When you’re designing a COB LED system, it can be difficult to compare COBs of different brands or models with one another. If you were to try to do it just by looking at the data sheets for each model, you likely wouldn’t quite end up with the exact numbers you were hoping to get, and would have to base your estimates on graphs, then do a bunch of math. Nobody likes math.
The easiest way to compare COB specs is by using the manufacturer’s simulation tool. Most manufacturers release a spreadsheet that allows you to input certain values and then spits out a number of different specifications. If you have an idea of the color temperature, voltage, or current you’d like to run at, you can plug these values into the spreadsheet and see how the various specs interact with one another.
One particularly important specification that will likely determine which COB you use (aside from price) is efficacy. Efficacy refers to the number of lumens the COB produces per watt of power. As discussed in this post on methods of measuring light, lumens are not the ideal unit of measure for horticultural applications (PPF or PPFD are), but they still serve as a solid way to compare one COB versus another. Efficacy is not to be confused with efficiency, which refers to the percentage of electrical power that an LED is able to convert to photons vs. the percentage of that power that’s wasted as heat. While efficiency is also an important metric, it’s much more complicated to derive, and is beyond the scope of this post.
Let’s have a look at the 3 most popular COB manufacturers’ tools:
- Cree’s Product Characterization Tool (PCT)
- Bridgelux’s Product Simulator
- CitiLED’s Selection Simulator
We’ll use these tools to compare a few of the most popular COBs on the market right now. Let’s say we want to use 36V 3500K COBs and throw them on a 1,400mA driver. We’ll examine Cree’s CXB3590, Bridgelux’s Gen. 7 Vero 29, and CitiLED’s CLU048 1212.
Cree Product Characterization Tool
Cree’s PCT can be found here. It’s a very versatile tool that allows you to compare a number of different specs from the 4 dropdown menus. We’ll just leave the default specs for now, since they have what we want to see.
How to Use It
- The first thing you need to do is select the model of COB you want to look at. We’ll use the CXB3590.
- Next, under “Flux”, choose the bin for the color temperature you want to compare. Cree “bins” its COBs based on their luminous flux output – for example, in the 3500K color temperature, the best and brightest bin available is the CD bin, which promises a minimum luminous flux of 12,000 and a maximum of 13,000. Make sure when you buy Cree COBs that you’re getting top bin.
- Set the case temperature. The default setting is “Tj”, which refers to the temperature of the actual junction in the diode. You need to switch to “Tsp”, which is the temperature of the solder point, or case. I use 50 degrees Celcius as my temperature, since they’ll definitely be warmer than the default of 25 degrees.
- Adjust the current range and set it to “V. Coarse”. If you don’t change this, the range will be too fine, and you won’t see any data since it’ll only show very low currents.
Now, simply move down the current column to find the value you want to compare. You can see that at our desired current, the CXB3590 produces 7,926.5 lumens at a voltage of 34.48V, which translates to 48.276 Watts (1.4A x 34.48V = 48.276W). At this current and case temperature, the efficacy of this COB is 164.2 Lumens per Watt.
Bridgelux Product Simulator
You can download Bridgelux’s Product Simulator here.
How to Use It
- Select your color temperature and CRI. We’re going to compare 3500K, and I’d recommend using 80 CRI (if you use 90 CRI, the luminous flux will be significantly reduced; output is more important than accurate color rendering in this application).
- Set the case temperature. We’ll go with 50 degrees Celsius, as we did with the Cree.
- Enter the current level you want to compare the COB at.
- Veros come in different array configurations (the individual diodes on the COB are wired in different series and parallel configurations) – find the configuration with the forward voltage most similar to the other COBs you’re comparing. In this case, we’re looking at the Vero 29-D, which has a forward voltage of 35.8V.
Once you enter the drive current, the fields will update for each COB. At 1,400mA and a 50 degree case temperature, the Vero 29-D produces 7,945 lumens at a voltage of 35.8V, which translates to 50.2 Watts (1.4A x 35.8V = 50.2W). At this current and case temperature, the efficacy of this COB is 158 Lumens per Watt.
CITILED Selection Simulator
You can find the Citiled Selection Simulator for Version 5 COBs here.
How to Use It
- Select your color temperature and CRI. We’ll use 3500K and 80 CRI, as we chose above.
- Set the forward current. Again, we’re looking at 1,400mA.
- Set the case temperature, same as the others.
- Find the row with the COB you want to compare. Here, we’re interested in the CLU-048-1212C4-353M2K1, since it uses about the same amount of power as the other COBs at this current.
The CLU-048-1212C4-353M2K1 produces 7,527 lumens at a voltage of 36.1V, which translates to 50.5 Watts (1.4A x 36.1V = 50.5W). At this current and case temperature, the efficacy of this COB is 149 Lumens per Watt.
Now that you know how to use each of these simulators, you can play around with all the variables and find out just about anything you need to know. You’ll notice that different COBs are more effective at different currents – the CXB3590 is very efficient at low drive currents like the 1,400mA value used above, but loses this advantage at higher currents. For example, as shown below, if you were to drive your COB at 2,100mA instead, the CXB3590 produces 147 lumens per watt. At the same current, the Vero 29-D produces 148 lumens per watt. Considering the Veros are about half the cost per COB, they would be the better bet at this current.
Another handy use for these simulation tools is to use them to find the exact forward voltages the COBs will run at, at a given current. For example, if you want to put 4x Cree CXB3590s on a single Mean Well HLG-185-C1400 driver, if you were to go by the typical forward voltage of a CXB3590, (data sheet says 36V), you might think that this won’t work. An HLG-185-C-1400 driver has a maximum voltage of 143V, so 4x 36V COBs in series would add up to 144V… one measly volt over the limit.
However, since we know that forward voltage of a COB will vary based on the amount of current running through it, we can use the PCT to check and see exactly how much voltage drop we can expect. The tool states the Vf will be 34.48 at 1,400mA, which means that 4 of these COBs will actually total 137.92V, which is within the limit of the driver.
Now fire up these tools, start plugging in some numbers, and buy your damn COBs already!