Proper voltage to power an LED with a constant current power supply.

Thread Starter

Mahonroy

Joined Oct 21, 2014
313
Hey guys, I am using a high powered LED from CREE (XHP70B-00-0000-0D0BP20E2). The datasheet says the forward current draw is 4.8 amps at 6.0 volts. I dialed in 6.0 volts on a bench power supply, and started dialing the current up. At 6 volts I was only getting 2.5 amp draw (if I turn the current up to max)... but as it heated up, the amp draw slowly increased. I found that if I increase the voltage up to 6.5 volts, then I could get the 4.8 amps.

So this brings me to my question: if I design a power supply that has a built in programmable peak current draw limit, for example using a TPS61088 or TPS61178, do I just set it to 6.5 volts, and 4.8 amp peak current limit? Or would I want some headroom and do something like 6.7 volts, and still limit current to 4.8 amps?

What if during runtime I want to dial the peak current limit down (to dim down the LED), to say 1 amp peak. Would the 6.7 volts still be ok in this situation? I am not quite wrapping my head around this. And the whole point is that I don't want to use a resistor, I want to limit the current via the power supply so its efficient and not generating more heat than necessary. Am I going about this all wrong?

Thanks and any help or advice is greatly appreciated!
 

Sensacell

Joined Jun 19, 2012
2,568
Stop thinking about controlling the voltage- LED's are current driven devices.
Stop watching those youtube videos where people apply constant voltage power supplies to LEDs- Yes, they light up, but this is terrible design practice, typically leading to failure.

You apply a controlled current, and the voltage is what it is.
Yes- your current control system needs to output enough voltage to turn the LED on, and this voltage varies with temperature and current as you have seen - and also from one LED to the next.

Your driver always needs to have enough overhead voltage to drive the LED - at it's worst-case current and temp.

If you are using a linear LED driver, then the excess overhead is always going to dissipate power, either in a resistor or a semiconductor, makes no difference.
If efficiency is an issue, then look at an LED driver that uses a switching inductor to regulate the current.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
313
Stop thinking about controlling the voltage- LED's are current driven devices.
Stop watching those youtube videos where people apply constant voltage power supplies to LEDs- Yes, they light up, but this is terrible design practice, typically leading to failure.

You apply a controlled current, and the voltage is what it is.
Yes- your current control system needs to output enough voltage to turn the LED on, and this voltage varies with temperature and current as you have seen - and also from one LED to the next.

Your driver always needs to have enough overhead voltage to drive the LED - at it's worst-case current and temp.

If you are using a linear LED driver, then the excess overhead is always going to dissipate power, either in a resistor or a semiconductor, makes no difference.
If efficiency is an issue, then look at an LED driver that uses a switching inductor to regulate the current.
Hi Sensacell, thanks for the reply. Isn't this what I just described though? I described scenarios that have a peak current limit in place (adjustable by me) that also use switching inductors (TPS61088 or TPS61178)
 

ElectricSpidey

Joined Dec 2, 2017
1,175
People obsess over current vs voltage when it comes to LEDs but the frequently overlooked factor is wattage.

The data sheet says 6 volts and 4.8A that would be 28.8 watts, but if it takes 6.5 volts to reach 4.8A then the wattage is 32.2 a little higher but something to consider.

Yea, it’s only 3.4 watts, but I would be cautious because I didn’t see a wattage rating in the sheet, only current and voltage.

You know…just do yourself a big favor, and don’t run it at maximum. And be sure to consider 30 watts is a lot of heat to dissipate.
 

SamR

Joined Mar 19, 2019
2,253
Also remember that devices, due to manufacturing variations (even with quality control culling), do not always exhibit the exact parameters found on the datasheet. The datasheet is a reference and not an exact performance test report. Particularly if you buy them from somewhere other than one of the major authorized parts suppliers.
 

Tonyr1084

Joined Sep 24, 2015
4,705
For a power supply to maintain constant current it must vary the voltage. Since the load remains relatively the same, for a desired current the voltage must go up or down.

Example:
You have a load (a resistor) of 100 ohms and you want to push 1 amp through it. You would need a voltage of 100 volts. If you wanted to dial that amperage up to 4.8 amps then the voltage would have to be 480 volts. If your load changes due to heating, say your 100 ohm resistor drops down to 80 ohms (just making up examples) then your constant current supply that is pushing 4.8 amps will drop the voltage to 384 volts.

E = I•R
1A (x) 100Ω = 100V
4.8A (x) 100Ω = 480V
4.8A (x) 80Ω = 384V

To maintain a particular set amperage the constant current supply will have to vary the voltage. As the properties of the load may change due to heating or cooling, the supply will compensate by adjusting the voltage.

