heatsink selection guide

The purpose of a heatsink is to dissipate heat away from a device such as a regulator or a transistor. In the type of applications we would consider here this would include; heatsinks for voltage regulators in power supplies, heatsinks for rf power amplifiers and heatsinks for audio power amplifiers.

HEATSINK SELECTION GUIDE

WHAT IS THE PURPOSE OF A HEATSINK?

As trite as it may sound the purpose of a heatsink is to dissipate heat away from a device such as a regulator or a transistor. In the type of applications we would consider here this would include:

Any active device such as a voltage regulator or a power transistor has a maximum junction temperature. If this junction temperature is exceeded then it is likely the device will fail, a common occurence. To prevent this occuring the device is thermally bonded to a "heatsink" in order that excess heat is diverted away as fast as possible.

HOW COME HEAT IS GENERATED BY AN ACTIVE DEVICE?

No device is perfect and more particularly the circuit in which we choose to operate the device has manifest inefficiencies. Foremost in my mind are Class "A" amplifiers, they are notoriously inefficient but have other compensations making their use worthwhile in low level circuits.

In audio power amplifier circuits, say class "B" designs, the efficiency might be around 66% so that for an amplifier delivering 200W RMS into a speaker load, expect another 100W of pure wasted energy which MUST be dissipated elsewhere. And this is before we consider safe junction temperatures. In case you were confused, the power supply must provide 300W, 2/3rds or 66% or 200W RMS is useful audio power, the remaining 1/3rd or 33% is wasted as heat. Fine on a cold day.

WHAT ARE THE SAFE LIMITS - ALSO CALLED "SOAR" - SAFE OPERATING AREA REGION?

Let's look at some relatively modern power amplifier devices such as the 2SK1058 and its complement 2SJ162. These are described in the application note as "Silicon N-Channel MOS FET's". All these comments and principles more or less apply to RF Power Amplifiers as well.

Now I've managed to obtain data sheets on both those devices (see later) and I'll quote, in part, from them.

With FETS, junction temperature is referred to as "channel temperature". With the 2SK1058 we have a quoted "maximum case temperature" of 150^o C. This is quite a common value.

You will note figure 1 depicts the "Power vs Temperature Derating" of a 2SK1058 which comes in a T0-3P package. At what is considered "ambient" temperature of 25C (77F) the device can dissipate around 100W. When the case temperature reaches 100C this drops down to about 40W.

If the metric temperatures of Celcius confuse you, here is the actual formula for converting into Fahrenheit.

Of course "all" calculations here will be on the Celsius scale. The Hitachi data sheet quotes all maximum ratings at a temperature of 25C.

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