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Required Heat Sink Thermal Resistance
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Power Dissipated
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Junction Temperature Rise
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Estimated Junction Temperature
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Check your regulator's datasheet for junction-to-case thermal resistance (θJC) values. Common values are 5°C/W for LM317T and 0.65°C/W for LT1084. Case-to-heatsink resistance depends on thermal interface material, typically 0.5°C/W.
Without adequate heat sinking, the junction temperature will exceed safe limits, causing thermal shutdown, reduced output current, or permanent damage. Most regulators have built-in thermal protection but this causes intermittent operation.
Power dissipation equals (Input Voltage - Output Voltage) × Output Current. For example, with 12V input, 5V output, and 1A load: (12-5) × 1 = 7 watts of heat must be dissipated.
Linear regulators dissipate power as heat equal to voltage drop times current. Switching regulators are more efficient (85-95%) so heat calculation involves efficiency: Power dissipated = (Output Power / Efficiency) - Output Power.
Choose a heat sink with thermal resistance equal to or less than the calculated value. Consider mounting orientation, airflow, and safety margin. Natural convection heat sinks typically range from 1-50°C/W.
Ambient temperature is the baseline from which all temperature rises are calculated. Higher ambient temperatures require better heat sinking to maintain safe junction temperatures. Design for maximum expected ambient temperature.
Yes, the thermal resistance calculation principles apply to any power semiconductor. You'll need the device's junction-to-case thermal resistance from its datasheet and appropriate case-to-heatsink values.
Use a 10-20% safety margin by choosing a heat sink with 10-20% lower thermal resistance than calculated. This accounts for manufacturing tolerances, aging, and unexpected operating conditions.