Thermal
Although switch mode power supplies have a very high power transfer efficiency, it is necessary to use heat sinks in high current applications. As a general rule of thumb, heat sinks should be used if the power dissipation exceeds 1 watt.
Most transistor parameters (such as current gain, leakage current, and power handling) are functions of temperature. Therefore, it is advisable to keep all electronic equipment operating at a constant temperature.
Heat Sinks
Heat sink size approximation
On Line Calculator
(The area in square centimeters is approximately equal to the square of the ratio of 50 divided by the tolerates temperature rise in centigrade for every watt dissipated)
The temperature (in centigrade) rise for every watt dissipates is approximately equal to the square root of the ratio of 50 divided by the heatsink area in square centimeters.
2N3055 Thermal Characteristics
The 2N3055 is a standard high power NPN transistor that might be found in a power supply. The derating curve shows how much the maximum power rating is affected by temperature.
Total Device Dissipation @ 25oC 115 watts
Derating above 25oC 0.65 watts/oC
Junction to case thermal resistance 1.52 oC/watt
TO-3 case thermal resistance 30 oC/watt
Maximum Junction Temp 200 oC
If all of the heat produced in the 2N3055 can be quickly removed, it can dissipate a maximum of 115 watts. However, if the case temperature is allowed to rise, the power handling capabilities will decrease by 0.65 watts for every degree above room temperature.
There is a natural reluctance or resistance to heat transfer. There are typically three components to this phenomenon: junction to case, case to heat sink and heat sink to ambient resistance. In mathematical terms:
The first two terms are specified in the semiconductor data sheets whereas the third is used as a design criteria to select an appropriate heat sink.
The maximum theoretical power dissipation (assuming zero thermal resistance between the case and ambient) can be determined as:
The maximum power dissipation is equal to the temperature difference between the transistor junction and case, divided by the thermal resistance between the junction and case. This relationship is very similar to Ohm’s Law.
For the 2N3055, this works out to be:
The junction thermal resistance specification means that the junction temperature will be 1.52oC higher than the case temperature for every watt dissipated.
The case thermal resistance specification means that the case will rise by 30 oC for every watt dissipated (unless the case is mounted on a heat sink). From this we can determine how much power the 2N3055 can dissipate if it is not mounted on a heat sink:
Since this means running the transistor at its maximum temperature, the life expectancy would be quite short. In most cases, a maximum junction temperature of 150oC would be used, but even this is not recommended.
Under normal operating conditions, the maximum power dissipation of a transistor mounted on a heat sink is:
By increasing the surface area, heat sinks lower the case temperature, hence the junction temperature.
To reduce the case to sink thermal resistance, a thermal compound is applied to the contact surface. This paste fills in all the irregularities in the surfaces and maximizes heat transfer.
Typical Thermal Resistance from Case to Heat Sink
Thermal Compound 0.1 – 0.2 oC/watt
Mica Washer 0.5 oC/watt
Since the power transistor case is electrically connected to the collector, a mica washer is generally used to electrically isolate it from the heat sink. The overall case to heat sink thermal resistance is therefore the sum of the two.
The thermal specification of the heatsink can be determined by solving the above equation for the heatsink to ambient thermal resistance:
7800 Regulator
The power dissipation curves for the 7800 series regulator.
Note that without a heatsink, this device can dissipate approximately 1 watt at room temperature.
