To make sure we're all clear on this, I whipped up some Paint magic. Let's discuss two scenarios. We assume we have a driver, and two models of LEDs:
1) The driver has a maximum voltage of Vmax, and a current of I.
2) LED A has a maximum current of Ia, and a voltage drop of Va.
3) LED B has a maximum current of Ib, and a voltage drop of Vb.
The first scenario is the most typical arrangement. We're putting a single string of 6 LEDs in series on the driver:
We have four of LED A, and two of LED B. Let's talk about limits, how the LEDs match to the driver, and so on. Firstly, choosing a current to drive the LEDs at. We need to make sure that the chosen current, I, is BELOW both Ia and Ib, the limits of our LEDs. Next, let's talk voltage. We need to make sure that the driver's max voltage, V, is above the sum of the LED's voltage drops, which in this case would be 4 * Va + 2 * Vb.
Let's look at a real-world example. Assume LED A is similar to a Cree XP-G. It's going to have a max of 1.5A and a Vf of 3.5v. LED B is similar to a Cree XP-E royal blue. It's going to have a max current of 1A, and a Vf of 3.3v (number off the top of my head, verify in the datasheets - see note below.). So, if we were to match this string to a driver, we'd need to make sure we were running the driver with a current BELOW 1A, because that's the lowest max in our string. And, we'd need to make sure that the driver's voltage limit was ABOVE 4 * 3.5 + 2 * 3.3, or 20.6v.
Now, example B. We have two of the above strings on a driver:
The voltage calculation is the same, but we MUST ensure that, at our target current, the voltage drop in the two strings is the same. For starters, we MUST compose them the same - notice I have four of LED A and two of LED B in the strings. I CANNOT have one string of LED A and a second string of LED B unless they have the SAME EXACT characteristics, which is generally never going to happen from color to color or model to model.
When calculating current, I need to choose my max current in each string the same way (it has to be below the lowest max in the string) but I need to choose a driver that will provide twice that chosen current, because I have two strings, and if the strings are of equal Vf, they'll split current equally.
A few notes on all of this. Firstly, when you're adding op the Vfs of your LEDs to determine the voltage capacity you need from the driver, make sure you're adding up the RIGHT Vfs. The Vf of a given LED will vary depending on the current. Most datasheets provide either a few datapoints and/or a graph. For instance, an LED might have a Vf of 3.5v at 1A, but 3.3v at 700mA.
Secondly, consider that the Vf of a given LED may vary considerably from the specified value. In other words, there's a lot of variation in this spec from individual LED to individual LED. in a typical setup where you have a single string of series LEDs on a driver, this is a non-issue, as long as you don't end up with an entire string of extra-high Vf LEDs and exceed the max voltage for the driver (in which case your LEDs would typically end up under driven, though it will depend on the design of the driver). This variation is MUCH MUCH more critical in the case of parallel strings of LEDs. This is why we're making so much noise about checking Vfs, putting resistors inline so you can monitor current, and so on - you MUST MUST MUST check, and compensate for, differences in Vf from string to string when building a parallel array on a driver in the real world.
Hope this clears things up instead of making them more cloudy!
Small differences but they were there nonetheless. I'd probably take kcress's suggestion and put a 1 ohm resistor in each line for easy checking of current balance.
