VISHAY SFERNICE www.vishay.com Film Resistors Application Note LPS Power Thick Film Resistors Mounting Instructions and Thermal Considerations 1. HEATSINK SPECIFICATION The mounting area on the heatsink and the bottom of the LPS must be free of particles. Surfaces in contact must be carefully cleaned in order to obtain the maximum thermal conductivity between the component and the heatsink. The heatsink must have an acceptable flatness: From 0.05 mm to 0.1 mm/100 mm. Roughness of the heatsink must be around 6.3 μm. 2. CHOICE OF THE THERMAL INTERFACE In order to improve thermal conductivity, surfaces in contact should be coated with a silicone grease or a thermal film. The function of this element is to minimise the thermal interface resistance by filling the potential air voids. Since the thermal resistance of air is very high, these voids will substantially degrade performance. Therefore, it is important to use a thermal interface material to fill these air voids. Several materials are available to reduce thermal resistance between the resistor and heatsink surface. Thermal grease is an addition of thermally conductive particles with a fluid typically, a silicone oil. The final consistency is like a grease. We recommend to use for the thermal grease Bluesil Past 340 from BlueStar Silicones (thermal conductivity at 25 °C = 0.41 W/mK - dielectric strenght = 15 kV/mm. Thermal interface can be applied either to the base area of the component or the area of the heatsink. We recommend to have a constant thickness of grease, to screen print or to use a rubber roller to deposit this element. Thermal film, an alternative to thermal grease, but with a lower efficiency for the thermal dissipation, is easier and faster to install than the grease. Moreover, you can use different times the same thermal films and this element allows to have uniform thickness. These thermally conductive pads are available in sheet form or in pre-cut shapes. These pads use silicone rubber binder combined with a variety of materials such as aluminum oxide, boron nitride or magnesium oxide to provide good thermal conductivity. For the thermal interface, we recommend to use Q-PAD II from Berquist (thermal conductivity = 2.5 W/mK; non-insulated). The Q-Pad II film is not electrically insulated. Q-Pad II is available with special dimensions for the LPS. Contact us directly for this thermal pad. 3. MOUNTING THE RESISTOR Avoid any movement of the resistor once positioned on the heatsink. The fixing screws are inserted and evenly tightened by hand (around 0.5 Nm) or by electric or pneumatic screwdrivers with a torque of 0.5 Nm. After, the screws are tightened again to the final torque (2 Nm). The use of torque wrenches with automatic release is recommended. The two step procedure must be strictly followed to allow the component base-plate to relax and conform to the heatsink. The bus-bars must be mounted onto the connections of the power resistor with the recommended torque. The cross sections of the bus-bars must be sufficiently large to avoid heating of the module by bus-bar resistive losses. Stress to the power resistor from bus-bar forces must be minimized during assembly, transportation and operation. SCREW TORQUE VALUES RECOMMENDED (Nm) Resistor on heatsink M4 2 Connexions M4 2 MOUNTING Note • Maximum torque: 2.5 mm Fig. 1 - Mounting assembly for LPS Revision: 10-Oct-13 Document Number: 52025 1 For technical questions, contact: [email protected] 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 APPLICATION NOTE MOUNTING INFORMATION Application Note www.vishay.com Vishay Sfernice LPS Power Thick Film Resistors Mounting Instructions and Thermal Considerations 4. THERMAL CONSIDERATIONS For reliable operation it is crucial not to exceed the maximum specified temperature for the resistive element THERMAL INFORMATION ON LPS RESISTORS MAXIMAL TEMPERATURE FOR RESISTIVE ELEMENT THERMAL RESISTIVITY BETWEEN RESISTIVE ELEMENT AND CASE Rth (j - c) LPS300 120 °C 0.112 °C/W LPS600 155 °C 0.112 °C/W LPS800 175 °C 0.112 °C/W LPS1100 200 °C 0.039 °C/W Excessive resistive temperatures will cause a drift of the resistance value or reduced component life. Proper thermal design followed by temperature measurements to verify the design, and consistent mounting procedures will avoid these problems. The film temperature (Tj) is related to the case temperature (TC) by the parameter “Thermal resistance” Rth (j - c). Thermal resistance is expressed in °C/W. In other words, the thermal resistance Rth(j-c) is the temperature rise (°C) between the film and the case per W applied. 