Rch vishay sfernice – C&H Technology RCH User Manual
Page 4
RCH
Vishay Sfernice
Power Resistors, for Mounting onto a Heatsink
Thick Film Technology
www.vishay.com
For technical questions, contact: [email protected]
Document Number: 50006
3
Revision: 09-Jun-08
RECOMMENDATIONS FOR MOUNTING ONTO A HEATSINK
Surfaces in contact must be carefully cleaned.
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. In order to improve thermal conductivity, surfaces in contact (alumina,
heat-sink) are coated with a silicone grease (type Sl 340 from Rhône-Poulenc or Dow 340 from Dow Corning).
The fastening of the resistor to the heatsink is under pressure control of two screws (not supplied).
In order to improve the dissipation, either forced-air cooling or liquid cooling may be used.
A low thermal radiation of the case allows several resistors to be mounted onto the same heatsink.
Do not forget to respect an insulation value between two resistors (dielectric strength in dry air 1 kV/mm).
In any case the hot spot temperature, measured locally on the case must not exceed 125 °C.
Tests should be performed by the user.
CHOICE OF THE HEATSINK
The user must choose the heatsink according to working conditions of the component (power, room temperature). Maximum
working temperature must not exceed 125 °C. The dissipated power is simply calculated by the following ratio:
P:
Expressed in W
T:
Difference between maximum working temperature and room temperature.
R
TH
:
(j-c): Thermal resistance value measured between resistance layer and outer side of the resistor.
It is the thermal resistance of the component (See Special Features table).
R
TH
:
(c-a): Thermal resistance value measured between outer side of the resistor and room temperature.
It is the thermal resistance of the heatsink depending on the heatsink itself (type, shape) and the quality of the
fastening device.
Example:
R
TH
: (c-a) for RCH 25 power rating 20 W at ambient temperature + 50 °C.
ΔT ≤ 125 °C - 50 °C ≤ 75 °C
R
TH
(j-c) = 1.4 °C/W (Special Features)
R
TH
(j-c) + R
TH
(c-a) =
=
= 3.75 °C/W
R
TH
(c-a)
≤ 3.75 °C/W - 1.4 °C/W ≤ 2.35 °C/W
Tightening Torque
on heatsink
RCH 5
RCH 10
RCH 25
RCH 50
0.5 Nm
0.6 Nm
0.7 Nm
1 Nm
P
ΔT
R
TH
j
c
–
(
) R
TH
c
a
–
(
)
+
[
]
------------------------------------------------------------------
=
1
( )
ΔT
P
-------
75
20
------