Thermal Equivalent Circuit for a Transistor. Junction Temperature Case TemperatureHeat Sink...

Thermal Equivalent Circuit for a Transistor

Junction Temperature

Case Temperature Heat Sink Temperature

Ambient Temperature

Thermal Resistance from Junction to Case

Thermal Resistance from Case to Heat Sink

Thermal Resistance from Heat Sink to Ambient

Total Average Power Loss

( )

C J T J C

S C T CS

A S T SA

J A T J C CS SA

T T P R

T T P R

T T P R

T T P R R R

Motorola MS-10 Heat Sink

Performance Under Forced Air Flow

Thermal Characteristic in Free Air

Top View of MS-10 Heat Sink

Hole Pattern for a TO-3 Case

Mica Insulator

Thermal Resistances for Mica Insulators

Clip-On Heat Sinks

Thermal Performance

Thermal Resistance for Clip-On

Bond-On Extrusions for DIP Packages

Thermal Performance

Thermal Resistances for Bond-On

Example

• TO-220 case style dissipating 5 watts

• RJC = 3.0 C / Watt (from device spec)

• TJmax = 150 C

• TAmax = 50 C

• Find the proper heat sink to keep the junction temperature from exceeding 150C in natural convection.

Need to determine a value for RCS – depends on the mounting technique.

If the device is mounted without an insulator, but only with Thermalcote, (silicone grease), check page 57, Figure 9 for TO -220 case style.

( )

J A

D

J C CS SA

J A

SA J C CS

D

T TP

R R RT T

R R RP

RCS = 1 C/W

No Insulator

Use Thermalcote

( )

(150 50 )(3 1)

5

16

J A

SA J C CS

D

SA

SA

T TR R R

P

C CR

W WC

RW

Thermalloy P/N 6030

Temperature rise above ambient@ 5 Watts Power Dissipation = 66 C

Thermal Resistance RSA

6613.2

5SA

C CR

W W

As long as RSA is less than 16 C/W, the requirement is met.

Thermalloy P/N 6030