How to properly power 100W COB LED?
I've been playing with some Luxeon COBs wit a 1.2A test current and 2.4A max. 

It's all about thermal management.  You cannot pump these guys with max current without extraordinary thermal management. 

On your part, the most important spec in the datasheet is the thermal resistance from junction to case (thermal pad).   On the Luxeon COBs I have the thermal resistance is only 0.29° C/W.

You cannot connect these in parallel.  I have tried.  Even with matched forward voltages, (42.0V and 42.2V) it did not work. 

I also tried 5 ohm resistors, I have thousands of 5.1 ohm 5 watt resistors left over from a telecom product I used to manufacture. I had to connect 4 them in a 20 Watt series parallel network so they didn't get so hot. 

Connecting the COBs in parallel what happens is due to their thermal sensitivity there becomes a mismatch in forward voltage and current. It happened over a 12 hour period and the thermal runaway go to the point where one got all the current and the other almost none. 

This COB has been to hell and back as shown here.  This has gotten so hot the twice solder on the power pad liquefied. That happened at about 1.5 Amp while mounted on a 0.125" thick, 1" wide, foot long copper bar.  That bar is screwed down to a 0.125" x 0.5" copper bar that is soldered to a 0.5" diameter copper water pipe.



With no fan and nothing flowing through the copper pipe the max current would be less than 0.5 Amp.  At 500mA the temperature of the copper bar rose to 73.4º C in half an hour. Had I allowed it top go, the temperature would have continued to rise.



I always use a constant current source.  My favorite is the Meanwell HLG series which range in price from $30 to $200, ranging from 60 W to 540 Watt. 

If you build your own the BOM for a driver is under $2 for a buck step down.

For slightly less ($1.40 qty 10) you can use an LM3466 LED string load balancer. I will be trying this myself in the next day or two. This week I bought the TI eval board for this part  AN-2182 LM3466 Demonstration Board Reference Design.  

With COBs there would only be one COB per string.  The COB is essentially a string of 12 LEDs (3v x 12).

      

I doubt you will be able to push them over 1 Amp without them burning up or liquid cool.  So a HLG-540-42B would be sufficient or 2 x HLG-240.   I recommend the B model because it  has a dimmer as well as voltage and current adjustment.    

Or if you want to go green.  You can try this: