The maximum permissible power dissipation of a semiconductor device is specified the . at a certain temperature either case temperature or ambient temperature. For example the maximum power dissipation of a power transistor may be given as 10 W at 25°C ambient, and that of another device as 20 W at 50°C case tempera ratings mean that each device can dissipate the specified power at temperat res u~’)to and including the given temperature, but not at greater temperatures. The maximum permissible dissipation decreases as a function of temperature when temperature increases beyond the given value. The decrease in permissible power dissipation at elevated temperatures is called derating, and the specifies a derating factor, in wrc, that is used to find the decrease in beyond a certain temperature.

**Example 16-4**

A certain semiconductor device has a maximum rated dissination of 5 W at 50°C case temperature and must be derated above 50°C case temperarure. The thermal resistance from case to ambient is SOC/W.

1. Can the device be operated at 5 W of dissipation without auxiliary cooling (heat sink or fan) when the ambient temperature is 40°C’?

2. If not, what is the maximum permissible dissipation, with no auxiliary cooling, at 40°C ambient?

3. What derating factor should be applied to this device. in watts per ambient degree?

**Solution**

1. We determine the case temperature when the ambient temperature is 40°C

Since the case temperature exceeds 50°C, the full 5 W of power dissipation is not permitted when the ambient temperature is 40°C.

2. The derated dissipation at TA = 40°C is

3. We must first find the maximum ambient temperature at which the device can dissipate 5 W when the case temperature is 50°C

Thus derating will be necessary when the ambient temperature exceeds 25°C. Let D be the derating factor. Then P t(max) = (5 W) – (TA – 25)D. Substituting the results of (2), we find

**Power Dissipation in Integrated Circuits**

All the heat transfer concepts we have described in connection with power devices apply equally to integrated circuits including derating. Of course a maximum junction temperature is not” specified for an integrated circuit, but a maximum device temperature or case temperature is usually given. Many integrated circuits arc available in a variety of case types, and the rated power dissipation may depend on the case used. For example, the specifications given in Chapter 13 f r the p.A741 operational amplifier show that the maximum dissipation is 500 mW for the metal package, 310 mW for the DIP and 570 mW for the Ilatpak. Note that these specifications require the use of derating factors above 70°C ambient and that the factors differ depending on case type

The total power dissipation of an integrated circuit can be calculated by measuring the current drawn from each supply voltage used in a particular application and c ~:!:’gthe product of each current with the respective voltage. For example, if an operational amplifier employing ±15-V-dc p wer supplies draws 10 mA from the positive supply and 8 mA from the negative: upply, the total dissipation is P, = (15 V)(l0 mA) + (15 V)(8 mA) = 270 mW.