Current Control for LED driver

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LED lighting in Residences, villas, streets and Traffic signals is replacing the conventional lighting due to following reasons:

1. Long life

2. All color illumination

3. Less power consumption

4. Less space to install

5. Other

LED(Light Emitting Diode) is nothing but a junction diode. The voltage drop in forward conduction in junction of LED will be around 3Volt to 3.5 Volt with passing current around 350mA.Therefore power consumed by LED is 3.5Vx350mA=1.225W. The whole power is converted in to light and gives very good lumens per watt. The LED forward current, at present, is limited around 350mA to avoid the junction heating that causes thermal run away or unstable operation.

Recently we installed in a villa complete LED Lighting of different types such as dimmable and all color generation by using RGB (RED, GREEN & BLUE) LEDs.

Now the question is designing the driver to LED.

If you are using one LED or many LED are in parallel, you do not need a driver but current of 350mA is require to limit by connecting a resistor in series across the supply DC Voltage. If you are using the AC voltage then first convert the AC into DC by diode rectifier then limit the current again by using resistor.

If you are using series of diodes, then you can use 6 diodes on 24V DC supply or 12 diodes on 48V DC supply. The generally available DC supply Voltages are 12V DC & 48V DC.

The diode or LED, when fails it will be short circuited between the two terminals but never be opened circuit. Utilizing this characteristic, LEDs are connected into series.

When you are using LEDs in series (say 12Number in series) then 48V DC supply shall act as a CONSTANT CURRENT of 350mA but VARIABLE VOLTGE SOURCE from Zero to 48V DC. All series connected LED draw 350mA constant current but voltage drop of 12LEDs( 12x3.5V=42V) in series will be 42V DC. Hence Variable Voltage source absorbs 6V DC. Therefore LED driver generates 350ma current at 42V DC.

Owing to some reason, 5LEDs from above 12LEDs get shorted. Then the LED driver shall generate 350mA at 23.5V DC (48- 5x3.5=23.5).

Series connected LED driver shall be constant current source but variable voltage from zero to 24V DC or 48V DC. This is the design principle used in LED driver for series connection of LEDs.

Mushtaq Hussain-INDIA

B.E., M.I.E., MS.

Solar Eagle: Thank you for the links. They are quite helpful.

Mushtaq Hussainh: Thank you for your lucid reply. So far I have been trying to run a simulation model of a buck converter with a 10V - 30 V input supply stepped down to 4-8 V. I'm using average current control to achieve 350mA current through the load. This is to just understand the basics before I try out a boost converter for a 48 V DC grid supplying an 80 V LED luminaire.

You've mentioned that a resistor needs to be connected in series with the supply to control the current flowing through the LEDs and no driver is required. If my understanding is correct, this will cause a color shift?

I also see that PWM is the most preferred method of dimming. Since I'm trying a simple circuit by myself, I would like to know if a normal square wave source can be used to generate the dimming signal?

I'm following the datasheets and application notes of Supertex to understand the working principle and find myself inundated by a lot of doubts.

Thanks again for your time and patience!

Mr. Nenya-03: As per your statement

"You've mentioned that a resistor needs to be connected in series with the supply to control the current flowing through the LEDs and no driver is required. If my understanding is correct, this will cause a color shift?"

I mean that Resistor(to limit the current to 350mA) is required only for single LED or parallel connected LED across the DC Supply. But not for the Series connected LEDs which requires a constant current (around 350mA) source DC supply

Mushtaq Hussain-INDIA

For better efficiency and in the case of series-parallel connected LED strings, is active current control the best option?

Thank you. Your answers are very helpful.