuck with off the shelf options in the event I want to distribute my light as a product in the future.24V 2A supplies are common and affordable, and output enough power to drive my LEDs, but I would presumably either need to cut my series down to 7, (requiring a redesign of my LED boards) or step the voltage up to use one.28V power supplies are more limited, more expensive, and tend to be overpowered for my needs.So at this point I'm wondering if I should bit the bullet and cut my strings down to 7, step the 24V up before the driver stage, or bump up for a higher voltage power supply. Also I'm wondering if a constant current topology exists that does not alter the input voltage.

author: ledtester
author_rank: Super Contributor
timestamp: June 12, 2020, 09:39:15 am
content: > Also I'm wondering if a constant current topology exists that does not alter the input voltage.There are buck+boost DC-DC converters (and led drivers) available, but you pay for it in efficiency. It would, however, allow the use of a wider range of input voltages.If you are going to have the end-user provide the power supply, I would make it as easy for them as you can. know I've passed up on a lot of components because of odd or difficult to source power requirements.Also, it's always good to leave "headroom" in your design to account for variances in components and aging. I don't know what a good margin would be in this situation, but consider that the forward voltage of the green leds could be as high as 3.3V so even a string of 7 could result in 23V which is cutting it close for a buck converter with a 24V input.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 12, 2020, 10:01:36 am
content: 36V and 48V are also fairly common power supply voltages.The framed power supplies (Alu. backplate, crome plated top) often have a voltage adjustment pot. The adjustment range may just be enough for you.In TV's it's fairly common to have long strings of LED's in series (Up to 100V or so total) and power them from a step-up current regulator. Some (most?) of these regulators do not go below 10% for dimming. I have no idea of efficiency of these things.

author: gcewing
author_rank: Regular Contributor
timestamp: June 13, 2020, 07:14:48 am
content: Does each string have to be all one colour? If you could put some of each colour into each string, you may be able to get the string voltage down within the capabilities of your supply.