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High Power LED Controller

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Thread starter #1
Hi everyone!

I'm pretty new to electronics and I'm trying to find a high power LED controller which can drive 5 Seoul Semiconductor P7 LEDs. The idea is to replace a standard halogen car headlamp with these very high-brightness diodes. The problem keeps falling back on the controller since I've had no luck at all in finding controllers which can put out enough power to drive them.

Ideally what I'd need is a controller which has following characteristics:

12vDC - 14vDC input
Current-mode controller
Capable of driving 5 LEDs, each drawing up to 2.8A current at up to 4.2V

So as you can imagine I haven't had such great luck finding somebody who produces such a controller. I'd really love to build one, though!

The datasheed for the diodes I'd be powering can be found here. I'd really appreciate any input that you guys could provide!

Cheers,

Corey
 

MikeMl

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#2
How much do these puppies cost?

Do you care if the lumens drop off if the alternator is not charging. If so, you could run three in series at 14.2 to 14.7V, and then brute-force linear regulate the current with only a few watts of dissipation in the regulator transistor.
 
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Thread starter #3
Hi Mike, thanks for the reply!

Normally I wouldn't mind it if the brightness were to drop off a little, but that operating range is quite a bit too high. The P7s will run up to 4.2V. I should have stated earlier that I'd typically want them running at around 3.6 - 3.7vDC so unfortunately that's a no go.

As for the cost of the LEDs, it's $13.47 per diode shipped in 10 unit quantities. Individually they cost $15.37 shipped. I'd post the link for you but I don't know if it's allowed in the forums.

Cheers,

Corey
 

MikeMl

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#4
Three in series at 3.7V, running at ~2A is still only 6.8W of dissipation in a transistor used as a current regulator starting from 14.5V. Sounds quite doable. Think about how much heat an incandescent wastes...
 
Thread starter #5
Yeah, that is true. Either way each LED will be dissipating about 10W of power each anyways so it's not so much the power dissipation in the regulator that I'm concerned about...

It's just that if I were to go the route of using a linear regulator, then what would happen if there were large transients for whatever reason? I'm assuming that the voltage across the LEDs would spike, right?

Additionally, I'm pretty sure that I really need 5 diodes per lamp to get a satisfactory light output. Typically the P7s will output around 700lm, which, multiplied by 5 diodes is 3500lm of raw light. So after all the optical losses I can expect to be at maybe 2000lm which is right on target.

Most of all I'm just afraid for my diodes, though. I know that LEDs are very sensitive to transients and I don't want them dying on me prematurely :)

Cheers,

Corey
 
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MikeMl

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#6
Here is a start at designing a controller. First thing, I need a model for the I/V behavior of the W724C0. I created a crude one. Compare mine to the data sheet plot.
 

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MikeMl

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#7
Here are three in series with just a resistor as a current limiter.
 

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Thread starter #8
Hello again Mike!

The I/V characteristics of the diodes are as follows:


It's actually pretty steep near the upper-end which is why I was mentioning that these diodes are really sensitive to transients.
 

MikeMl

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#9
Compare your plot to mine. I tried to make my model fit your plot at 2.5V/0A and 3.6V/2.8A.

I will play around with a constant-current regulator later when I get some time...
 
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audioguru

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#10
How will you cool the LEDs?
Liquid nitrogen?
That is a lot of heat in a small space. The LEDs will fry in a couple of seconds.
 

MikeMl

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#11
... The LEDs will fry in a couple of seconds.
No they wont. Just mount them on the rear of the housing. The dissipation of a single LED will be 4.4W at the recommended operating point. Not much heat to get rid of considering the 5000lb heatsink it is bolted to.
 

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MikeMl

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#12
Corey,

Here is a hack at making a linear constant-current regulator for a string of three of the W724C0 LEDs using my favorite LM431. I configured it to deliver ~1.6A. Note that it keeps the LED current constant while the battery voltage sweeps from 12.2 to 15V. Below 12.2V, the NFET is saturated, and the current decreases. R1 sets the current.

In the third plot pane, see the power dissipation in one of the LEDs, the FET, and the resistor R1. At 14.5V (motor running alternator voltage), the dissipation in the FET and R1 are about 4W each; quite manageable.

Let me know if this seems to be heading in the right direction. If it is, when I get some more time, I will simulate the effects of transients.
 

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MikeMl

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#13
I made it adjustable by adding a pot.
 

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#14

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#15
My 0.02: I used "10 W" LEDs very similar to this on my car's reversing lights.

I strung 3 in series, and utilised the car's exisiting wiring as a 'current regulator'. They lasted a suprisingly long time, some weeks, and well long enough to evaluate them.

My conclusion was the coloUr rendering was a bit poor, so I didn't persue it any further.

A plus with these LEDs is the efficiacy, 70 lumens / Watt, a lot better than incandesant at 5 to 10 lm/W
 
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MikeMl

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#16
See page 34.
http://www.electro-tech-online.com/custompdfs/2009/07/41215a-1.pdf

Obviously you can use a discrete comparator rather than a MCO and add a pull-up resistor if it has an open collector output.
This circuit is wrong. It won't oscillate as drawn. You would have to reverse the inverting and non-inverting inputs to the comparator to make it oscillate. It also has an awful start-up transient until C1 charges which causes the current through the load to be very high initially.
 
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#17
I think SoftStart was ommitted for clarity of illustration. I'd say the LED would tolerate the high starting current anyway, going on the longievity of my "10 W LEDs setup" in my earlier posting.

The 'Comparator' doesn't have to be a comparator, it can be a transistor. It doesn't need to compare accurately, just fast.

Could the posted circuit have its flaws fixed enough to become a practical circuit?
 
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#18
This circuit is wrong. It won't oscillate as drawn. You would have to reverse the inverting and non-inverting inputs to the comparator to make it oscillate.
Yes you're right, another option is to use a N-channel MOSFET and move it to the low side.

It also has an awful start-up transient until C1 charges which causes the current through the load to be very high initially.
That shouldn't happen, the current ramps up until the voltage across the sense resistor exceeds the reference slightly, the MOSFET is turned off and the current falls. This makes the start up time slow which is only a problem if you want to PWM it which would be a rubbish way of varying the brightness, a more sensible way is to vary the reference voltage.
 
#19
Hysteretic converters like this are good but they do have their limitations, namely that they can be unstable under certain loading conditions. Not a problem if you just want to drive a couple of LEDs though.
 

Mr RB

Well-Known Member
#20
This 3 transistor constant current smps oscillates/regulates around load current and could be adapted for 12v-3v or 12v-6v use;
2-transistor Black Regulator

You could use a logic level PFET for the main switch device to get currents up to a few amps with no problems.
 
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