The SOAtherm model distributed with LTspice software and simplifies Hot Swap and Surge Stopper designs by verifying directly within a circuit simulation that a particular MOSFET’s Safe Operating Area (SOA) is not exceeded. This article assumes a basic understanding of the SOAtherm model. If you are not familiar with SOAtherm, please refer to LTspice: Modeling Safe Operating Area Behavior of N-channel MOSFETs.

Typically, a circuit designer uses the LTspice SOAtherm-NMOS symbol standalone to verify that a particular MOSFET’s SOA is suitable for a given application; no additional heat sink or PCB thermal model is necessary. However, in some particularly demanding applications, especially those where high power transients last longer than 10 milliseconds, it may be desirable to take advantage of the extra thermal capacity and dissipation provided by a heat sink or the PCB. Previously, this was implemented by connecting a resistor-capacitor network to the SOAtherm-NMOS model’s Tc pin.

Now, with the SOAtherm-PCB and SOA-HeatSink symbols, it is possible to model heat sink and PCB thermal behavior by specifying a few physical parameters rather than calculating an array of component values from formulas.

SOAtherm-PCB

To use the SOAtherm-PCB symbol, connect it to the Tc pin of the SOAtherm-NMOS symbol as shown below.

 

 

In most simulations, it is only necessary to provide the following information:

When using the SOAtherm-PCB symbol, a conservative practice is to change the SOAtherm-NMOS RthetaJA parameter to a large value, such as 1k, to eliminate the MOSFET’s default power dissipation value from the simulation.

 

 

Keep in mind that thermal behavior, especially with airflow, involves complex interactions between many variables. While SOAtherm-PCB is a useful tool in the circuit designer’s arsenal, it is not meant as a substitute for more sophisticated software that implements finite element analysis of a PCB layout in conjunction with three-dimensional airflow behavior and radiation to adjacent components.

SOAtherm-HeatSink

To use the SOAtherm-HeatSink model, connect the symbol to the Tc pin of the SOAtherm-NMOS symbol.

 

 

You can specify whether the heat sink is copper or aluminum by right-clicking on the heat sink symbol, and then double-clicking on the “SpiceModel” field to produce a drop-down menu.

 

 

In most simulations, you only need to provide the following information:

Note that Rtheta is the thermal resistance found in the heat sink datasheet and includes the effects of airflow. For example, in the Aavid ML26AAG datasheet, the following plot is provided.

 

 

At 200 feet per minute air velocity, the thermal resistance is 10°C/W.

With this information, the SOAtherm-HeatSink model is able to provide a first-order estimate of the transient thermal behavior of the heat sink. It is not meant to replace more sophisticated finite element software.

Advanced Topics

The above information is sufficient to begin running SOAtherm-PCB and SOAtherm-HeatSink simulations, but the diligent engineer will soon question what is being modeled and what simplifications have been made.

SOAtherm-HeatSink model

The SOAtherm-HeatSink model is fairly straightforward. It pretends that the heat sink forms a bar of copper or aluminum with a cross section that matches the contact area of the MOSFET tab (Area_Contact_mm2). The length of the bar is determined from the specified volume of the metal (Volume_mm3) divided by the contact area. The thermal interface material (thermal grease, sil-pad, etc.) is modeled by a single resistor of the value provided by the Rinterface parameter. If that parameter has not been specified, a default value is calculated based on (100°C/W) / Area_Contact_mm2. The opposite end of the bar connects to the ambient environment through a single resistor of value Rtheta. That models the power dissipation to the environment, as described above.

Since the bar is modeled as a series of resistor-capacitor taps in a Cauer thermal model, the transient as well as steady state behavior is reflected in this simple model.

SOAtherm-PCB model

The SOAtherm-PCB model assumes that the PCB has only a single layer of copper on one side of the PCB, and the total area of that copper is defined by the Area_PCB_mm2 parameter. The copper is modeled as a circle with the MOSFET in the center. The copper circle is further divided into ten concentric circles, with the smallest circle’s inner radius determined by the Area_Contact_mm2 parameter. The SOAtherm-PCB thermal model lumps each circle’s thermal resistance and thermal capacity into an R-C pair forming one of the ten taps of the Cauer thermal model.

The power is assumed to dissipate from the top and the bottom of the PCB through both convection and radiation. The convection and radiation are modeled independently at each of the ten taps of the Cauer model described above. This provides a more accurate result than closed form equations that attempt to describe convection and radiation of an entire PCB.

Note that a natural convection (no airflow) heat transfer coefficient value of 1.1625e^-5 W/(°C•mm²) is assumed in the default convection model. This value may vary depending on the structure shape, orientation of the PCB, laminar vs. turbulent airflow, etc. If desired, this value may be overridden with the hconv0 parameter.

Airflow is modeled by adjusting the heat transfer coefficient of the convection model according to the equation below:

hconv = hconv0 (1 + 0.013 • LFM0.8)

A complete set of parameters that can be used with the SOAtherm-PCB model are listed below. In most cases, only the subset of parameters listed in the previous section of this document are necessary.

Recommendations for Further Reading:

• LTspice: SOAtherm Tutorial
• LTspice: Modeling Safe Operating Area Behavior of N-channel MOSFETs
• LTC4226 Parallel MOSFETs in Hot Swap Circuits