Junction temperature
Junction temperature, short for
Microscopic effects
Various physical properties of semiconductor materials are temperature dependent. These include the diffusion rate of dopant elements, carrier mobilities and the thermal production of charge carriers. At the low end, sensor diode noise can be reduced by cryogenic cooling. 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 calculation
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. This in turn is used to select an appropriate heat sink if applicable. Other cooling methods include thermoelectric cooling and coolants.
In modern processors from manufacturer such as
An estimation of the chip-junction temperature can be obtained from the following equation:[3]
where: = ambient temperature for the package [°C]
= junction to ambient thermal resistance [°C / W]
= power dissipation in package [W]
Measuring junction temperature (TJ)
Many semiconductors and their surrounding optics are small, making it difficult to measure junction temperature with direct methods such as thermocouples and infrared cameras.
Junction temperature may be measured indirectly using the device's inherent voltage/temperature dependency characteristic. Combined with a Joint Electron Device Engineering Council (JEDEC) technique such as JESD 51-1 and JESD 51-51, this method will produce accurate measurements. However, this measurement technique is difficult to implement in multi-LED series circuits due to high common mode voltages and the need for fast, high duty cycle current pulses. This difficulty can be overcome by combining high-speed sampling digital multimeters and fast high-compliance pulsed current sources.[4]
Once junction temperature is known, another important parameter,
Junction temperature of LEDs and laser diodes
An
Junction heating can be minimized in these devices by using the Continuous Pulse Test Method specified in LM-85. 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.[6]
See also
References
- ISBN 9789401798075.
- ^ Rudolf Marek, "Datasheet: Intel 64 and IA-32 Architectures", Software Developer's Manual Vol.3A: System Programming Guide
- ISBN 9780387257624.
- ^ "Measuring LED Junction Temperature (Tj) - Vektrex". Vektrex. 2017-01-06. Retrieved 2017-10-17.
- ^ "Thermal Measurements Products & Solutions - Vektrex". Vektrex. Retrieved 2017-10-17.
- ^ "3 Steps to Improved LED Light Measurements: Accuracy - Vektrex". Vektrex. Retrieved 2017-10-17.