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
(T
MAX)

Maximum Output Current
(I)

Maximum Power Dissipation
(PMAX @ 25C)

 Device

Junction to Air (no heatsink)
(θJA)

Junction to Case
(θJC)

Case to Heatsink (θCHS)

 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

Regulator Heatsink Calculator
Voltage Regulator Type =
Input Voltage = V
Output Voltage = V
Output Current = A
Ambient Temperature = C
 

 
Heatsink Required =

C/W

FETs

Thermal Co-efficients

Maximum Permissible Junction Temperature
(T
MAX)

Maximum Output Current
(I)

Maximum Power Dissipation
(PMAX @ 25C)

Maximum Resistance when switched ON
(RON)

 Device

Junction to Air (no heatsink)
(θJA)

Junction to Case
(θJC)

 IRF630 80 C/W 1.67 C/W 150 C
(100C used)
9.0A @ 25C
6.0A @ 100C
75W @ 25C
30W @ 100C
0.4 Ω
FET Heatsink Calculator
Voltage Regulator Type =  
Current = A
Ambient Temperature = C
 

 
Heatsink Required =

C/W

Explanation:

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 (TΔ) needs to be determined to ensure the regulator will not overheat can be determined using the following equation where TAMB = 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 (θJA) it will need an external heatsink. The heatsink required can be calculated using the following equation, where θCHS is the thermal coefficient between case and heatsink (normally taken as 0.5C/W when using an insulating washer and thermal grease):

 

Example:

Return to Plans & Designs Page