Application Note

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: sfer@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
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: sfer@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
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: sfer@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
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: sfer@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
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: sfer@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