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RTOP

Vishay Sfernice

Document Number: 50045

Revision 14-Jan-04

For technical questions, contact sfer@vishay.com

CHOICE OF HEATSINK

The user must choose the heatsink according to the 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.

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 Table Special Features).

RTH: (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:

RTH: (c-a) for RTOP 200 power rating 130W at ambient temperature + 30°C.

Thermal resistance (see table 1) RTH (j-c): 0.5°C/W

∆T ≤ 125°C - 30°C - ≤ 95°C

RTH (j-c) + RTH (c-a) =

= 0.73°C/W

RTH (j-c) ≤ 0.5°C/W

RTH

(c-a) ≤ 0.73°C/W - 0.5°C/W ≤ 0.23°C/W

∆T

[RTH (j-c) + RTH (c-a)]

(1)

RECOMMENDATIONS FOR MOUNTING ONTO A HEATSINK

Surfaces in contact must be carefully cleaned.

The heatsink must have an acceptable ﬂatness: from 0.05mm to 0.1mm/100mm.

Roughness of the heater must be around 6.3µm.

In order to improve thermal conductivity, surfaces in contact (alumina, heatsink) are laid on with a silicone grease (type SI 340

from Rhône-Poulenc or Dow 340 from Dow Corning).

Tightening torque on heater: 2 Nm

For the electrical connections, it is recommended to use M4 x 6 screws and if necessary a washer of 1mm thickness. The

recommended screw tightening torque is 1 Nm.

130

∆T

Power Resistors for Mounting onto a Heatsink

Thick Film Technology

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