Application Note Calculating thermal resistance Light Emitting Diode of LEDs Contents 1. Objective 2. Features 3. Measurement Method and Procedures 4. LED Mounting 5. Calculation 6. Conclusion This document contains tentative information; the contents may change without notice. 1/8 (SE-AP00014A) Jan. 6, 2015 Application Note 1. Objective The thermal resistance of LEDs can be obtained by the junction temperature (TJ) estimated from the forward voltage (VF) and the input power, since VF changes due to the ambient temperature (TA). To ensure accuracy and repeatability, this application note describes the other measurement methodology of the thermal resistance of LEDs in natural convection as described in JESD51-2 by JEDEC (Joint Electron Device Engineering Council). 2. Features Measurements of the thermal resistance vary depending on the ambient temperature, the LED-mounting method, and the board construction. To minimize the measurement variation, Light Emitting Diode Nichia utilizes the measurement methods by controlling the following items: Table 1 Measurement Conditions and Advantages Control Item Box as described in JESD51-2 Temperature (no airflow , natural convection) Voltage Measurement Qty of LED Board Condition Design Advantages Stable temperature 4 wire channel No impact of contact resistance 1 pc. Stable temperature Unified design of the board size and Minimizing the variation due to the pattern the board design 3. Measurement Method and Procedures The temperature coefficient and the change ratio of VF can be measured as follows. 3-1. Measurement of Temperature Coefficient Put an LED into a constant temperature chamber and measure the VF according to the TA change. - Increase the TA from 25°C at a regular interval of 20°C. - To minimize the heat generation from the LED die and increase the measurement repeatability, apply 1 mA to each die. The measurement current (IM) is determined by being multiplied by the circuit connected in parallel. (e.g. 2 dice in series and 3 dice in parallel; IM = 1 mA × 3 =3 mA) * The TA shall be increased till it exceeds the TJ at the rated current. * To regard TA as being the same as TJ, measure the VF after a while. Figure 1 shows the relation between TA and VF. Herein, the slope of the line corresponds to the temperature coefficient. This document contains tentative information; the contents may change without notice. 2/8 (SE-AP00014A) Jan. 6, 2015 Light Emitting Diode Forword Voltage（Ｖ） Application Note Ambient Temperature(℃） Figure 1 Relation between TA and VF 3.2 Change Ratio of VF The change ratio of VF can be obtained by increasing TJ at the rated current. 1) Put an LED into natural convection within an enclosure (cf. Figure 2). 2) 3) Apply IM to the LED and measure the VF1. Make the LED generate heat by applying IF. (IM + IF = rated current) 4) Measure the VP and VF2 and stop applying IM. Please refer to Figure 3 (Page 4) for the measurement timing in 2) to 4) above. * Apply IF at the pulse interval of 10µ, 100µ seconds,・・・; 10 times longer. * To measure RƟJS, connect a thermocouple to the Ts measuring point with solder paste and measure the Ts. (cf. Section 4) This document contains tentative information; the contents may change without notice. 3/8 (SE-AP00014A) Jan. 6, 2015 Light Emitting Diode Application Note Test Fixture (Front View) Test Fixture (Side View) Position of LED (the center of the box) TA Measuring Point Test Fixture (Front View) The temperatures shall be measured in a closed environment with the front covered by a board. The support fixture depends on the test board; Left: FR-4, Right: Aluminum. Figure 2 Test Enclosure Assembly in accordance with JESD51-2 This document contains tentative information; the contents may change without notice. 4/8 (SE-AP00014A) Jan. 6, 2015 Application Note Voltage VP VF 1 VF 2 Light Emitting Diode Time Heating Time （10μsec,100μsec,…） I M ON I F ON I M OFF I F OFF Figure 3 Measurement Timing of VF 4. LED Mounting Mount an LED on a test board. When the input power is high, mount the LED on an aluminum board (cf. Figure 4). On the other hand, when the input power is low, mount the LED on an FR-4 (cf. Figure 5) (*TJ= 85°C) TS Measuring Point Patterns on Copper Foil Resist Picture Board Depth: 1.5mm, Insulating Layers’ Depth: 120μm, Copper Foil Depth: 105μm Figure 4 Aluminum Board (NVSW119B) This document contains tentative information; the contents may change without notice. 5/8 (SE-AP00014A) Jan. 6, 2015 Application Note φ0.85 hole × 108 B D Close up TS Measuring point Light Emitting Diode 1st Layer 2nd Layer 4th Layer 3rd Layer Picture -Completed Board - Board Depth: 1.6 mm, Copper Foil Depth: 70 μm (1st and 4th layer), 35 μm (2nd and 3rd layer) - Diameter of Through Holes: 85 μm, Diameter of Through Hole Land: 125 μm - The 4th layer above was seen from the reverse side of the board. - The electrical conduction is provided by the connection between anode (at B on 2nd layer) and cathode (at D on 3rd layer) through the via holes (Through Holes). Figure 5 Glass Epoxy Board (e.g. NS2W757A) * Ts measuring point shall be 1mm outside the land pattern. * Please refer to the specifications concerning the recommended location of copper pattern and Ts measuring point, since they depend on each model. This document contains tentative information; the contents may change without notice. 6/8 (SE-AP00014A) Jan. 6, 2015 Application Note 5. Calculation RƟJA and RƟJS can be obtained by the following formulae: - K (℃／ V ) where, T A 2 T A1 V F (T A1 ) V F (T A 2 ) K = temperature coefficient TA1, TA2 = TA in measuring each temperature coefficient VF(TA1), VF(TA2) = VF(TA) in measuring each temperature coefficient VF(TA2) < VF2 < VF(TA1), TA1 < TA2 - ΔTJ (℃) (VF 1 VF 2 ) K Light Emitting Diode - P (W ) ( I M I F ) VP - RθJA (℃／W ) ΔTJ P - T J (℃) ΔT J T A Herein, TA is the ambient temperature before the test. - RθJS (℃／W ) TJ TS P RƟJA= thermal resistance between TJ and TA, RƟJS = thermal resistance between TJ and TS RƟJA and RƟJS shall be calculated in a thermally stable condition after the LED is operated. When the transient thermal resistance should be measured before the thermal stability, RƟJA can be obtained by the above described formula. Please refer to Figure 6 for the transient thermal resistance curve. Figure 6 Transient Thermal Resistance Curve This document contains tentative information; the contents may change without notice. 7/8 (SE-AP00014A) Jan. 6, 2015 Application Note 6. Conclusion Light Emitting Diode If you are concerned, please contact us. This document contains tentative information; the contents may change without notice. 8/8 (SE-AP00014A) Jan. 6, 2015