The difference is equal to the amount of heat transferred from the junction to case multiplied by the junction-to-case thermal resistance.
On the high end, the resulting increase in local power dissipation can lead to thermal runaway that may cause transient or permanent device failure.
Maximum junction temperature (sometimes abbreviated TJMax) is specified in a part's datasheet and is used when calculating the necessary case-to-ambient thermal resistance for a given power dissipation.
In modern processors from manufacturer such as Intel, AMD, Qualcomm, the core temperature is measured by a network of sensors.
If the applied mechanisms are not compensating enough for the processor to stay below the junction temperature, the device may shut down to prevent permanent damage.
= power dissipation in package [W] Many semiconductors and their surrounding optics are small, making it difficult to measure junction temperature with direct methods such as thermocouples and infrared cameras.
This difficulty can be overcome by combining high-speed sampling digital multimeters and fast high-compliance pulsed current sources.
An LED or laser diode’s junction temperature (Tj) is a primary determinate for long-term reliability; it also is a key factor for photometry.
An L-I sweep conducted with an Osram Yellow LED shows that Single Pulse Test Method measurements yield a 25% drop in luminous flux output and DC Test Method measurements yield a 70% drop.