Heatsink Requirements
31 Aug 14 - Converted to JavaScript.
One thing that needs to be taken into account is ensuring that your regulator or FET doesn't overheat. Regulators are fitted with an internal over temperature cut-out, but you don't want the power dying when you trying to use the output.
The following table gives the thermal and power ratings for devices used on this website.
Regulators |
||||||
xx = Output Voltage |
Thermal Co-efficients |
Maximum Permissible Junction Temperature |
Maximum Output Current |
Maximum Power Dissipation |
||
Device |
Junction to Air (no heatsink) |
Junction to Case |
Case to Heatsink (θ_{CHS}) |
|||
LM317T | 50 °C/W | 5 °C/W | 0.5 °C/W | 125 °C | 1.5A | 20W |
LM317LZ | 180 °C/W | N/A | 160 °C/W | 125 °C | 100mA | 625mW |
78xx | 65 °C/W | 5 °C/W | 0.5 °C/W | 150 °C | 1.5A | 15W |
LT1084CP-xx | N/A | 0.65 °C/W | 0.2 °C/W | 125 °C | 5A | 45W |
LT1084CT-xx | N/A | 0.65 °C/W | 0.2 °C/W | 125 °C | 5A | 30W |
FETs |
||||||
Thermal Co-efficients |
Maximum Permissible Junction Temperature |
Maximum Output Current |
Maximum Power Dissipation |
Maximum Resistance when switched ON |
||
Device |
Junction to Air (no heatsink) |
Junction to Case |
||||
IRF630 | 80 °C/W | 1.67 °C/W | 150 °C (100°C used) |
9.0A @ 25°C 6.0A @ 100°C |
75W @ 25°C 30W @ 100°C |
0.4 Ω |
The following shows the equations to calculate the need and necessary heatsink, with a worked example is at the bottom for a LM317T regulator. The maximum allowable temperature rise (T_{Δ}) needs to be determined to ensure the regulator will not overheat can be determined using the following equation where T_{AMB} = the Ambient Air Temperature around the device.
For a voltage regulator,
For a FET,
The thermal coefficient required is given by:
If the value of θ calculated by this equation is lower than the devices own heatsink effect (θ_{JA}) it will need an external heatsink. The heatsink required can be calculated using the following equation, where θ_{CHS} is the thermal coefficient between case and heatsink (normally taken as 0.5°C/W when using an insulating washer and thermal grease):
Example: