LTA 50 www.vishay.com Vishay Sfernice CHOICE OF THE HEATSINK The user must choose according to the working conditions of the component (power, room temperature). Maximum working temperature must not exceed 175 °C. The dissipated power is simply calculated by the following ratio: ΔT (1) P = ---------------------------------------------- R + R + R TH (j - c) TH (c - h) TH (h - a) P: expressed in W. ΔT: difference between maximum working temperature and room temperature. RTH (j - c): thermal resistance value measured between resistive layer and outer side of the resistor. It is the thermal resistance of the component. RTH (c - h): thermal resistance value measured between outer side of the resistor and upper side of the heatsink. This is the thermal resistance of the interface (grease, thermal pad), and the quality of the fastening device. RTH (h - a): thermal resistance of the heatsink. Example: RTH (c - a) for LTA 50 power rating 10 W at ambient temperature +25 °C Thermal resistance RTH (j - c): 2.5 °C/W Considering equation (1) we have: ΔT = 175 °C - 25 °C = 150 °C Δ 150 R T TH (j - c) + RTH (c - h) + RTH (h - a) = ----- = -------- = 15 °C/W P 10 RTH (c - h) + RTH (h - a) = 15 °C/W - 2.5 °C/W = 12.5 °C/W with a thermal grease RTH (c - h) = 1 °C/W, we need a heatsink with RTH (h - a) = 11.5 °C/W. OVERLOADS In any case the applied voltage must be lower than the maximum overload voltage of 750 V. Accidental overload: the values indicated on the graph below are applicable to resistors in air or mounted onto a heatsink. ENERGY CURVE Axis Title 100 10000 10 1000 ine (J) ne ne 1 nergy 1st li 2nd l 2nd li E 100 0.1 0.01 10 0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 Overload Duration (s) Revision: 23-Jan-2025 3 Document Number: 50086 For technical questions, contact: sferfixedresistors@vishay.com THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
