Junction temperature measurement by non-contact method (electrical)
By combining high-speed digital multimeters and high-precision fast current pulses, the junction temperature can be measured in multi-LED circuits. The results allow the thermal resistance to be calculated under specified conditions. The Electronic Industry Association EIA / JEDEC JESD51-1 describes the “Electrical Test Method”, a junction temperature measurement technique based on direct voltage measurement for diode testing. This method uses two levels of current: a measuring current, a low level, and a high-current heating current. The measuring current level is large enough to ensure the diode conduction state, but sufficiently low to not produce significant self-heating, typically several tens of milliamps. For LEDs, the heating current is usually chosen to be nearly the same as the operating current – hundreds of milliamps or amps.
Example of a junction temperature measurement circuitry in a luminaire with separate power supply together with measuring instruments. The tested device has LEDs in series, mounted on a heatsink in order to remove heat. In this example, the junction temperature is measured from a single LED in the array, and the thermal resistance (Rθ) of the junction is calculated.
In the test method, first the change of the direct voltage (Vf) to the temperature is determined by feeding the LED with the measuring current and the temperature variation. The resulting points are represented graphically and the relation is reduced to a single slope factor, called the factor K: K = ΔVf / ΔT
For single LEDs, K ranges from 1 to 3 mV / ° C. Using this factor, a change in LED junction temperature can be calculated by measuring the corresponding change of voltage Vf and division by K: ΔT = ΔVf / K The electrical test method specifies a dynamic technique for measuring the junction temperature (Tj) under high-current heating conditions. The LED is powered by a large heating current for a period of time and then quickly switched to the low current. Vf is measured immediately after switching to the low current and compared with the measured Vf voltage before applying the heating current. Using the above equation, ΔT is obtained and added at the temperature of the known case (Tcase) of the LED to obtain the actual junction temperature: Tj = Tcase + ΔVf / K
Although the test method is quite simple, implementation in LED lighting circuits presents significant challenges for both the pulse source and the measuring instruments. The pulse source must provide fast and precise high intensity pulses in a power circuit containing cables and then quickly return to low current levels to allow for junction polarization stresses. Impulse currents up to 10A and voltages of hundreds of volts are typical values. As for small currents, there are three to five orders of magnitude below these levels. Similarly, the measuring instrument must rapidly acquire voltage values, typically up to 200 V, to accurately measure millivolts in tens of microseconds at each trigger.
These requirements are met by customized measurement and data acquisition systems, therefore pulse sources are dedicated to this type of test. The characteristics of these specialized equipment are not met by most measuring instruments, usually used for this type of measurement.