It is completely understandable to be confused by this! The definitions of “case” and “temperature limits” in power electronics datasheets are notorious for causing headaches.
To answer your primary question right away: No, T_c (Case Temperature) generally does not refer to the top plastic housing. For standard power devices, it refers to the metal backplate.
Here is a straightforward breakdown of how these temperatures work, what the datasheet means, and how testing labs evaluate them.
1. What Exactly is T_c (Case Temperature)?
In standard discrete power packages like TO-220, TO-247, or D2PAK, the “case” refers strictly to the mounting surface that transfers heat to the heatsink.
- For non-isolated packages: T_c is the temperature of the metal backplate. This is the primary thermal path for the heat generated by the silicon die (junction).
- For isolated packages (e.g., TO-220F): The entire outside is covered in plastic. In this specific scenario, T_c refers to the back plastic surface that touches the heatsink, not the top plastic housing facing away from the board.
When you measure the temperature on the top plastic housing (the part you can easily point a thermal camera at or tape a thermocouple to), you are actually measuring T_t (Top Temperature), not T_c.
2. What Does T_c = 100°C Mean in the Datasheet?
When a datasheet specifies a continuous drain current (e.g., I_{D}) at T_c = 100°C, it is a conditional rating, not an absolute maximum limit.
It means: “If you use a heatsink good enough to keep the metal backplate at exactly 100°C, the device can continuously carry this much current without the internal silicon junction (T_j) exceeding its absolute maximum limit (usually 150°C or 175°C).”
The metal backplate can absolutely reach and exceed 100°C in real-world operation, provided the internal junction temperature (T_j) remains below its maximum rating.
3. TUV / SGS Testing: Maximum Allowable Plastic Temperature
When safety laboratories like TUV, SGS, or UL test your product (usually under standards like IEC 62368-1 or IEC 60601-1), they are looking at two main things regarding internal component temperatures: Material Safety and Manufacturer Specifications.
They do not have a single universal temperature limit just for “plastic housings.” Instead, the limits are determined by:
- The Component’s Absolute Maximum Junction Temperature (T_{j(\max)}): Labs will measure the case or top plastic temperature and calculate the internal junction temperature using thermal characterization parameters (like \Psi_{JT}, the junction-to-top thermal characterization parameter). If T_j exceeds the datasheet maximum (e.g., 150°C), the test fails.
- The Plastic’s Relative Thermal Index (RTI): The black epoxy molding compound of the MOSFET/IGBT has a UL-certified RTI, which is the maximum temperature the plastic can withstand long-term without degrading. For most power devices, the molding compound has an RTI of 130°C to 150°C. The plastic housing cannot exceed this RTI limit during normal operation.
- Touch Temperatures: If the device’s plastic housing is physically accessible to the end-user, strict touch temperature limits apply (e.g., 85°C for internal parts that might be briefly touched during maintenance, or lower for external enclosures). If it’s purely internal, the touch limits don’t apply, and the RTI/T_j limits take over.
In Practical Terms:
If you measure the top plastic housing of a TO-247 operating in a power supply, and it reads 115°C, the internal junction is likely sitting around 125°C to 135°C (depending on power dissipation). Most testing labs will pass this, provided the molding compound’s RTI is rated for \ge 130°C and the die T_{j(\max)} is 150°C or higher.