Tackle Thermal Design At the System Level

Jun 1, 2008 12:00 PM
By Kim Gauen, Applications Engineer, Freescale Semiconductor, Kokomo, Ind.

Designing a cost-competitive electronic system requires careful consideration of the thermal domain as well as the electrical domain, especially where power is involved. Overdesigning the system adds unnecessary cost and weight, but underdesigning may lead to overheating and even system failure. Finding an optimized solution requires a good understanding of how to predict the operating temperatures of the system's power components and how the heat generated by those components affects neighboring devices, such as capacitors and microcontroller units.

Same Goal, Different Perspectives

While the semiconductor supplier and system designer may have the same goal — a successful end product — when it comes to thermal analysis, they are not usually on the same page. Semiconductor manufacturers know the thermal aspects from the semiconductor's perspective. In most cases, semiconductor users have their techniques to evaluate the entire system. However, when the two interface, there can be problems.

One reason may be an educational perspective: The thermal aspect is in the mechanical engineer's realm. For a successful module design, many electrical engineers find themselves handling packaging and thermal aspects — areas in which they may not have the appropriate or sufficient background to address. Without a little additional insight, they may have a painful and unnecessary learning experience.

While impossible to address in a single technical article, a brief discussion of a few key areas can get the system designer on track and headed in the right direction.[1] But first, consider a couple of system examples that further demonstrate the need for understanding thermal design.

For automotive systems, reducing the weight and size of everything has become a high priority. As a result, for the electronic modules, plastic has become a common housing material instead of the cast aluminum housing with a good-sized aluminum heatsink that many systems used previously.

For instance, a lighting control module built using advanced power ICs with highly efficient power switches conducts 55 A, yet is housed in a plastic case with no heatsink. The only heatsinking is that provided by the module's pc board. While not unusual, this could not have been possible just a few years ago. However, a complete thermal analysis and evaluation was required.

In another case, a module initially designed with relays was converted to use the latest analog and mixed-signal intelligent power devices to control lighting. To handle up to 80 A in a plastic case without any heatsink, the copper wiring harness was used to remove heat from the module. In this case, the module designer had to confront thermal issues that were not present in the relay-based design.

Start with the Data Sheet

Looking at the device data sheet sounds simple, but it can reveal as many as five or six different thermal ratings for some ICs. To choose the right one, the designer should use the value for the primary thermal path for heat removal. The table shows three thermal resistance (R_θ) parameters for two different packages. The two packages require different techniques to measure the temperature. Fig. 1 shows the required approach for each.

It turns out that temperature measurements are not easy, especially as the IC package size shrinks. To estimate the junction temperature based on the temperature at the top of the package, JEDEC has standardized the terms for psi-junction to top of package (Ψ_JT) and psi-junction to board (Ψ_JB). These are thermal characterization parameters, not true thermal resistances. However, psi is a complementary parameter that can be used in parallel with the traditional ratings.

Someday systems engineers may use thermal characterization involving psi frequently, but today suppliers do not consistently provide information on psi on new product data sheets, and existing data sheets rarely have this information. The tried-and-true R-theta approach is one where the required data is available, and the designer needs to use several tools and methods to support his or her evaluation.

Without additional thermal consideration, a manufacturer's data sheet showing 71°C/W, a worst-case JEDEC R_θJA rating, would not be used in many applications. However, with a little additional copper on the board, as shown in Fig. 2, and by using copper in the wiring harness, the application could provide a 50°C/W rating and be quite acceptable. Fig. 3 shows data frequently found on a power IC data sheet indicating how additional copper reduces the thermal resistance.

Semiconductor suppliers cannot provide a thermal resistance value for the end application, because the application stack-up impacts and determines the thermal path, as shown in Fig. 4. A large portion of the thermal resistance is in the thermal interface itself. Simply routing the heat out of the back of a package, such as a PQFN, to a minimal pad size results in quite high thermal resistance. With a larger pad or vias or even a heatsink on the back of the board, the thermal resistance can be reduced from 70°C/W to as low as 5°C/W.

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