Performing a thermal analysis in Spice allows you to study a circuit's electrical performance and the accompanying thermal effects simultaneously. For example, a Spice simulation that includes thermal analysis allows you to study the effects of reactive loads on amplifier dissipation or driver waveforms on power-supply switching-transistor dissipation and even secondary breakdown prediction. Spice maintains accurate results in temperature regions where the dominant method of heat transfer is by conduction and convection, where most electronic systems operate.

To perform a steady-state or transient thermal analysis in Spice requires a straightforward transformation of thermal elements to their electrical counterparts (Table 1). The circuit in Fig 1, implemented in PSpice (MicroSim Corp, Irvine, CA), provides an example of the concept. (A copy of the netlist is attached to DI_SIG #1576.) Q1, Q2, D1, D2, and surrounding components comprise a typical class A/B power amplifier for audio and servo applications. D1 and D2 establish a quiescent current in Q1 and Q2 to reduce the crossover distortion common to true class-B amplifiers. Vin and R1 model the driver stage, and emitter resistors Re1 and Re2 create local feedback to help stabilize the amplifier's dc operating point.

Current-controlled voltage sources He1 and He2 measure Q1's and Q2's emitter currents, respectively, and convert these currents into voltages. Voltage-to-current multipliers G1 and G2 multiply the instantaneous transistor collector-to-emitter voltage by the transistor emitter current. This product is a conservative estimate of the instantaneous power that each transistor dissipates.

The network that follows G1 and G2 models the thermal elements of the transistor cases and heat sink. Each multipliers' output is a current source that represents the instantaneous power generated within each transistor, where the scale factor is 1A=1W. Rjc is the thermal resistance between the transistor junction and the case expressed in degrees centrigrade (or kelvin) per watt. Rch is the thermal resistance between the transistor case and heat sink. Cc is the thermal capacity of the transistor case, and Ch is the thermal capacity of the heat sink expressed in watt-seconds per degree centigrade. Thermal capacity equals the product of the mass (in kilograms) and the material's specific heat (in watt-seconds per kilogram°C). Rha is the thermal resistance between the heat sink and ambient. Rhc has a lower value for forced-air cooling than for convection cooling. Fig 1 models the ambient temperature using an ideal voltage source, Vambient, with a dc voltage equal to the ambient temperature. Vambient can be in kelvin or degrees centigrade.

Fig 2 illustrates the thermal effects of applying a sine-wave signal to the power amplifier in Fig 1. The heat sink and transistor case rise in temperature vs time. These curves illustrate the effect of the thermal capacity of the transistor case and the heat sink. The high-thermal-capacity heat sink required considerable time to heat and limits the rate of temperature rise of the smaller thermal-capacity transistor case. ( DI #1576)