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fifahaywire
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LED Driver

11/29/2011 8:12 AM

Iello. I'm currently carrying out a project where I am suppose to implement an LED driver.

So, basically my circuit starts off with a rectifier, and then to a RC filter and then through a buck converter. The output is a series of power LEDs with voltage drop of 3.5V and rated forward current of 350mA for each LED (I will be using 5 LEDs). Also it is a 1.4W LED.

I'm currently having problems with finding the internal resistance of the LEDs as I need to model it in Pspice.

Vo = IRin + 5xVf

Where, Vo is the output voltage of buck converter and Vf is the forward voltage of LED.

Or should i use P=V^2/R = I^2R to get the resistance I need?

I'm still new and this is my first project.

Thanks :)
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PWSlack
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#1

Re: LED Driver

11/29/2011 8:32 AM

If LEDs had a fixed forward resistance then they wouldn't be LEDs; they would be LERs.

They are diodes. They pass whatever current is presented to them subject to sustaining the forward voltage drop - provided that the forward current is not large enough to burn them out. That is why it is usual to introduce a series resistance, or failing that run them only from a high impedance supply.

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fifahaywire
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#2
In reply to #1

Re: LED Driver

11/29/2011 8:50 AM

Thanks PWSlack,

i understand that now :)

but is the resistance of a random value?

i'll try to simulate it out with 50 ohms then.

thanks again.
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PWSlack
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#4
In reply to #2

Re: LED Driver

11/29/2011 8:53 AM

The apparent resistance is a function of the forward current, which could be anything from zero to the maximum rating of the device. Without knowing what the supply is limited to providing, plucking resistance values out of cyberspace, or anywhere else for that matter, is both nonsensical and abstruse.

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fifahaywire
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#5
In reply to #4

Re: LED Driver

11/29/2011 9:07 AM

PWSlack,

i was also researching while posting here. i found this regarding dynamic resistance. http://www.national.com/appinfo/webench/led/Rd.pdf

this is what you mean, right?

i'm sorry for taking your time.

thanks!

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fifahaywire
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#3

Re: LED Driver

11/29/2011 8:53 AM

Hi SolarEagle,

thank you for the fast reply. i'm checking out the website now.

thanks a lot
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Usbport
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#6

Re: LED Driver

11/29/2011 11:24 AM

In my experience, you can apply Ohm's law as a first approximation when the LEDs are operated per the manufacturer's recommendations.

V = I x R; Since V = 3.5 volts and I = 0.35 amps, R must equal 10 Ohms.

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PWSlack
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#7
In reply to #6

Re: LED Driver

11/29/2011 11:35 AM

...and if the forward current is 0.1 amp instead, then R must equal 35 ohms. The point is that they are LEDs, not LERs; the apparent resistance varies with the forward current QED.

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Randall
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#8

Re: LED Driver

11/29/2011 3:36 PM

For once I disagree slightly with PWSlack. Look at this VF against IF graph for the first diode I found a spec. for:-

Just look at the typical 25° C curve for now. The dynamic resistance of the diode is the slope of the curve around the area that you want to use it.

Not too accurately: from 250 mA to 450 mA the Voltage rises by about 0.2 V so the dynamic resistance is 0.2/0.2= 1 Ohm (that seems like too much of a coincidence). Extend the tangent to the curve around the area you want to use it to the x axis and you get 0.8V

So for your model V=0.8 + I*1

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PWSlack
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#9
In reply to #8

Re: LED Driver

11/30/2011 4:31 AM

Respect.

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Winfield Hill
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#10
In reply to #8

Re: LED Driver

11/30/2011 8:41 AM

Right idea, wrong part. When fifahaywire says 3.5 volts, that tells us it's a white LED he's using. These use far-blue LEDs to excite a white phosphor, and physics tells us they need nearly 3-volts to create the energetic blue photons and operate.

Philips Luxeon is a big supplier, and I grabbed an I-V plot from one of their 350mA LEDs (right). These LEDs show a dynamic resistance of 1.1 ohms, up to about 400mA.

rs = dV/dI = (3.0-2.8) / (390-210mA) = 1.1Ω

Just above 400mA the plot shows the dynamic resistance beginning to decrease, to perhaps 0.7 ohms. That matches the datasheet spec of 3.2 volts for 700mA pulsing. (But another plot shows that doubling the current only increases the light by about 1.7x.)

If we extend the 100-to-350mA portion of the line down to 0V, it crosses at about 2.55V. So the dynamic I-V formula, for ID from 100 to 400 mA, would be

VF = 2.55 + 1.1 ID

The datasheet tells us the LED also has a -2 to -4 mV/°C temperature coefficient.

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fifahaywire
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#11
In reply to #10

Re: LED Driver

11/30/2011 9:22 AM

thank you, guys for the explanation. it now seems clear to me already :)

Winfield_Hill, yes you're right. i'm using a white LED.

http://uk.farnell.com/hlll/t21d1/led-pwr350-to220-daylight-white/dp/1226709

but the one im going to use doesnt have IV characteristic, so i can't really calculate the dynamic resistance. but thanks for the detailed explanation

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Winfield Hill
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#12
In reply to #11

Re: LED Driver

11/30/2011 9:34 AM

Your T21 LEDs are probably similar, maybe a little lower resistance. I like the TO-220 package.

Philips make a series of LEDs in sort of TO-247 packages, that are convenient. They're rather serious 14-watt LEDs, p/n LAFL (link). They're meant for automotive "forward lighting". Ahem!

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fifahaywire (4); PWSlack (4); Randall (1); Usbport (1); Winfield Hill (2)

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