The LED thread - High Power LED drivers

I was browsing CE and it hit me that we really miss a dedicated thread about LEDs. Does that matter? No, it doesn't. But, I thought it would be great to have a thread about LEDs when there are so much about them happening around us. (This thread is about High Power LEDs and methods to drive them).

Warning: This is a long post!

You probably are staring into an array of LEDs directly or an illumination created by them right now when you are reading this (I meant your LED and LCD displays 😁). Light Emitting Diodes emerged from being mere indicator lamps to seven and more segment displays to ultra awesome OLED screens rapidly. One LED in an OLED screen is super small. But, LEDs are not just getting smaller, they are growing bigger too. High power LEDs! They are taking over conventional Fluorescent Lamps and Compact Fluorescent Lamps quite fast (I rule out Incandescent lamps here as those have almost already been replaced by CFLs or are being replaced. Behold, here comes LED!). This post is about them - High Power LEDs and driving them.

A word about LEDs
Light Emitting Diode, as the name says, is a p-n junction diode that emits light when energised. They conduct only when forward biased and will be open in reverse biased condition. When forward biased, electrons and holes in this semiconductor device recombine emitting light energy, and that is called electro-luminescence. The wavelength of emitted light is dependent on the band gap. You probably remember connecting the positive of the battery to that longer leg of LED, which we call anode, and the negative to the other one, cathode, while playing with those red, green and yellow ones many years back. The greatest things about LEDs are that they are very sturdy and consume very little energy to function. Same is true with high power LEDs when compared to other artificial light sources.

You probably are familiar with bulb-shaped LED lamps designed to replace CFLs and Incandescent lamps for interior lighting. Those are made up of a few high power LEDs working together under that white diffuser. By saying high power LED, I mean, those which consume around one watt or more power. Typical indicator LEDs are rated at around 50mW of power.

It is fun dealing with High Power Light Emitting Diodes (HPLED). Those little, blindingly bright devices can make way for amazing DIY projects and save energy in real. Many newer automobiles are using HPLEDs even as head lights! Gone are the days of LED brake lights 😁 That is pretty cool! Commercially available LED lamps do seem to be pretty costly. Though in the long run, it turns out that LED lamps have the least running cost. But, yeah, as engineers we can build it cheaper ourselves, right?

Here are a few facts you probably didn't know before.
1. LEDs generate heat, a fair amount of heat.
It is true that LEDs don't emit Infra Red rays unless it is an IR LED you are dealing with, which makes their light cool. But, the p-n junction gets hot, very hot in the process. More than 70% of electrical energy an LED consume is converted to heat. (Don't sound very efficient, eh? well, they are, when compared to other light sources of equivalent power.) Well designed heat sinks should be used to remove heat from the semiconductors, or it will fail!

2. All LEDs do not have huge efficacy (lumens/watt output).
Most commercially available LED lamps which are not so costly has around 70lm/watt efficacy. But, if you are ready to pay more, branded LEDs are available with 130 or more lumens/watt efficacy.

3. All LEDs do not produce superior quality light than any existing artificial light source.
The quality of lighting is measured based on Colour Rendering Index (CRI) and warmth of light output. CRI represents the ability of a light source to reproduce colour of an object when compared to an ideal light source. Warmth - LEDs are available with a wide range of colour temperature these days. Better quality LEDs have very good CRI and consistent colour temperature. But again, you have to pay more for these qualities.

Now, let us get to driving LEDs.
The biggest problem when dealing with LEDs is that they should never have the current increase beyond the maximum value even for once. If current increases beyond that critical value, the device will be permanently damaged in 99% of the cases! For this reason, LEDs aren't run at their maximum rating, but 50mA or 100mA or even more lower than the maximum. So, if I have an LED with 700mA maximum rating, I'd probably run it at 500mA or 550mA or even less. It'll depend on how good a heat sink I use too.