You said you needed 6.5 volts to get 4.8 amps. 6.5 ÷ 4.8 = 1.35Ω I'm suspicious you might not be factoring in the forward voltage of the LED. I haven't looked at the data sheet so I don't know what the forward voltage would be for that LED. But it could be that some genius engineer decided 4.8 amps was correct but failed to account for the LED Vf (forward voltage) when he calculated the voltage for the device.

Honestly, you don't NEED to run it at 4.8 amps. A slightly lower amperage and you'll probably not be able to detect any drop in output. And you'll also run a little cooler. A cooler LED lasts longer.
 

kubeek

Joined Sep 20, 2005
5,724
Hi Sensacell, thanks for the reply. Isn't this what I just described though? I described scenarios that have a peak current limit in place (adjustable by me) that also use switching inductors (TPS61088 or TPS61178)
If you mean peak current limiting inside the switch mode power supply, then that is at all not the same thing as regulating that power supply to the average current going through the LED (instead of the usual way of regulatingthe output voltage).
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
313
I guess I'm confused then. Looking at TPS61088 and TPS61178, they are constant voltage and constant current boosting voltage regulators. You can set the specific output voltage, as well as the specific peak current draw. Its not varying the voltage to maintain a specific current.
 

djsfantasi

Joined Apr 11, 2010
6,504
I guess I'm confused then. Looking at TPS61088 and TPS61178, they are constant voltage and constant current boosting voltage regulators. You can set the specific output voltage, as well as the specific peak current draw. Its not varying the voltage to maintain a specific current.
Read what you wrote carefully... You don’t set the constant current provided, you set “the specific peak current draw”. Or in other words, you set the specific output (ie constant) voltage and limit the (changing) current.

The result of Ohms Law is if one parameter is constant (ie current), any change is reflected in both of the remaining parameters. You can’t keep two parameters constant and change the remaining one.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
313
Read what you wrote carefully... You don’t set the constant current provided, you set “the specific peak current draw”. Or in other words, you set the specific output (ie constant) voltage and limit the (changing) current.

The result of Ohms Law is if one parameter is constant (ie current), any change is reflected in both of the remaining parameters. You can’t keep two parameters constant and change the remaining one.
So why would limiting the current draw here not work? In my original example I mentioned keeping 6.5 volts, and then adjusting the maximum current down to say 1 amp max, or even less. I don't understand why this would not work then?
 

kubeek

Joined Sep 20, 2005
5,724
I guess I'm confused then. Looking at TPS61088 and TPS61178
none of those shows any constant current application. Peak current limiting is used in order to not saturate the inductor, but doesn´t have direct corellation to output current.
Here you can see how it is typically done, the FB pin is connected to the sense resistor (or through an amplifier), instead of a voltage divider. https://www.google.com/search?q=led+constant+current+boost&tbm=isch&ved=2ahUKEwj448uKwavmAhVKIFAKHcXmCzQQ2-cCegQIABAA&oq=led+constant+current+boost&gs_l=img.3...7404.7966..8030...0.0..0.88.337.4......0....1..gws-wiz-img.n8PZPN-fV9I&ei=xsjvXfjoDsrAwALFza-gAw&bih=1023&biw=1920&client=firefox-b#imgrc=hXBSbOwPQVF7QM
 

djsfantasi

Joined Apr 11, 2010
6,504
So why would limiting the current draw here not work? In my original example I mentioned keeping 6.5 volts, and then adjusting the maximum current down to say 1 amp max, or even less. I don't understand why this would not work then?
Ohms Law! That’s why it won’t work. V=IR comes into play. R represents your load. If you set the voltage at 6.5V, then you have
6.5 = I x R
or I = 6.5 / R​
You want to change the current, I. But you’ve set the voltage at a constant 6.5V and your lead resistance can’t change. So, you still have
I = 6.5 / R​
You cannot keep BOTH current and voltage constant. If you change one, by definition the other will change.

That is unless you live in an alternate universe where our laws of physics don’t hold. :)
 

djsfantasi

Joined Apr 11, 2010
6,504
Ohms Law! That’s why it won’t work. V=IR comes into play. R represents your load. If you set the voltage at 6.5V, then you have
6.5 = I x R
or I = 6.5 / R​
You want to change the current, I. But you’ve set the voltage at a constant 6.5V and your lead resistance can’t change. So, you still have
I = 6.5 / R​
You cannot keep BOTH current and voltage constant. If you change one, by definition the other will change.

