# LED mixing: 2 off-line drivers vs 1 powersupply + 2 buck converters

I want to drive two sets of 3 LEDs. They are all 35V but the two sets differ in color spectra and current: the first set needs 900mA the other 500mA.

I want to freely mix the two sets using PWM, so that the total power consumption always stays between 72W and 100W.

One easy way to do this would be to connect each set in series and use two dimmable drivers (e.g. ELG-100-C). But this seems to be quite a waste in terms of cost and size; and probably also efficiency considering one of the drivers might be much less utilized at any given time.

It seems to me a cleaner way is to use a 100W power supply and 2 buck converters.

But I have trouble finding an off the shelf constant current buck converter that supports PWM dimming and the 105V needed. Nor any reference design for such a thing.

Using boost converters seems to be more common - but this is likely to be less efficient. Another possibility around this would be to drive the sets in parallel and use a current mirror (or a more fancy LED balancer) - not sure how this fares efficiency wise.

My main questions are:

• Is using 2 buck converters a good idea worth investigating further? (Any pointers would be nice!)
• Is there another easy solution I'm missing?
• Am I in way over my head? I had some EE classes during my Computer Engineering Bachelor, and now again skimmed Chapter 9 of the Art of Electronics.

I am willing to dive into the material but want to avoid to find out I spent months working on an idea that was doomed from the start - because I won't be able to match (or exceed) the efficiency of two professional 100W LED drivers.

• Easy is subjective. Also, this is not a design service. I think you need to narrow your question down. As of now, it seems you're just throwing a problem at us along with some components (current limiters, power FETs etc) and expect us to solve it for you. – Dampmaskin Apr 24 '18 at 13:19
• @Dampmaskin Thanks, these are good points! I tried to fix it. – ian Apr 24 '18 at 14:59
• Why must it be a PWM signal? I assume you use a microcontroller as signal source. Why don't you output a dimming control signal like DALI and buy one of the numerous ECGs with a matching current/voltage/power rating? – Ariser Apr 24 '18 at 15:22
• @ian: No, I meant the control interface, not the output of the ECG. If it's only the output of the ECG which must be PWM there might be some solutions though. – Ariser Apr 24 '18 at 16:22
• Next question: Is it really necessary to put the modules in series? 35V modules tend to be as stable that you can put them in parallel if you take care of contact resistance and put them onto a common heat sink. – Ariser Apr 24 '18 at 16:36

The easy way is to use an HLG-40H-36B (750mA) and HLG-60H-36B (1.7A) at about $30 each. Powering 2 buck drivers with a single supply is a very valid method. I use a TI LM3414HV which is a simple inexpensive circuit. But that's too much like work for a one shot deal. You can use an eval board, from Infineon Technologies Part Number VALLEDILD6150TOBO1. LINK: Infineon VALLEDILD6150TOBO1 This is a very nice buck driver for$18 each. Up to 60V and 1.5A.

## UPDATE:

I was not sure when you said "They are all 35V" whether "They" meant sets or LEDs. I assumed sets because the rest of the sentence was about sets. So you have two sets of three 35V LEDs?

If you could find them the 2 buck drivers will likely cost more than using an additional LED power supply driver.

The only driver chip I could find above 105V is the Diodes Inc. AL1676 The datasheet example is an AC input but it can also be used as a high voltage DC-DC CC driver. They do not have an eval board for this part.

The current balancing the strings will be less efficient as it is the equivalent of a linear driver. You could use the TI LM3466 to balance the three LEDs in parallel to keep the voltage down.

But I don't think you want to design your own drivers.

The cost of two buck drivers is more than an ELG.

Your original idea to use the two ELG-100H-C was your best option. That is if you are powering it with over 200VAC, otherwise it is a 70W driver. The HLG-120H-C is a good alternative with 94% efficiency (vs ELG 90%) and an additional two years warranty.

• Thanks! The eval board is indeed beautiful, but I need 105V if I put the 3 LED modules in series. As mentioned, I have trouble finding buck drivers or reference designs that support this voltage. Or do you suggest hooking them up in parallel? – ian May 2 '18 at 20:55
• see my update.. – Misunderstood May 5 '18 at 20:14