5. CHOICE OF THE HEATSINK The user must choose the heatsink according to the working conditions of the component (power, room temperature). Maximum working temperature must be not exceeded. The dissipated power is simply calculated by the following ratio: T P = ------------------------------------------------------------------------------------------R TH (j - c) + R TH (c - h) + R TH (h - a) P: Expressed in W T: Difference between maximum working temperature and room temperature or fluid cooling 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: (See §4 for the thermal resistivity of each LPS model). 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. APPLICATION NOTE Rth (h - a): Thermal resistance of the heatsink. Example: Rth (c - h) + Rth (h - a) for LPS 1100 power dissipation 850 W at + 18 °C fluid temperature. T 200 °C - 18 °C = 182 °C 182 Rth (j - c) + Rth (c - h) + Rth (h - a) = T ------ = -------- = 0.214 °C/W P 850 Rth (j - c) = 0.039 °C/W Rth (c - h) + Rth (h - a) = 0.214 °C/W - 0.039 °C/W = 0.175 °C/W Revision: 10-Oct-13 Document Number: 52025 2 For technical questions, contact: [email protected] 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 Application Note www.vishay.com Vishay Sfernice LPS Power Thick Film Resistors Mounting Instructions and Thermal Considerations 6. MECHANICAL PROPERTIES (for all LPS models) The clearance distance in air is defined as the shortest direct path between the terminals and the base and between terminals. The surface creepage distance is the shortest path along the plastic housing between the terminals and the base and between the terminals. PARAMETER Dimensions Clearance distance in air Surface creepage distance VALUE UNIT 65.2 x 60 x 25.8 mm Termination to base 14.7 min. mm Termination to term 40 min. mm Termination to base 30 min. mm Termination to term 83 min. mm 7. OVERLOAD In any case the applied voltage must be lower than UL = 6600 V for LPS 1100 and UL = 5000 V for LPS 300, LPS600, LPS 800. APPLICATION NOTE Accidental overload: The values indicated on the following graph are applicable to resistors in air or mounted onto a heatsink. For more information about the calculations from this energy curve, see the Application Note "Pulse Capabilities for Thick Film Power Resistors” (www.vishay.com/doc?50060). Revision: 10-Oct-13 Document Number: 52025 3 For technical questions, contact: [email protected] 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 Application Note www.vishay.com Vishay Sfernice LPS Power Thick Film Resistors Mounting Instructions and Thermal Considerations 8. LIGHTING PULSES Typical calculations are for the lightning pulses. The lightning is an atmospheric discharge of electricity resulting from an accumulation of static charges. The pulses are defined in the IEC 61000-4-5: V Peak 50 % 0 t1 t2 ..t3 2 x t3 t 2 x t1 2 V x - --------------- - --------------- 1 V E = -- x ----- x t 1 + ----------------- x e - e 3 - 2 x R R 2 Lightning pulse With: E = Energy (J) V = Peak Voltage (V) R = Resistance () t1 =Time to peak voltage (s) t2 =Time to 50 % of peak voltage (s) t3 = Time to negligible voltage (s) (minimum 20 x t2) = Exponential rate of decay = - (t2 - t1)/ln(0.50) Using the energy curve and with the limitations for LPS300, LPS600, LPS800 at 5000 V and for LPS1100 at 6600 V, you can apply the following voltages according the ohmic values: APPLICATION NOTE For pulse 1.2/50 (1.2 μs for the increase time and 50 μs for the half-height): Revision: 10-Oct-13 Document Number: 52025 4 For technical questions, contact: [email protected] 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 Application Note www.vishay.com Vishay Sfernice LPS Power Thick Film Resistors Mounting Instructions and Thermal Considerations APPLICATION NOTE For pulse 10/700 (10 μs for the increase time and 700 μs for the half-height): Revision: 10-Oct-13 Document Number: 52025 5 For technical questions, contact: [email protected] 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