Tackle Thermal Design At the System Level

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

High Temperatures

The main goal in thermal analysis is avoiding problems from excessive temperatures, including thermal runaway where the junction temperature increases until the device fails. High junction temperature has many other electrical and mechanical effects. A few of these effects are:

• Increased leakage currents

• Increased degradation of gate oxides

• Increased mechanical stresses

• Increased MOSFET on-resistance

• Reduced MOSFET threshold voltage

• Reduced transistor safe operating area.

Knowing a few of the critical thermal milestones can provide more insight into selecting components and diagnosing failed devices:

• 130°C — common FR4 circuit board maximum temperature rating

• 150°C — typical maximum junction temperature rating of power ICs

• 165°C to 185°C — typical power IC and power transistor overtemperature shutdown

• 155°C to 190°C — mold compound's glass transition temperature

• 183°C — melting point of Sn₆₃Pb₃₇ solder (63% tin, 37% lead, standard eutectic solder)

• 217°C to 220°C — melting point of Sn_96.5Ag_3.0Cu_0.5 (96.5% tin, 3% silver, 0.5% copper, high-temperature solder)

• 280°C — typical melting point of die attach solder

• 660°C — melting point of pure aluminum (metallization and wirebonds are often aluminum)

• 1400°C — melting point of silicon.

To investigate the thermal aspects of a particular system, three analysis tools can be used in combination to answer questions about temperatures: empirical testing of a prototype or of a thermal mock-up; analysis using manufacturers' thermal ratings and characterizations; and thermal modeling.

Characterizing Systems

In the product development schedule, the use of empirical testing to supplement the data from thermal resistance ratings and thermal models enhances the understanding of the system. Common tools for empirical testing include measurements with thermocouples, infrared scanning and measuring a device's junction temperature by monitoring a temperature-sensitive parameter (TSP) of one of its components. Following are a few tips for each.

For thermocouple measurements, using the smallest gauge wire possible reduces the heatsinking effect of the leads. Monitoring as many points as practical improves the overall understanding of the circuit and enhances the chance of detecting unexpected hot spots.

Infrared scanning is very helpful because it provides information about the entire scanned area. Discovering unexpected hot spots, such as undersized pc-board traces or connector pins, with an infrared scan is not uncommon.

A diode's forward voltage is one of the most commonly used TSPs, and diodes may be readily accessible as electrostatic-discharge structures on a logic pin, for example. Using a TSP requires establishing the TSP's variation over temperature, which must be done at near-zero power.

A thermal mock-up that mimics the final module's behavior with similar mechanical and thermal characteristics, but little or no electrical similarity, can provide a quick evaluation tool to assess the total module power budget, thermal coupling, effects in changes to the primary thermal path, and more.

Models and Ratings

Several simple thermal models use the supplier's thermal ratings and the actual application interface. However, all have limitations because each inherently attempts to simplify a complex thermal circuit using a few lumped components.

To model transient conditions, the thermal circuit needs to include capacitance as well as the thermal resistance values for junction to case, case to heatsink and heatsink to ambient. The values of the R's and C's can be estimated using the system's material properties and physical dimensions, or they can be extracted from empirical tests.

For example, when the system is powered and in steady state, the thermal resistances can easily be derived from the power dissipation and the temperatures at the three thermal nodes. Characterizing the transient response requires monitoring temperature response to a step input of power. As the number of power devices increases, this approach becomes quite complex.

For a module with multiple power devices, a model that accounts for each device's power dissipation, junction temperature and thermal resistance, but not thermal capacitance, is quite popular. As shown in Fig. 5, each device is treated independently with a common internal air temperature separated from the housing temperature by a single internal air-to-housing thermal resistance and from the ambient temperature by a housing to ambient thermal resistance.

The designer estimates the module's internal air temperature rise above ambient based on experience and adjusts to account for the module's size as well as its other thermally significant features. Finally, device junction temperatures are estimated from the device's R_θJA, its power dissipation and the module's internal air temperature, which is the ambient temperature for the power device.

Fig. 6, which depicts a design that has been simulated using Flomerics' FLOTHERM simulation software, illustrates the compromise that results from using a single internal ambient temperature. In this example, external temperature is 85°C and the maximum temperature is 123°C, but this temperature occurs on only one of five power devices.

Yet another approach accounts for thermal coupling between devices and is practical for systems with up to four or five power devices. The improved accuracy comes at the cost of additional empirical characterization and the need to solve several or even many simultaneous equations. As the number of power components increases, this rapidly becomes more difficult.