Current / Temperature Correlation

[davegLED]

Hi All,

Just had my first play round with a power LED:

http://catalog.osram-os.com/catalogue/catalogue.do?favOid=00000004000111de024c0023&act=showBookmark

I drove the LED from a voltage source, 5V with a resistor array giving 3.3ohms = 500ma.

I realise they should be driven from a constant current source but this was just a quick test.

What I want to know if how much heatsinking I will need for my application. When in use the LED will be driven with 1000ma but for very short durations. The LED will be surface mounted to a standard PCB - I presume this will be OK for flash operation?

Cheers,
Dave

 

[LukeA]

What's the duty cycle of the flashes? If it's less than 5 seconds on 25 seconds off it should be ok. But you should still get as much aluminum on there as you can.

 

[davegLED]

Hi Luke, thanks for your quick reply.

The duty cycle is small - on for a couple hundred milliseconds and off for several seconds at very least. It is actually a camera flash.

I tried running the LED at 500ma for a minute, the datasheet quotes 500ma as the max DC voltage. The LED got very hot and began to smoke a little. I quickly turned it off! Must have got into thermal run-away since I was using the constant voltage source. My first glimpse of the magic smoke I have read about!

The next task is so calculate the maximum allowable DC current for 'torch mode' when using a constant current source:

http://www.national.com/pf/LM/LM3551.html

As I mentioned since the LED is to be reflowed straight onto a PCB the limiting factor is heat dissapation.