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Hot Topic: Electronic Thermal Design

One aspect of successful circuit design is getting the thermal analysis right: How much heat will be produced under various operating conditions, and will any component exceed its rated limit? Often this process is handed off to thermal/packaging engineers who have expertise in this area. While that know-how is a big plus, the process disconnect is a big minus, a barrier to first-pass success. In this article, I’ll discuss an integrated electronics + thermal design environment that can give electronics engineers “first pass, sanity check” answers to these thermal design questions.

The IEEE Standard 1076.1 (VHDL-AMS), which natively supports not only analog and digital electronic hardware modeling, but also thermal behavior and the interactions between those aspects, is the key to having an integrated system viewpoint. Several examples of how this modeling capability provides timely visibility of important electro-thermal interactions are shown below. These examples are available in SystemVision Cloud, a free online circuit simulation platform. I encourage you to click on each schematic’s associated link and open the “live” version of any of these circuits. There, you can view other signals and component parameter values; or create a copy of the circuit and make desired changes, then run a new simulation and immediately see the result of those changes.

Linear Regulator Temperature Finder

The first example is a “virtual thermal calibration” circuit. It can help designers predict the temperature of a linear regulator when configured for a specific part in its intended usage context. The configuration is based on readily available information from the component manufacturer's datasheet. In SystemVision Cloud, the user can adjust the regulator model's parameters to match the electrical and thermal characteristics for a particular part number. This includes the output voltage, VDO and current limit, as well as the junction-to-case and junction-to-ambient or heat-sink thermal resistance values. Notice that the red terminal on the regulator model is its “thermal” connection, from which the internally dissipated heat flows into the external thermal network.

In this example, an L78S05 with direct case-to-ambient heat transfer is modeled (i.e., no heat sink). The datasheet specifies the junction-to-case resistance to be 5o C/Watt, and the junction-to-ambient resistance for a T0-220 package to be 50o C/Watt. Therefore, the difference of 45o C/Watt is assumed to be the case-to-ambient thermal resistance. This value is assigned as the “heat-sink” resistance in the example circuit. If an actual heat sink is being used, its published thermal resistance can be used instead.

If the heat-sink thermal capacitance is also provided, that value can be entered and then the simulation will predict not only the steady-state operating temperature, but also temperature response to input voltage and load transients. The input voltage function generator can apply any time-varying input voltage profile. The load current level can be adjusted, or a custom dynamic load model can be used. Together, these can accurately represent the expected operational context for the regulator.

Thermal Modeling of AC-DC Power Adapter with Current Boost Regulator

The second example shows a similar but more complete linear regulator circuit. The 5 V regulator is driven from a 120 VAC/60 Hz input using a transformer/rectifier circuit to step down to a 9 V DC-link voltage. The required load current capability is 5 A, which is well above the current limit of the linear regular component itself (1 A). The current difference is provided by the load sharing role of the MJ2955 PNP bypass transistor. (Note: This design is based on an application circuit shown in Figure 11 of the On Semiconductor Datasheet MC7800/D, November 2014 – Rev. 27 ).

Note that all of the power dissipating electronics models, including 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, have the red “thermal” terminal and their heat flows are combined in the external thermal network. That network includes the heat-sink's heat capacitance (0.1 J/o C) and heat transfer resistance to the ambient (1o C/Watt), as well as the datasheet published values for the junction-to-lead thermal resistance of each of these active electronic components. An assumed value is used for the thermal capacitance of the BJT (0.005 J/o C). This value was not provided by the manufacturer, but rather was selected to give a fast thermal time constant for illustration purposes only.

Electronics 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 self-heating electronic device (e.g., a laser), during fast-changing power dissipation conditions. An NTC-type 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. The drive voltage to the self-heating “laser,” which is simply modeled with an electro-thermal resistor, is step increased to several operating levels (blue waveform). The temperature of the laser (red waveform) is seen to be held at the regulation set-point of 25o C, with only momentary disturbances during the power level transitions.

Summary

In the physical world, the electrical and thermal aspects of the system are coupled. Is that reality properly assessed when the electrical and thermal analyses are done independently? Do the thermal engineers account for the heat dissipation of each component at each operating state, or is it easier to assume they are all running at full power? Do the electrical engineers know which parts of the circuit the thermal engineers are struggling to keep cool, and that it would help to tell them that Part A is off when Part B is on? IEEE Standard VHDL-AMS models support simulation of the electronic and thermal aspects together, helping to bridge these awareness gaps and prevent surprises in the production hardware.

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