LED drivers. Why, how and what?

I am a hobbyist in electronics with knowledge of circuits and fabrication. But lacking knowledge about this topic. I would be greatful if some one answers my silly questions mentioned below.
Today we have ample LED,s available in 1 watt, 3 watts and 5watts in "star" package with different optics. I recently bought a 1watt x 3 LED's fitted in a spot light enclosure. A driver is also supplied with that which connects to the 220 volts power source.
My question is when a LED can be lit with a series resistor to a DC or battery source, why we require a driver to drive the same? What is the advantage of using a driver? What is the function of a driver?
If i am intending to make a driver for 3watt Star LED, which will power it with 220 volts, How can i move ahead? Are there any reference designs or links?
What is the difference between a regular SMPS DC power supply and a LED driver? Can a SMPS power supply be used in place of a LED driver? If yes, How? and if no why?
Thanks lot...Neha

The LED doesn't really care where it's power comes from so the problem is how to efficiently provide it when the source is the wrong voltage.

Take a typical LED like you show, the forward voltage may be 6V and it's current requirement may be 1A, it varies from one type to another so I'm using these figures as an example.

If you just used a bridge rectifier and dropper resistor, it would need to be:

Resistance = V/I = (220 - 6) / 1 = 214Ω
Power rating = V x I = (214 / 1) = 214W

So you have a super efficient LED but an overall efficiency of less than 1% and you need a resistor the size of a horse and a cooling fan to stop it overheating.

So the alternatives are to use a transformer to drop the 220V to a more manageable level or an SMPS to do the same. Given the size and weight of a transformer and it's likely mounting position n a ceiling, SMPS wins hands down.

The only difference between a normal SMPS and one for driving an LED is that LED characteristics are set by current and not voltage so by various means, they stabilize the output current to match one or more LEDs in series rather than stabilize the voltage which would still need a series resistor to limit each LEDs current.

Brian.

Thanks for the guidance Brian. But i have still some silly questions as i am still unable to understand the concept of a driver for a LED.

For ex, When a LED requires 2 volts as a forward voltage, and when the is fed, the led starts glowing. But will not it consume only the current in proportion to its resistance when a regulated voltage is fed? Then why a current control is required?

If a current control section is designed for 100 ma, and we connect a led which consumes 150 ma, what shud be the result of the driver output? wether it shud shut off or go Dimm?

Another quetion is , if i have an LED which is not supplied with any datasheet, how can i find out the forward drop of the same?

Thanks a lot...
Neha

Try to think of an LED as being a perfect stabilized voltage source (it isn't of course, but bear with me) and the voltage it produces is 'Vf', the forward voltage drop. Now imagine that inside the package is a small resistor, wired in line with the LED and it's connecting wires.

If you connected a variable voltage across the LED and start to increase it from zero upwards, up to the point where you reach Vf, no current would flow because you have not yet reached a voltage higher than the Vf voltage. As you apply a voltage higher than Vf, the current rises very quickly, all you are doing is applying it across that small resistor.

That is greatly over-simplified but hopefully it demonstrates how the voltage and current interact with each other. The resistor doesn't physically exist in the LED, but is a property of the materials in its semiconductor junction. However, the heat dissipated by this imaginary resistor is very real and causes the LED to heat up when current flows through it.

Now you see why LEDs are current driven rather than voltage driven. If you control the current, you can control the dissipation in the LED to keep it within safe limits. If you tried to put a fixed voltage across the LED, it would try to draw as much current as that imaginary resistor let it and the result would likely be catastrophic. To put the problem in proportion, if Vf was 2.0V, it would likely damage the LED if you applied as little as 2.1V.

Because they have a constant voltage drop, you decide the current they can draw. The manufacturer will specify the safe limit and as long as you stay below it, the LED should have a long and happy life. If you allow too much current to flow, the LED might be a little brighter but it would overheat and be damaged. Your LED rated at 150mA will just be less bright at 100mA but well inside its safe limits.

To find the forward voltage drop of an LED, you need to pass a current through it and measure the voltage across it. The voltage across it when it is glowing is Vf. Note that imaginary resistor is inside the LED so Vf will change slightly as you alter the current but it will stay fairly constant over a wide current range. When you see a manufacturers data sheet it will always tell you the current Vf is measured at.
Note that most small LEDs, the 5mm ones are usually only rated at 20mA or so keep the current low when you take a measurement. As a general rule assume that Vf is about 2V (it varies according to LED type and color) and add a series resistor of (Vsupply - 2)/0.02 Ohms to keep the current at about 20mA.

Brian.

The pictures and discussed current levels suggest, that you are refering to lighting applications of LED. In this field
efficiency is the keyword and a sufficient reason to avoid resistors and also linear regulators for LED supply.

Temperature dependance and type variation is another reason to use (mostly switched mode) constant current drivers.

But you surely know these simple LED flashlights, that are powering a LED just from a battery. They work as well.

Thanks Brian for bieng so patient to expalin it in very detail. Thanks to Frank too for the cute little explanation.