That is unless you live in an alternate universe where our laws of physics don’t hold. :)
You CAN set a current limit, but typically in that case it’s a protective limit and the power supply will shut down.
 

Tonyr1084

Joined Sep 24, 2015
4,705
An LED is dependent upon the current going through it. Take, for instance, a single common LED with a forward voltage of 3.2 volts. You can power it from a 5 volt source, but you should not exceed 30 mA (0.03 amps). Let's shoot for a current of 20 mA. You can power it from 9 volts, 12 volts, 18 volts, 24 volts, and yes, you can even power it from 120 VAC. However, when powering it from an AC source you have to add a blocking diode because a common LED can't handle a whole lot of reverse current.

So with a 3.2 forward voltage drop, you would calculate the various voltages listed above in the following manor:

5V - 3.2V = 1.8V
1.8V ÷ 0.02A = 90Ω

9V - 3.2V = 5.8V
5.8 ÷ 0.02A = 290Ω

12V - 3.2V = 8.8V
8.8V ÷ 0.02A = 440Ω

18V - 3.2V = 14.8V
14.8V ÷ 0.02A = 740Ω

24V - 3.2V = 20.8V
20.8V ÷ 0.02A = 1040Ω

In each case you maintain 0.02 amps (20 mA). As the voltage changes so must the resistance. THIS is the very nature of Constant Current. Voltage changes to maintain the current. In my above examples I had to change the resistance to maintain the constant current because the voltage was changing.

You change the voltage to achieve a constant current. Likely your LED has a built in resistor. But if not - then it is designed to operate at a max current far above the ordinary common LED. But it is designed around current and not voltage. The above example is meant to show that the operation of an LED is not voltage dependent but current dependent. In every case above, the current is held to the same 20 mA level. Since your LED either has a resistor or is designed for a constant current the voltage must change.
 

Audioguru again

Joined Oct 21, 2019
1,577
An LED part number has a range of forward voltage because the ones made on a Friday have a different voltage than the ones made on a Tuesday etc. When you buy an LED you get whatever they have not the forward voltage that you want.

In a Chinese LED flashlight manufacturer, somebody measures and groups together LEDs that have the same forward voltage so that they can be connected in parallel. Some LEDs you can buy that are already grouped together into forward voltage "bins".
 

Reloadron

Joined Jan 15, 2015
5,504
Something else to note other than 4800 mA being the Maximum current on a 6 volt mode. This is your data sheet: Cree® XLamp® XHP70.2 LEDs . In addition to Maximum drive current: 4800 mA (6 V), 2400 mA (12 V) the nice people at Cree also point out:
Note: * Data for the 6‑V configuration is calculated and for reference only.
So that 4800 mA is based on being twice (2X) the current used in the 12 volt version. Page 29 of the linked data sheet pretty clearly illustrates both the 6 volt and 12 volt configurations. The die actually contains 4 LEDs and my guess would be each has a forward working voltage of 3 volts. Also note how a zener diode is in the configuration. The current on a 6 volt configuration with 6.0 volts applied will likely be about 4800 mA, do not expect it to be exact as it never will be.

Ron
 
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Thread Starter

Mahonroy

Joined Oct 21, 2014
313
So what would happen if I set the TPS61088 or TPS61178 to 6.5V, and set the peak current limit to 1 amp. Are you saying that it would not regulate 1 amp current draw through the LED?

Are you also saying that ontop of changing the peak current draw setting, that I would also need to be adjusting the voltage real time as well?

I understand what you are saying about the resistors. Like I mentioned in the original post, I don't want to use current limiting resistors, I want to use a current limiting boosting voltage regulator to control the LED and set it to different brightnesses. It sounds like you are saying that the voltage regulator should be allowing me to specify the current limit, and then the voltage just ends up being some arbitrary number... but like I mentioned with the TPS61088 or TPS61178, they don't seem to operate this way, so that's confusing.

The only other thing I can think of, is am I supposed to modify the application circuit of the TPS61088 or TPS61178, so that there is an inline resistor with the LED (for current draw sensing), run this to an op amp, along with a analog signal from a microntroller, and then run the output to the feedback pin of the TPS61088 or TPS61178? This way the voltage changes with current draw, and I can control the current draw by outputting different analog values from the microcontroller?
 

Tonyr1084

Joined Sep 24, 2015
4,705
You can set either the voltage OR the amperage. Not both.

Wait! Re-reading your question.

OK, after re-reading - you can set a voltage and specify a limited current. My statement originally was that you can not set a voltage AND a current. Either the voltage or the current will depend on the load and one of the two settings, volts or amps.
 
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