SimScale CAE Forum

LED Thermal Simulation


#1

Hi Guys,
I’m trying to make some thermal simulations.
I would like to ask you some tips.
Usually, which is the best material to set up for LEDs?
I try Silicone. But I’m not very satisfied.

How to calculate the temperature that the LED produce?
I only have the current as a value (700mA) and the LED datasheet.

Why the FR4 material was removed from material list?
I would like to make a comparison between aluminum PCB’s and fr4 PCB’s


#2

Hi @sn1px!

You can have a look at this project if you like: Heat sink LED.

I would have to look up the formula for the LED temperature for specific currents and materials can be adapted if needed. Once you have uploaded your model you can share it with us and we’ll comment on it. Also it would be good to look for data sheets of the LEDs.

Best,

Jousef


#3

Thank you for your fast reply.
Now i check the project suggested.

Meanwhile:
This is my first project with simscale: https://www.simscale.com/projects/sn1px/led_heatsink/
This is the LED Datahasheet: https://www.lumileds.com/uploads/610/DS109-pdf

Current: 700mA


#4

This is my progress in the calculation of Tj of LED:

The formula was found on internet is:
Tj = TA + (ƟJA)×(Pd)

ƟJA = Junction-to-ambient thermal resistance
TA = Ambient operating temperature (temperature grade of the actual device)
Pd = Voltage (V) × Current (I). Typical Voltage and Current values are found in the datasheet

ƟJA = 3.66°C/W (maximum value in LED datasheet)

The operating conditions are defined by:

Voltage = 3.27 V
Current = 700 mA

What do you think? :slight_smile:


#5

Hi @sn1px!

Looks good at first glance but would have to look that up myself and see if this formula is a special case and if there are other formulae for this purpose. What I would do in your case is to take that formula, put everything in and see if the simulation fits the data (if available from the sheets you provided at the top).

Best,

Jousef


#6

Hi @sn1px

Your thermal calcs aren’t quite right. The formula you have would be correct if the LED was rated for operation in free space, then the datasheet would spec a ƟJA value. But it specs a ƟJC (junction-case) value. There is no spec for a ƟJA so this implies it must be heat-sink mounted.

The correct formulae are therefore :

Tj = Ta + (Ɵjc + Ɵh)×(Pd)
Tc = Tj - Ɵjc x Pd

where
Ɵjc = junction-case thermal resistance
Ɵh = heat-sink thermal resistance; and
Ta = ambient air temperature around the heat-sink
Tc = case temperature at the thermal pad

This device MUST be run on a heat-sink. By way of examples:

A 40mm x 40mm x 1mm flat aluminium plate mounted vertically in free still air has a thermal resistance of about 46°C/W which would give a junction temperature of 135°C at 25°C ambient (and a case temperature about 10°C lower at 125°C).

A 25mm square, single sided, 25 micron thick, copper area on a PCB as a heat-sink, mounted horizontally and radiating from the top surface only is about 245°C/W and would allow a maximum wattage of 0.45W (140mA approx) for the same temperatures.

Note also that the 700mA limit for the lower spec LUXEON F PC Amber LED is an absolute maximum. From the data sheet you shouldn’t normally operate it above 350mA and 80°C case temperature to avoid reduced life span/premature failure.

Good heat-sink design is a fine art! Hope this helps a bit.

You might also find this site useful: Heat-sink calculator

Best,
Irving