Say, we have an LED with forward voltage 2V at current 20mA. So connecting it across a 2V DC supply in forward bias should illuminate it, right? Yes, in a perfect world it'll. In a perfect world a 2V supply will always supply 2V with zero error and a 2V LED will have always 20mA flowing through it when 2V is applied across the terminals. But that cannot be the case, there are errors in the world we live! The source might have an output of 2.03V or the LED will have 20mA flowing at 1.98V itself. Those are enough parameters to overdrive the LED and make it live drastically less. An LED exhibits similar characteristics as that of a p-n junction diode for it being a diode too. Once turned ON, a very small change in forward voltage will cause a large change in the current flow in a diode. If the source is stable, the LED can be directly connected to the source (not at all recommended). But if it is not, the good old current limiting resistance comes to the rescue.

1. Series current limiting resistance driving (This method isn't good to be used with HPLEDs)
When the voltage source from which an LED has to be driven is higher than its forward voltage or not stable, a series resistance is connected to the LED to keep current flow within the maximum rating. The series resistance is designed to drop the extra voltage and thus keep things safe.

If a 6V battery was to be used to drive that 2V, 20mA LED in the previous example, we can figure out the resistance as given below,
R = (Vs-Vf)/If
R: Series resistance
Vf: Led forward voltage
Vs: Source voltage
If: Forward current
So, R = (6-2)/0.02 = 200ohms
A slightly higher value should better be chosen to keep the current below the maximum.

If a series resistance was to be used with HPLEDs, it would dissipate a lot of power. A few hundred milliamperes will be flowing through the resistor. Unwanted wastage of power there and will need a resistor with large power rating.
Consider a 3V, 400mA HPLED is to be powered from a 6V source in the same way.
R = (6-3)/0.4 = 7.5ohms
Power dissipated in resistor, I^2R = 0.4X0.4X7.5 = 1.2watts
Now, that is crazy. Resistor has to dissipate as much power as the LED consumes (3V X 0.4A = 1.2watts).

So we use much better and fairly efficient alternatives - a buck-boost converter or a constant current driver.

2. Buck/Boost/Buck-Boost converters
A Buck or Boost converter is an electronic circuit which can very efficiently step down or step up DC voltage as desired, respectively. A semiconductor switch (like MOSFET) is operated at high frequency in series with the DC voltage source to have a variable DC at the output. The magnitude of output voltage is determined by the turn on and turn off duration of the semiconductor switch. The only loss encountered in this circuit is the switching loss which s quite low and that makes it very efficient. A buck-boost converter can drive HPLEDs very efficiently by supplying a stable voltage across them as required and keeping the current within the limits. Read more about buck, boost and buck-boost converters here (wikipedia links): Buck Converter, Boost Converter, Buck Boost Converter

3. Constant current drives
Drive current of LEDs has a lot to do. You have the control over current, you decide how much light the diode gives out, how much heat it generates and hence how long it'll last. Constant current drive is an excellent way to drive high power LEDs.

I was in the process of designing one full fledged buck-boost or at least a buck converter when I encountered a very simple, cheap and fairly efficient constant current drive circuit. For quite some time now, I've been driving a bunch of 3watt LEDs using it, with slight modifications to suit my own needs, from various constant and variable sources. Very stable and efficient with good control over drive current, a very good place to start. Thanks to Dan from instructable.com #-Link-Snipped-#. Complete instructables page here: #-Link-Snipped-#

We have a lot of crazy heads here. Join hands and share what you know and learn what others share. Share and discuss about drive circuits you might already know or have in mind. May be we can try make a fairly simple yet efficient and cheap HPLED circuit, or say a buck converter. As a second step, make it work with DC as well as AC inputs.

Oh, Yeah! Don't forget the heat sink. If you are to drive a HPLED, make sure you fix them to a heat sink for drive currents above 150mA. The fingers are quite useful too, to see if it's getting hot; really hot! 😁