Constant current (1A)Driver for "Varying Input Volatge" (16-32).

[AshwinNambiar]

I need to design a constant current LED Driver of 1A to drive an LED array (Green) 3 in parallel and 4 in series.
"The input voltage will vary from 16 to 32 and irrespective of this a constant current driver is to be designed.

1.I had tried using linear circuits-LM317,where there was too much heat dissipation (owing to the drop created from "16-32" input).
2.I used a TIP41C transistor and at the base provided a 60mA constant current device(AL5809-60P1-7),to obtain a constant collector current (Ic).However, the current was not constant because of the varying supply voltage,which caused a variation in Hfe cause of variation in Vce.

I have now decided to design an switching circuit.Avoided so far because wanted to avoid EMI issues.
**I'll be glad to receive help/guidance /suggestions as to how to go about with the design of constant current driver with varying input voltage.**

 

[KlausST]

Hi,

EMI. Please read carefully the design guides in the datasheet.
Don't try to build it on a breadboard...it will not help much, if ever you bring it to work
***
There are IC manufacturers they build constant current controllers for LED applications.

Usually they have interactive selection guides.

*** in case you don't find wat you need, you could use a buckconverter.
Device selection:
Look for a buck = step down converter. Voltage rating ... current rating.
Use one with adjustable output voltage. Those have a feedback pin. The regulaor regulates the output in a way that there is a given (in the datasheet) voltage at this feedback pin. Look for a converter with low feedback pin voltage. Below 1V. Some are below 0.8V.
The lower the better for the dissipated heat.

Calculate a shunt (connected between feedback pin and GND): R = U(fb) / I(Led).
Dissipated power is: U(fb) * I(Led). Use a resistor with higher rating.

Connect your Leds between output capacitor and shunt.

How it works:
The regulator increases output voltage. Current begins to flow. This current causes a voltage drop across the shunt. This voltage isuse for feedback. The regulator increases current until it reaches the nominal feedback voltage.

Klaus

 

[AshwinNambiar]

Hi,

EMI. Please read carefully the design guides in the datasheet.
Don't try to build it on a breadboard...it will not help much, if ever you bring it to work
***
There are IC manufacturers they build constant current controllers for LED applications.

Usually they have interactive selection guides.

*** in case you don't find wat you need, you could use a buckconverter.
Device selection:
Look for a buck = step down converter. Voltage rating ... current rating.
Use one with adjustable output voltage. Those have a feedback pin. The regulaor regulates the output in a way that there is a given (in the datasheet) voltage at this feedback pin. Look for a converter with low feedback pin voltage. Below 1V. Some are below 0.8V.
The lower the better for the dissipated heat.

Calculate a shunt (connected between feedback pin and GND): R = U(fb) / I(Led).
Dissipated power is: U(fb) * I(Led). Use a resistor with higher rating.

Connect your Leds between output capacitor and shunt.

How it works:
The regulator increases output voltage. Current begins to flow. This current causes a voltage drop across the shunt. This voltage isuse for feedback. The regulator increases current until it reaches the nominal feedback voltage.

Klaus

But how do i keep the current constant.The current is boosted everytime the voltage is bucked.Added to that is the issue of dissipation.

I can do this:Use an LM317,get a constant voltage and then give this constant voltage to a transistor to get a constant Ic.But severe dissipation problems.The LM317 gets heated up.
Any value addition to these as to what can be done would be great.

 

[KlausST]

Hi,

But how do i keep the current constant.

The regulator regulates FB voltage to 0.8V (for example)
If the shunt value is 0.8Ohms, then the current is (according Ohm's law) : I = U / R = 0.8V / 0.8 Ohms = 1.0A.
The power dissipation is U x I = 0.8V x 1A= 0.8W. --> use a 2W.

******

LM317 solution:
Assuming 8V LED voltage and 24V input voltage..
Power dissipation in the transistor is: (24V - 8V) × 1A = 16W.

Choose what you find more suitable.

Klaus

 

[treez]

Re: Constant current (1A)Driver for "Varying Input Volatge" (16-32).

Please put the leds in series, and feed them through a sepic led driver as attached.
The attached runs in the free download ltspice simulator.

The excel design file is also attached....as you know, you design a sepic as a 1:1 flyback then if you want uncoupled, you just double the inductor value.

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also attached is the word file saying why parallel leds are a bad idea

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Or if you want something which is not using such a fine-pin-pitch chip then go for the hv9910b, and to be simple, just use a hv9910b based boost converter to get to 50vdc, then buck down from there with a hv9910b based buck converter to the leds.


HV9910B
https://ww1.microchip.com/downloads/en/DeviceDoc/20005344A.pdf