Transistor power dissipation is often listed using two closely related metrics, including ambient temperature (T_A) and case temperature (T_C). As an example, consider the datasheet design-maximum ratings for the onsemi TIP41BG NPN transistor as highlighted in Figure 1. The power dissipation is listed as 65 W using case temperature assumptions. The power dissipation drops to 2 W using ambient temperature assumptions. Understanding the difference between these two specifications is essential for reliable equipment design.
Figure 1: Excerpt from the onsemi TIP41 datasheet with total power dissipation sections highlighted in red.
Tech Tip: The total power dissipation is an absolute design maximum rating – not to be exceeded under any circumstances. For long reliable equipment life, stay far away from the design maximums. Also, avoid simultaneously stressing the component across multiple dimensions. A design that attempts to simultaneously operate the TIP41BG with V_CE at 80 VDC and 65 W total power dissipation is unlikely to provide long-term reliability.
What is transistor ambient temperature?
Ambient temperature is the temperature of the air in the enclosure surrounding the electronics. A room-temperature benchmark of 25˚ C (77°F) is universally used for semiconductors. Note that this assumes still (unmoving) air with no fans or other forced cooling mechanisms.
A better way to look at this situation is to assume a temperature probe is placed in the enclosure. The temperature probe measures the air temperature, with no physical connection to the transistor under consideration.
Tech Tip: The ambient power dissipation metric assumes an absence of a heatsink. Under these conditions, the featured TIP41 transistor can dissipate no more than 2 W.
What is transistor case temperature?
Case temperature is measured directly on the transistor. For the featured TIP41, we can take the temperature reading directly on the metal tab of the transistor.
A useful way to understand this benchmark is to assume that an ideal (infinite) heatsink is used to maintain the transistor’s case at 25˚ C. Under these ideal conditions, the featured TO-220 transistor can sustain 65 W dissipation.
Tech Tip: In all cases, the maximum power dissipation metrics link back to the core temperature of the semiconductor die. By working backwards from T_A and T_C, we find the same maximum die junction temperature T_J. Consequently, the T_A and T_C metrics provide a convenient way to visualize the transistor’s functionality considering the junction-to-case and junction-to-ambient thermal resistance.
Derating for elevated temperature
The datasheet specified maximum power dissipation is accompanied by a derating clause. In this example, the total ambient power dissipation is specified as 2.0 W with a derating of 0.016 W/˚ C while the case temperature derating is 0.52 W/˚ C. We can use these numbers to calculate the transistor’s maximum power dissipation at an elevated temperature.
Elevated ambient temperature example
Let the ambient temperature be 50˚ C which is a realistic residential temperature for electronics packaged in a small enclosure.
The total power dissipation is calculated as:
P_D = 2 – 0.016 (50-25) = 1.6 W
If we allow a 25% safety margin, the maximum recommended continuous power dissipation is 1.2 W.
Elevated case temperature example
A similar example may be used for elevated case temperature calculations. Using the same 50˚ C example, we calculate maximum power dissipation as:
P_D = 65 – 0.52 (50-25) = 52 W
With our 25% safety margin the transistor maximum power dissipation is about 40 W. Like before, we assume a quality heatsink that can maintains a worst-case temperature of 50˚ C. Refer to this excellent article for more information about heatsink calculations. You will find that the 40 W continuous rating can be difficult to achieve when we consider the thermal interface materials as well as the size and cost of appropriate heatsinks.
Parting thoughts
Determining the maximum power dissipation requires careful analysis to ensure long equipment life. We begin with a recognition that die temperature is the limiting factor for a transistor’s power dissipation. Ambient temperature and case temperature are simply convenient ways to express the same information. Ambient does not include a heatsink while case temperature assumes an ideal heatsink. Be sure to review this article for assistance with real-world calculations.
Keep your semiconductors cool.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.