Thermal - Can I use Spice to perform Luminus LED thermal modeling?

Yes you can.

We will give an example calculation using the SST-10-SB Sky Blue LED and model the time-dependent thermal response for continuous wave (CW) operation with typical system component values and then show how it can be used to model how the LED junction temperature, Tj, changes if a different component is used.

Define the thermal stack: we will use a single LED operating at the maximum rated power in a room temperature environment. The thermal stack will consist of the LED package soldered to a small MCPCB, a thermal interface material (TIM), and a small heatsink. To construct the Spice model, we need to following information:

Component	Rth (K/W)	Mass (kg)	Cp (J/kgK)	Cth (J/K)
LED package	5.3	2.5e-5	780	1.93e-2
Al-MCPCB (1 cm2)	0.3	2.7e-4	921	2.53e-1
TIM (1-cm2)	0.6	2.9e-5	710	2.04e-2
Heatsink (40 mm dia)	3.7	50e-3	921	4.61e+1

• Rth is the electrical thermal resistance of each component and can be found on datasheets.
• Cth is the thermal capacitance of each component.  This value can be estimated from the mass and specific heat (Cp) of the predominant material in each component.  Cth is estimated as
  Cth = mass * Cp 

Spice Model for CW operation

Here is the output of an LTspice model using a 5 watt CW LED power level as input [1].  The green line is Tj.  It jumps immediately to 53 C and then slowly rises to a steady state value of 74.5 C after 1200 seconds of run time (20 minutes).  The maximum Tj rating for this LED is 115 C so this system is operating at safe levels. 

The temperatures shown below are at the interfaces, so we know that the LED has a Spice-modeled
26.5 C temperature difference between the junction and the solder point after reaching steady state. If we use the datasheet value of 5.3 K/W and 5 W input power, we also get 26.5 C. 

Similarly, we know that the temperature difference between the bottom of the LED and the top of the heatsink is 4.5 C. which illustrates the value of using MCPCBs and TIMs in high power LED applications.

If I replace the MCPCB with a 0.7 mm thick FR-4 substrate by changing the PCB Rth from 0.3 to 20.4 K/W, the steady state Tj increases to about 170 C, definitely an unsafe operating temperature [2].

Run 1.  Al-based MCPCB Spice model with annotations showing the temperature probe locations.

Run 2. FR-4 PCB Spice model.

[1] The free program LTspice has good help files so we will not discuss the mechanics of creating this model.  Please note the directive statements in the lower left.  These two statements are required to perform a transient analysis.  In particular, .IC V(N003)=25, sets the initial conditions and if omitted, you will get flat lines for 1200 seconds.

[2] We are using the 2040 K-mm2/W value found on page 7 in the Texas Instruments application note, https://www.ti.com/lit/an/tida030/tida030.pdf and a 1 cm2 area to estimate the Rth value to use in the second model. 

The correct area to use in this case is an engineering judgement call and is undoubtedly smaller since
FR-4 has poor thermal spreading.  This refinement would increase the Rth value since you will then be dividing by a smaller number.  I also omitted changing the value of the thermal capacitance. Since I have already shown the LED gets too hot, I skipped all these needed steps to refine the model results.  

The TI article discusses how the effective Rth value can be improved by using thermal vias in FR-4 PCBs. The small size of the 3535 LED precludes using this strategy. The TI device under discussion has room for 33 thermal vias directly under the device.

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Where to buy Samples of Luminus LEDs: https://www.luminus.com/contact/wheretobuy.

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