Heat sinks

Heat sinks for transistors

Heat sinks are needed for
transistors
passing large currents.

Why is a heat sink needed?

Heat sink Photograph © Rapid Electronics

Waste heat is produced in transistors due to the current flowing through them.
If you find that a transistor is becoming too hot to touch it certainly needs a
heat sink! The heat sink helps to dissipate (remove) the heat by transferring it
to the surrounding air.
The rate of producing waste heat is called the thermal power, P. Usually the
base current I_B is too small to contribute much heat, so the thermal
power is determined by the collector current I_C and the voltage V_CE
across the transistor:
P = I_C × V_CE   (see diagram below)





Insulation kit

The heat is not a problem if I_C is small or if the
transistor is used as a switch because when 'full on' V_CE is almost
zero. However, power transistors used in circuits such as an audio amplifier or
a motor speed controller will be partly on most of the time and V_CE
may be about half the supply voltage. These power transistors will almost
certainly need a heat sink to prevent them overheating. Power transistors
usually have bolt holes for attaching heat sinks, but clip-on heat sinks are
also available. Make sure you use the right type for your transistor. Many
transistors have metal cases which are connected to one of their leads so it may
be necessary to insulate the heat sink from the transistor. Insulating kits are
available with a mica sheet and a plastic sleeve for the bolt. Heat-conducting
paste can be used to improve heat flow from the transistor to the heat sink,
this is especially important if an insulation kit is used.

Heat sink ratings

Heat sinks are rated by their

thermal resistance (Rth) in °C/W. For example 2°C/W means the heat sink (and
therefore the component attached to it) will be 2°C hotter than the surrounding
air for every 1W of heat it is dissipating. Note that a
lower thermal resistance means a better heat sink.
This is how you work out the required heat sink rating:

1. 
   
   
   
   Work out thermal power to be dissipated, P = I_C
   × V_CE
   
   If in doubt use the largest likely value for I_C and assume that V_CE
   is half the supply voltage.
   
   For example if a power transistor is passing 1A and connected to a 12V
   supply, the power P is about 1 × ½ × 12 = 6W.
   
   
   
   


2. 
   
   
   
   Find the maximum operating temperature (Tmax) for the
   transistor if you can, otherwise assume Tmax = 100°C.
   
   
   
   


3. 
   
   
   
   Estimate the maximum ambient (surrounding air) temperature
   (Tair). If the heat sink is going to be outside the case Tair = 25°C is
   reasonable, but inside it will be higher (perhaps 40°C) allowing for
   everything to warm up in operation.
   
   
   
   


4. 
   
   
   
   Work out the maximum thermal resistance (Rth) for the heat
   sink using: Rth = (Tmax - Tair) / P
   
   With the example values given above: Rth = (100-25)/6 = 12.5°C/W.
   
   
   
   


5. 
   
   
   
   Choose a heat sink with a thermal resistance which is
   less than the value calculated above (remember lower value means better
   heat sinking!) for example 5°C/W would be a sensible choice to allow a
   safety margin. A 5°C/W heat sink dissipating 6W will have a temperature
   difference of 5 × 6 = 30°C so the transistor temperature will rise to
   25 + 30 = 55°C (safely less than the 100°C maximum).
   
   
   
   


6. 
   
   
   
   All the above assumes the transistor is at the same
   temperature as the heat sink. This is a reasonable assumption if they are
   firmly bolted or clipped together. However, you may have to put a mica sheet
   or similar between them to provide electrical insulation, then the
   transistor will be hotter than the heat sink and the calculation becomes
   more difficult. For typical mica sheets you should subtract 2°C/W from the
   thermal resistance (Rth) value calculated in step 4 above.
   
   
   
   
   

If this all seems too complex you
can try attaching a moderately large heat sink and hope for the best. Cautiously
monitor the transistor temperature with your finger, if it becomes painfully hot
switch off immediately and use a larger heat sink!

Why thermal resistance?

The term 'thermal resistance' is used because it is analagous to
electrical resistance:

• The temperature difference across the heat sink (between the transistor
  and air) is like voltage (potential difference) across a resistor.


• The thermal power (rate of heat) flowing through the heat sink from
  transistor to air is like current flowing through a resistor.


• So R = V/I becomes Rth = (Tmax - Tair)/P


• Just as you need a voltage difference to make current flow, you need a
  temperature difference to make heat flow.