Get the Heat Out! … Electronic-Thermal Design Modeling

Mike Donnelly
25 March 2017

We have recently added a number of new electro-thermal models to our Component Library. These models represent devices that are either temperature sensing/control elements, or electronic components that dissipate significant power and may require thermal analysis. These models include:

  • Peltier Thermo-Electric Cooler
  • Thermistor
  • BJT (NPN and PNP)
  • MOSFET (N- and P-channel)
  • Diode (Rectifier and Zener)
  • LED and Incandescent Lamp
  • Linear Regulator and OP-AMP

With this new set of models, in addition to our existing passive thermal network elements, our users can more effectively assess the thermal aspects of all their electronic designs, and explore temperature control concepts to mitigate thermal problems.

Modeling Transistor Amplifier Self-Heating using a Thermal Network

The circuit shown above is a very simple transistor amplifier, intended to show the basics of electro-thermal modeling. The "design" includes an 8 Ohm pull-up resistor and an active pull-down NPN BJT. Both of these models are from the “Thermal and Electro-thermal” Category of the Components Library. They both have a thermal port that can connect to an external thermal network, and they output all power dissipated in the device as a thermal heat-flow into that network.

The thermal network includes the heat-sink's heat capacitance (0.1 J/degC) and heat transfer resistance to the ambient (10 degC/Watt). This network configuration represents a system in which the resistor and transistor contribute heat to the same heat-sink, and thermal “cross-talk” or mutual- as well as self-heating is modeled. The transistor's thermal heat-flow path also includes an 8.8 degC/Watt resistance, representing the junction-to-lead thermal resistance published in the device datasheet (see Diodes Inc. FZT869).

From the simulation results it is clear that the heat-sink temperature rises to nearly 120 degC (purple waveform), causing the transistor's junction temperature to approach 150 degC (average, red waveform). This is significantly higher than the value predicted in a companion design example: "Modeling Transistor Amplifier Self-Heating - Hot Part Monitor"). In that system, the two devices are assumed to be thermally isolated, and the steady-state junction temperature < 90 degC.

Electro-thermal Trade-off Analysis for AC-DC Converter with Current Boost Regulator

The second example shows a more practical electronic circuit with electro-thermal aspects modeled. The design is a 5V regulator (non-switching), driven from a 120 Vac/60 Hz input and using a transformer/rectifier circuit to step down to a much lower DC-link voltage. The load current capability is 5A, which is well above the current limit of the linear regular component itself. This is thanks to the load sharing role of the bypass PNP transistor. The design is based on an example application circuit shown in Figure 11 of the On Semiconductor Datasheet MC7800/D, November 2014 - Rev. 27.

All of the power dissipating electronics' models are from the "Thermal and Electro-thermal" Components Library. This includes the rectifier diodes, the linear regulator and the BJT, as well as the current sense resistor and the effective winding resistance of the transformer primary and secondary.

The thermal network includes the heat-sink's heat capacitance (0.1 J/degC) and heat transfer resistance to the ambient (1 degC/Watt), as well as the datasheet published values for the junction-to-lead thermal resistance of each of the active electronic components. An assumed value for the thermal capacitance of the BJT (0.005 J/degC, not provided by the manufacturer) was added for purposes of illustration. Obtaining the actual value would require deeper analysis or measurement of this important component characteristic. However, in this example, both thermal capacitance values were selected solely to give sufficiently fast thermal time constants, so that steady-state operating temperatures could be reached with minimal simulation time.

This design is based on another shared design “AC-DC Power Adapter with Current Boost Regulator”. That design has been modified not only to add the electro-thermal aspects, but it was also adjusted to improve its thermal performance. For example, the DC-link capacitance (c3) was increased from 4700uF to 22000uF, to allow a reduced DC-link operating voltage (i.e. to improve efficiency) while avoiding regulation drop-out at AC zero-crossings under heavy load conditions.

Laser Temperature Regulator System using a Peltier TEC

The third example shows a complete thermal control system in which a Peltier Module, or Thermo-Electric Cooler (TEC), is used to actively transfer heat away from a "laser" or any other electronic device, during fast-changing power dissipation conditions. A thermistor, which has resistance that is highly sensitive to temperature, is used in a Wheatstone bridge configuration. The bridge produces a differential voltage that is amplified by an op-amp circuit. The op-amp output voltage is approximately proportional to temperature over a limited range, and 180 degrees out of phase.

The rest of the control loop is modeled using ideal mathematical control blocks. This abstraction allows the designer to focus on the overall performance of the regulator, and to assess the choice of PID gains during actual transient operation. These gains were selected to ensure stability of the loop using a companion design: Laser Temperature Regulator TDFS. In that design, a TDFS loop-stability analysis is performed at a nominal operating point. But because many of the components in this loop are non-linear in nature (e.g. the TEC, the thermistor/bridge, even the op-amp with rail voltage limiting), it is good practice to simulate performance during large-signal transients, as shown here.

In this case, the drive voltage to the self-heating “laser” (i.e. modeled with the electro-thermal resistor) is step increased to several operating levels (blue waveform), and the temperature of the laser (red waveform) is seen to be held at the regulation set-point of 25 degC, with only momentary disturbances during the power level transitions.

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