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Thermal control

Heat and power design

The whole point of heat sinking is to keep the transistor junction (or the junction of some other device) below some maximum specified operating temperature. For silicon transistors in metal packages the maximum junction temperature is usually 200 deg C, whereas for transistors in plastic packages it is usually 150 deg C.

Heat sink design is then simple: Knowing the maximum power the device will dissipate in a given circuit, you calculate the junction temperature, allowing for the effects of heat conductivity in the transistor, heat sink, etc., and the maximum ambient temperature in which the circuit is expected to operate. You then choose a heat sink large enough to keep the junction temperature well below the maximum specified by the manufacturer. It is wise to be conservative in heat sink design, since transistor life drops rapidly at operating temperatures near or above maximum.

Thermal resistance

To carry out heat sink calculations, you use thermal resistance, defined as heat rise (in degrees) divided by power transferred. For heat transferred entirely by conduction, the thermal resistance is a constant, independent of temperature, that depends only on the mechanical properties of the joint. For a succession of thermal joints in the total thermal resistance is the sum of the thermal resistances of the individual joints. Thus, for a transistor mounted on a heat sink, the total thermal resistance from transistor junction to the outside (ambient) world is the sum of the thermal resistance from junction to case the thermal resistance thetaJC from case to heat sink thetaCS, and the thermal resistance from heat sink to ambient thetaSA. The temperature of the junction is therefore:
TJ = TA + (thetaJC + thetaCS + thetaSA) * P ,  ( oC )
    P - dissipated power in [W]

    thetaJC -thermal reistance from junction to case [oC/W] ; thetaCS -thermal reistance from case to sink [oC/W]

    thetaCS -thermal reistance from sink to ambient [oC/W]

    TA - ambient temperature [oC].

Example: From unregulated power supply +15 V we want to supply 2 A on 5 V. We have 10 V over the voltage regulator =>
    P = 10 V * 2 A = 20 W (dissipated power)
    TA = 25 oC and 85 oC
    thetaJC = 1.5 oC, thetaCS = 0.3 oC, thetaSA = 2.3 oC
    TJ = 25 + (1.5 + 0.3 + 2.3) * 20 = 107 oC
    TJ = 85 + (1.5 + 0.3 + 2.3) * 20 = 167 oC - if we want to work at higher temperatures, we need better heatsink !

Comments on heatsink

1. Where very high power dissipation (several hundred watts, say) is involved, forced air cooling may be necessary. Large heat sinks designed to be used with a blower are available with thermal resistances (sink to to per ambient) as small as 0.05 oC to 0.2 oC per watt.
2. When the transistor must be insulated from the heat sink, as is usually necessary (especially if several transistors are mounted on the same sink), a thin insulating washer is used between the transistor and sink, and insulating bushings are used around the mounting screws. Washers are available in standard transistor-shape cutouts made from mica, insulated aluminum, or beryllia (BeO). Used with heat-conducting grease, these per watt add from 0.14 oC to about 0.5 oC per watt.
3. Small heat sinks are available that simply clip over the small transistor packages (like the standard TO-5). In situations of relatively low power dissipation (a watt or two) this often suffices, avoiding the nuisance of mounting the transistor remotely on a heat sink with its leads brought back to the circuit.
4. Sometimes it may be convenient to mount power transistors directly to the chassis or case of the instrument. In such cases it is wise to use conservative design (keep it cool), especially since a hot case will subject the other circuit components to high temperatures and shorten component life.
5. If a transistor is mounted to a heat sink without insulating hardware, the heat sink must be insulated from the chassis. The use of insulating washers is recommended (unless, of course, the transistor case happens to be at ground). When the transistor is insulated from the sink, the heat sink may be attached directly to the chassis. But if the transistor is accessible from outside the instrument (e.g., if the heat sink is mounted externally on the rear wall of the box), it is a good idea to use an insulating cover over the transistor (e.g, Thermalloy 8903N) to prevent someone from accidentally coming in contact with it, or shorting it to ground.
6. The thermal resistance from heat sink to ambient is usually specified for the sink mounted with the fins vertical and with unobstructed flow of air. If the sink is mounted differently, or if the air flow is obstructed, the efficiency will be reduced (higher thermal resistance); usually it is best to mount it on the rear of the instrument with fins vertical.

This page is a copy from "The art of electronics - 2nd Edition" by Paul Horowitz and Winfield Hill.
Please buy the book and read all the examples for better understanding.
The purpose of this page is for faster finding of appropriate information about power design thermal considerations.

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