The case for constant current drivers:
If you take a look at high powered LEDs, one unique characteristic is the exponential relationship between the applied forward voltage to the LED and the current flowing through it. You can see this clearly from the electrical characteristics of the Cree XP-G2 below in Figure 1. When the LED is turned on, even the smallest 5% change in voltage (2.74V to 2.87V) can create a 100% increase in current driven to the XP-G2 as you can see at the red marks current went from 350mA to 700mA.
Now higher current does make the LED brighter, but it also will eventually over-drive the LED. See Figure 2 for Cree’s specifications of the maximum forward current and the de-rating curves in different ambient temperature conditions. In the example above we would still be alright driving the XP-G2 LED at 700mA, however, if you didn’t have a current limiting device, the LED would draw more current as it’s electrical characteristics changed due to temperature increase. This would
eventually push the current way above the limit…especially in hotter environments. The excess forward current would result in extra heat within the system, cut down on the LEDs lifespan, and eventually ruin the LED. We call this thermal runaway which is explained in more detail here. This is the reason the preferred method of powering high powered LEDs is with a constant current LED driver. With a constant current source, even as the voltage changes with temperature the driver keeps the current steady while not over driving the LED and preventing thermal runaway.
When do I use a constant voltage LED driver?
The above example is with high powered LEDs and on a smaller scale as we only talked of using one LED. With lighting in the real world, it isn’t convenient or economical to build everything by hand from a single diode, LEDs are usually used together in series and/or parallel circuits to create the desired outcome. Fortunately for lighting designers, manufacturers have introduced many LED products to the market that have multiple LEDs already assembled together like LED rope light, LED strips, LED bars, etc.
The most common LED strips are designed with a group of LEDs in series with a current-limiting resistor in line with them. The manufacturers make sure the resistors are of the right value and in the right position so that the LEDs on the strips will be less prone to the variation of the voltage source as we talked of with the XP-G2. Since their current is already being regulated, all they need is a constant voltage to power the LED(s).
When LEDs or an array of LEDs are constructed like this they will typically state a voltage to be run at. So if you see that your strip takes 12VDC, don’t worry about a constant current driver, all you will need is a 12VDC constant voltage source as the current is already being regulated by on board circuitry that the manufacturer has built in.
Advantage of using a constant current LED driver
So when you’re building your own fixture or working with our high powered LEDs, it is of your best interest to use constant current drivers because:
They avoid violating the maximum current specified for the LEDs, therefore avoiding burnout/thermal runaway.
They are easier for designers to control applications, and help create a light with more consistent brightness.
Advantage of using a constant voltage LED driver
You use a constant voltage LED driver only when using an LED or array that has been specified to take a certain voltage. This is helpful as:
Constant voltage is a much more familiar technology for the design and installation engineers.
The cost of these systems can be lower, especially in larger scale applications.
Feel free to take a look over at our guide to LED strips that has a lot of devices that can be run from constant voltage. Also, if you need help selecting a constant current LED driver, head over to our helpful post on how to choose the right one.