Temperature measurement by the non-contact method (electrical)
By combining high-speed sampling digital multimeters and high-precision fast-pulse current sources, the junction temperature in multi-LED circuits can be measured. The results allow to calculate the thermal resistance in the mentioned 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, the variation of the forward voltage (Vf) with respect to temperature is first determined by supplying the LED with the measuring current and varying the temperature. The resulting points are graphically represented and the relationship is reduced to a single slope factor, called the K factor: 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