The forward voltage (Vf) is the voltage across the terminals of the LED. The rated Vf (which is what binning information is based on), is the voltage across the LED when the rated current is flowing through the LED.
Increasing the current will result in a higher Vf, and decreasing the current will result in a lower Vf. Vf and current are tied together along a curve that the LED operates at. This is the I/V curve.
There are two ways of looking at it, depending on how you think of powering the LED.
First, let's look at things in terms of powering an LED properly, with a constant current power source. In this case, the Vf depends on the current through the LED. So, The I/V curve looks like this: (this is a TV1K Luxeon III)
When controlling the current, the Vf changes only a little across a wide current range. In fact, when powering the LED with a constant current source, Vf isn't of too much concern, which is a good thing (I'll explain this a bit later)
Now, lets look at driving the LED in terms of supplying a constant voltage - which is what we're used to for driving things like motors, light bulbs, etc...
This is the same data, only I've flipped the graph, so now the current varies depending on the voltage across the LED - as if you used a voltage supply to power the LED.
Now, if driving an LED with a constant voltage supply, things get quite a bit more sensitive. Changing the voltage by less than 1V results in a 10X change in current. It would be rather difficult to ensure that an LED is being properly driven by just supplying a constant voltage to it.
That's not the only challenge. Every single LED has a different Vf. Binning ranges of Luxeon LEDs are pretty big. For instance, the K-bin range is 3.51V-3.75V. For Luxeon III LEDs, this measurement is taken at a current of 700mA. So, lets say we want to drive this particular Luxeon at 700mA, using a fixed voltage supply. We would have to set our power supply to about 3.68V. That might be fine for that one particular Luxeon, but if you wanted to set up a bunch of different Luxeons to be driven at 700mA, you would have to tune each voltage source to match that particular Luxeon's characteristics. You'd also have to make sure that your power supply could supply that exact voltage without drifting due to temperature, load, etc. A very difficult method.
But there's more
The Vf varies, not only on an individual basis, but with temperature. As the LED heats up, the Vf decreases, and increases as the LED temperature decreases. The change is only a vew millivolts per degree Centigrade, but, as you can see in the 2nd graph, very small changes in voltage result in large changes in current.
In the most extreme case, the LED heats up a bit, the Vf drops a bit; which, when combined with a fixed voltage source, results in more current going through the LED. Thus, the LED gets hotter. So the Vf drops more, drawing more current, etc, etc. This is called "thermal runaway", and if not stopped, results in the destruction of the LED, either from too much current, or too high of temperatures.
We're still not done yet. /ubbthreads/images/graemlins/tongue.gif
Vf also varies with time. Luxeons experience a reduction in Vf over their operational lifetime. The vast majority of the reduction happens in the first 100 hours or so of operation. I have done experiements verifying and quantifying this change in Vf. Below, is a graph that shows the above Luxeon after its Vf shifted after 100 hours:
And, the two plots together (initial Vf curve in blue, 100 hours later in red)
So, if you had started by using a fixed voltage supply to power this Luxeon, set at 3.68V, you would no longer have 700mA of current. You would now have about 1900mA of current through the Luxeon, or over 2.5 times the initial current. If left at that current level for long, that Luxeon is well on its way to destruction!
So, to conclude:
- Vf is the voltage across the LED
- The rated Vf (what you will see stated for an LED, or what a Luxeon will be binnned as) is the voltage across the LED at the LED's rated current.
- Vf and current are tied to each other across the I/V curve. The LED operates somewhere on this curve
- Vf varies on an individual LED basis. Two Luxeons from the same Vf bin will probably have different Vfs. Vf binning information is really provided to give a hint at characteristics, not specific information for powering. That's why Vf bins are in a range instead of exact values.
- Vf varies with the temperature of the LED. Higher temperatures lower the Vf (temporarily)
- Vf changes with use time (at least in Luxeon LEDs, probably in 5mm LEDs too). As LEDs age (while powered up), the Vf drops. This is a permanent change.
- Driving LEDs with a constant voltage sources is a bad idea. Direct driving LEDs from batteries is just one step from this configuration.
- Driving LEDs with a constant current source is a good idea. Changes in Vf are of no concern when using a current source.
Probably more information than you bargained for /ubbthreads/images/graemlins/tongue.gif