Running Multiple LED Strings With Equal Power

[LED Boatguy]

Here's a pretty good idea from an Engineer over at On Seniconductor. Usually, when we run multiple strings (arrays) we run the risk of having current hogs and cascading failures when a string drops out. This is because there is a single sense resistor that sets the total output and leaves each LED string to battle it out.

Sooooooo this guy simply divided up the sense resistor by three and ran each string with one plus a 10 cent transistor.

OK, forget the 317 is there and pretend there is a real CC power supply feeding this array. It'll certainly work with linears, switchers may be a different matter. I'll be spending some quality time with this as I've declared war on the T-12 fluoros around the house that keep burning out.

http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF

Edit: Ya know; these strings are switchable. Imagine how this could work in a 3-way light :thumbsup:.

 

[2xTrinity]

Interesting -- this is defintely a tough problem, and IMO this is a great solution for regulating say lots of strings of 5mm LEDs cheaply. For power LEDs, IMO it's still not a bad idea to use separate regulators on each string, and simply gang the controls all together, or something like a switching mode voltage regulator to bring the voltage down to close to the Vf of the LEDs, then using a LDO linear regulator to regulate current to each string.

There are a a caveat to the circuit designed. For example, the PT4105 switching regulator IC used in the "Kennan" at Kaidomain, and several other similar drivers, operates using a 0.2V current sense feedback. However, in order to trigger the transistors as shown in this diagram, the sense resistors will have to be dropping at least 0.7V. In a high power application with 1A through each string, that's 0.7 watts lost in each sense resistor. That's a fair amount of power loss, though if you really needed to, yo ucould use something like a quad comparator or opamp chip to amplify the sense voltage to be enough to drive the transistor on.

 

[LED Boatguy]

Well, I'm gonna get a chance to try the straight array thing as I just ordered a couple 40W 1750ma Xitaniums using the Live.com 30% off Ebay special.

Anyways, it'll be interesting to see how the 80% efficient Xitaniums stack up against level 4 and 5 efficiency high voltage switching power supplies driving loooooooong strings of LEDs.

PS I have dozens of 22 X 3 5mm arrays out there fed by a single $1 linear 450ma PS with no current balancing at all. Not a single failure to report. Looks like I'm going to get a chance to find out if the big boy LEDs play nice together.

 

[CM]

This technique has been around for a long time. For example,

http://preview.tinyurl.com/3faemp

Those skilled in the art, to borrow from patent speak, can extend the idea for distributed driving.