Electrical design is just the beginning of an electronics product. Thermals are an essential component in the creation of a reliable design. This blog introduces thermals as a brush up for established engineers as well as a new concept for hobbyists.
Semiconductors are not 100% efficient. The power lost is transferred as heat. Because semiconductors are energy enhanced devices, thermals play a major part in both performance and product survival. This is true whether or not the device is analog or digital. Switching power loss is based on frequency. With today’s advancement in speed, power losses go up as well. Therefore, it’s good to know how much heat a device creates as well as how to dissipate the heat favorably.
Prior to getting into the subject of junction temperature, realize that there is thermal impedance and thermal resistance. Thermal impedance is time based. Thermal resistance is based on steady state operation. Picture a pan on the stove. It takes a while to heat up, doesn’t it? The same is true for a junction. It takes a while for the heat to escape the junction. Knowing this is key to avoiding a device meltdown.
Power dissipation mainly occurs due to conduction, switching, and transient events where voltage and current are present simultaneously. Power dissipation has the units of watts. Voltage multiplied by current gives power (watts = volts x amps). Compute it for a small period and you have the transient thermal temperature. Average it out over time and you have the steady state temperature rise of the junction.
We now have the units of watts that give us power loss. Thermal resistance is given in degrees Celsius per watt. Using the factor label method and borrowing from this most excellent publication by Diodes Inc. we get:
“Rth(JX_ Θ ) = (Tj –Tx) / P
Where P is the dissipated power (heat) that flows from the junction to the point ‘X’. Ideally, during this measurement, close to 100% of the power should flow from the junction to the point ‘X’. This figure depends only on the physical properties of the heat flow path and is independent of the amount of power dissipated or the size of board the device is mounted on.”
Note that the Greek symbol “theta” is the thermal resistance in degrees Celsius per watt. Bouncing over to Wikipedia, we can see how the thermal resistance along a path is nothing more than a series of resistors as shown in the following figure.
Now we have a basic idea of the temperature rise or fall along the way. Of course there are unfair instances such as high ambient temperatures, air flow or lack thereof, and surrounding components that are also contributing heat. This equation does however give a basic idea of the temperatures at certain points which may just explain why that surface mount package keeps unsoldering itself from the printed circuit board (PCB).
As in electrical analysis, resistors are not components that have time varying characteristics. In order to do that, capacitances are placed in parallel with the resistors to create exponential curves that mess up the first order of thought processes based upon basic straight-line algebra. In a way, I think electrical engineers do this to get back at mechanical engineers for those pesky thermal dynamics courses.
That is about as far as I want to go in this initial, introductory blog. I encourage feedback as always to steer future blog subjects on thermals. I have provided some valuable references that describe this aspect of design for proper junction temperature. The TI application note explains packages, surface mounting, and a host of other goodies. The Diodes Inc, University of Colorado, and Cheggs references go more into the device physics.
Physics? Yuck! Mom, he just swore. A fun degree to have now but a pain to obtain. Glad I did it now lay off my kid as he struggles through chemistry. I never used that useless subject so why bother him with it if he wants to lay out automobile designs for a living? My trigger finger is twitching at the thought of it and those tests where I averaged in the 30 percentile. Memorize this! Practical problem solvers choose to look it up and not memorize something you’re going to forget eventually anyway.
- Semiconductor and IC Package Thermal Metrics, Application Report SPRA953C–December 2003–Revised April 2016
- Understanding Semiconductor Thermal Resistance Data, Siva Uppuluri, Applications Engineer, Diodes Inc.
- “The term, kT/q , appears often in semiconductor device equations…” Chegg Study Textbook Solutions
- Principles of Semiconductor Devices, 4.2. Structure and principle of operation, Chapter 4: p-n Junctions B. Van Zeghbroeck, 2011