Learning LTspice on a Mac

The best way to learn an electrical circuit is to simulate it. Up until 2010, PSpice (Personal (computer) Simulation Program with Integrated Circuit Emphasis) was the tool of choice. But then PSpice got purchased by Orcad, which made PSpice part of a huge software package and made the free version only available on a limited time basis. Orcad then got purchased by Cadence. Schools started to look at other vendors such as National Instruments (NI). If schools already had a site license with NI’s LABVIEW, then it was only a matter of paying a few extra hundred dollars a year to acquire the license to NI’s MultiSim, which also provides a one year evaluation version for all students. This may explain those technology fees that students see get added to their tuition bill. Those outside of the classroom or students whose schools don’t have software licensing should consider LTspice, which is truly free and is supported by Linear Technology, which was recently bought by Analog Devices. It has all of the features of the original PSpice software and Analog Devices keeps adding models for their own new chips (op-amps, voltage regulators, etc.). Special thanks to my former college professor on this background of PSpice.

LTspice has a version for Mac at website. In this example, an electrical circuit with a current source was used as a model to explain the thermal dynamics of heat sinking for a metal-oxide-semiconductor field-effect transistor (MOSFET), which can be used for power switching applications. A MOSFET generates heat during its switching and run stages, which results in the build up of heat. Because MOSFET's have temperature limitations, this heat must be dissipated. A MOSFET is surrounded by different types of materials which serve to dissipate this heat through convection. As the heat is dissipated, a particular temperature variance exists across the material. Therefore, these variances must be considered in the selection of materials that are needed to dissipate a certain amount heat. This variance per heat is known as the thermal resistance property which is specified as ℃/W. An element with a thermal resistance of 1 ℃/W means that a temperature variance of 1 degree exists across the element when dissipating 1 Watt of power. If a MOSFET has a thermal operating limit 100 ℃ and the ambient temperature is 40 ℃, then what is the heat sink’s maximum thermal resistance to prevent overheating of the MOSFET, considering that the junction thermal resistance is 1.03 ℃/W and case thermal resistance is 1.09 ℃/W?

The action of having temperature variances across materials could be thought of as a voltage drop across a resistor. Temperature could be thought of as voltage. Temperature variance per power could be thought of as resistance. Power could be thought of as a current source. Ambient temperature could be thought of as a battery source. With these analogies, an electrical circuit could be created to model the temperature variances across different materials. It is important to realize that the electrical circuit is only a model and is not an actual electrical circuit. When the circuit is run through the simulator, the operating temperature of the MOSFET can be determined. Also, the circuit model uses a current source, which establishes a fixed current throughout the circuit regardless of any voltage elements in the circuit. So, circuit analysis must be based on current source behavior.

A MOSFET is typically surrounded by three surrounding layers of materials that can dissipate heat: junction, case, and heat sink. The junction typically has a thermal resistance of 1.3 ℃/W and the case typically has 1.9 ℃/W.  Since the MOSFET cannot exceed 100 ℃, the circuit can be set to run at max conditions. The combined thermal resistances of all heat transferring materials must have a temperature variation variance of 60 ℃. With the circuit drawn in LTspice (see below), Ohm's Law was used to calculate a maximum thermal resistance of 15.982 ℃/W for the heat sink. This value was plugged in the circuit and simulated. The voltage node Tj, which represents the temperature at the junction, was found to be 100 ℃! This proves that the MOSFET is not overheated. Overheating can only happen if the MOSFET's total power losses go up (the current source in the model) or if the thermal resistances of materials go up (the resistors in the model). This is why it is important for materials to have high thermal conductivity which is the reciprocal of low thermal resistance.

Article by DScrobeIII