Thermal measurements can be used in a variety of applications involving LEDs, laser diodes, and other semiconductors. Thermal measurements are useful at all stages of semiconductor production from design and development to quality assurance. For instance, on-the-fly thermal measurements can detect production failures. Vektrex offers products and solutions to meet these needs.
- Thermal Modelling
The process of developing a mathematical model of an LED, laser diode, or semiconductor’s thermal structure.
- Junction Temperature (Tj)
The operating temperature of the LED, laser diode, or other semiconductor.
- Thermal Resistance (RΘ or Rth)
Defined as the rate of temperature increase for the dissipated power.
- Structure Functions
Calculations that transform thermal transient measurement results into a thermal resistance versus thermal capacitance graph.
This technique, often called the Electrical Test Method, uses a two-level pulsed drive current to measure junction temperature and thermal resistance.
- CIE TC 2-63
The International Commission on Illumination’s (CIE) recommended methods for the optical measurement of high-power LEDs. Learn more about the standard.
- IESNA LM-85
This standard outlines the methodology to be used for the measurement and characterization of high-power LEDs. Purchase a copy of the standard.
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. For example, a typical white LED output declines 20% for a 50 °C rise in junction temperature. Because of this temperature sensitivity, LED measurement standards, like IESNA’s LM-85, require that the junction temperature is determined when making photometric measurements.
The small size of an LED or laser and surrounding optics makes Tj impossible to sense directly. Instead, it is measured indirectly, using the LED or laser diode inherent voltage/temperature characteristic.
Key Products in the MarketplaceThe table below provides a quick look at some key products in the LED/laser thermal measurement marketplace and the measurement applications supported by those products.
|Vendor||Tool||Tj||RΘ||Thermal Modeling||Permissible Pulse Graphs||Transient Pulse Graphs|
|Vektrex||Current Source with Bias + Software||✓||✓||✓||✓||✓|
|Aurotek||JD-2020 LED Thermal Resistance Analyzer||✓||✓||✓|
Turn-Key SolutionsIf you're looking for a turn-key solution for applications from R&D to production, refer to the table below.
|Vendor||Tool||Production Application||R&D Chip Design Application||R&D Packaged Device|
|Aurotek||JD-2020 LED Thermal Resistance Analyzer||✓|
Thermal Resistance, RΘ
Thermal resistance is the most widely used LED/laser thermal property. Thermal resistance (Rth or RΘ) is defined as the rate of temperature increase for the dissipated power. In the case of LEDs and lasers, RΘ is a measure of the capability of the device to conduct heat. Some thermal resistance measurements include all the power applied to the device, but actually, true RΘ is related only to the portion of the dissipated power that is not transformed into light. Tj Utility directly calculates RΘ.
Structure functions are calculations that transform thermal transient measurement results into a thermal resistance versus thermal capacitance graph. The thermal resistance versus thermal capacitance graph acts as a visualization tool and provides insight into each layer the heat passes through. The calculations needed to produce structure functions are complex and special 3rd party tools are generally used to perform them.
The most common Tj measurement technique is the Joint Electron Device Engineering Council’s (JEDEC) JESD51-51 method. This technique, often called the Electrical Test Method, uses a two-level pulsed drive current. The high current level, called the heating current, heats the junction to its operating temperature. After an extended drive period, the current switches briefly to the measurement level. During this low-level interlude, Vf measurements are made using a sampling voltmeter. Usually, the device under test is mounted on a temperature control platform during the test to maintain a constant case temperature.