Driver for Cree CXA1304

Photo of schematic here :

I can't find any info on the LED. I don't know if they are single units or multiple LED arrays. What is important is to know the voltage and the current needed to drive them. LEDs are essentially current mode devices so you need to design a constant current driver. It would be best if you drive each one separately, otherwise you need series resistors to balance the current in each one. The LM2940 is a linear regulator and so will drop significant voltage and current at the same time generating a lot of heat. Switchers drop voltage with little current and pass current with little voltage drop, so little heat.

I don't have time to find a part for you and I don't have the specs on the LED, but you should be able to find switching LED drivers if you look around for them. Using a switching driver will solve the heat problem in the regulator. The only thing you can do about the heat in the LED is to reduce the current along with the light output.

Is that a 9 volt part? How much current do you want?

A switcher would be efficient, but a bit more trouble than a linear reg. The LEDs will need to be kept cool no matter what regulator is used.

If you're only dropping a few volts, and need to heat sink the LEDs anyhow, a linear reg should be OK.

You could just buy some 12 volt LED bulbs with built-in regs, but no design fun. The car side/turn light replacement bulbs are cool.

The LEDs are :

Yes there would be no fun if I just went and brought a part ready to go, although it is getting a little frustrating now.

Are you saying that for a given current, the LED's will reach a certain temperature T which will be the same, regardless of whether driven by linear or PWM?

So far this heat sink has been remarkably ineffective :

when the total circuit current is 0.8A.

I have a heat sink on the LM2940 which seems to be coping at about 50 deg C.

Unfiltered PWM will be less efficient than linear or a filtered switcher. Most of the power applied to an LED gets converted to heat.

Very wild guess for that little piece of aluminum, 30 K/W unless it's glued to something bigger. Each LED is 9 volts * 0.8 amps = 7.2 watts.

2 of them is 14 watts. Say 10 watts of heat, since some of the power comes out as light. 30 K/W * 10W is a temp rise of 300C.

The numbers are rough, but suggest the problem.

Worse, LEDs get less efficient when they are hot. Spritzing an LED with freeze spray can be dramatic.

Higher supply voltage will make the reg dissipate a lot more power.

Well I can go bigger and bigger on the heatsink but no matter how big I go, eventually it will hit thermal equilibrium, so whats the solution if bigger doesnt fix it?

On the reg side - how about something like :

If "thermal equibrium" is 10C above ambient, bigger works.

Cree wouldn't sell them if they didn't work. They need a big enough heat sink.

I guess that's a switcher. The MeanWell stuff is good, if that's not a fake.

There's not much in the way of surface area on that compared to an LED heatsink. The LEDs you have look very much like ones used in domestic luminaires, maybe 10W or 5W and they have lots of fins.

They do get warm, you can't avoid that. Your PSU could be made more efficient using a switcher, but with 9V LEDs running from 12.5V the game's hardly worth the candle. Or candlepower.

I don't know what your application is, but multiple smaller LEDs spread out over a larger area might be an option. Or an LED panel.

P=VI so that's approximately true, because V is only approximately the same.

The extra "efficiency" of PWM translates to more heat in the LEDs and less heat in the regulator.

Some leds are available in a Star package. Which is easier to cool.

Pretty much. LEDs are roughly constant voltage devices so that power dissipation is directly proportional to average current. You need heatsinking or they will get hot. Datasheet should include details.

I generally just bolt them onto a rectangular sheet of 3mm aluminium.

You would be better off with 1A rated white butterfly LEDs (3 in series is about 12v nominal). They lend themselves to being bolted onto a simple metal plate heatsink and then a modest dropper resistor or better still a constant current source in series with the chain.

The way you have it now a fairly high proportion of waste heat is in the linear regulator. A better match of LED device would cut that a lot.

For a one off and starting from where you are now this switching part might be the path of least resistance if you can find it. OnSemis NCP3065

brands are available)