Every electronics constructor and engineer knows that electronic devices like IC’s voltage regulators and CPU heats up. Temperature is critical parameter for reliable system operation. System reliability is very closely related to device temperature high or low. While temperature increases the reliability drops exponentially. If you look in device datasheets you will find a recommended Operating temperature range. For instance Voltage regulator 7805 can normally operate at 0 – 70°C temperature range. When maximum Junction Temperature can reach up to 150°C. So how is operating temperature range calculated. It is not as difficult as it may look like. We know, that temperature is analogous to voltage, thermal resistance changed to electric resistance, power dissipation is as current, then:

Temperature=Power*Thermal_Resistance;

Temperature menas the rise in °C, Power in Watts and Thermal_Resistance (°C/Watts). The calculation can be don using simple Ohms law. Lets analyse real example by using simple 7805 voltage regulator. We know that output produces 5V. Lets assume that input is 9V and output provides 1A current. Then regulators power dissipation is:

P=V·I=(9V-5V)·1A=4 Watts

So datasheet says that thermal resistance of junction to case (TO-220) is 4°C/W and case(without heat-sink) to ambient is 50°C/W. Then total thermal resistance is Ө=4+50=54°C/W. Now we can calculate the temperature rise above air surrounding temperature:

T=P· Ө =4W· 54°C/W=216°C

Well this leads to device failure as operation range is above 70°C. In this case you need to add heat sink. Then regulator and heat sink to ambient air thermal resistance is about 8°C/W. Then over all regulator thermal resistance would be Ө=4+8=12°C/W. Again calculate the temperature rise above air surrounding temperature:

T=P· Ө =4W· 14°C/W=32°C

This is pretty critical temperature. If ambient temperature is about 20°C then 32°C+ 20°C=52. 52°C is < 70°C – Working temperature is OK. But if ambient temperature is 40°C, then working temperature would rise up to 32+40=72°C this is bit over from recommended. So you should consider using better heat-sink or fan to force air flow or lower input voltage of voltage regulator.

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This entry was posted on Monday, January 22nd, 2007 at 2:36 pm and is filed under Electronics Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.