Hitachi IGBT Module Application Manual

5.6.3 Heat Dissipation Design

This section presents a basic procedure for selecting a heat sink based on steady state and transient state

considerations.

5.6.3.1

Steady State

Figure 37 represents the thermal equivalent circuit and includes the parameter notations used in the

equations which follow.

Figure 37. Thermal Equivalent Circuit

The junction temperature ( T

j

) can be estimated using Equation 25:

Equation 25:

T

j

= P

{

R

th

(j- c) + R

th

(c-h) + R

th

(h-a))} + T

a

Also, the change in junction temperature (

∆

T

j

) can be calculated using Equation 26:

Equation 26:

∆

T

j

= P

!

( R

th

(j-c)+ R

th

(c-h) + R

th

(h-a))

Here, the measurement points for T

c

and T

h

are as shown in Figure 36.

Note:

Always select a heat sink having characteristics that assure T

j

will

never exceed the maximum junction temperature ( T

j max

) of the

IGBT module(s).

5.6.3.2

Transient State

Generally, it is sufficient to consider the steady-state junction temperature T

j

for radiation design.

However, because the power dissipation is actually swinging with pulse state, so that T

j

relates to the

temperature ripple based on T

c

as shown in Figure 38.

Figure 38. Temperature ripple of T

j

In this case, the ripple peak value of the junction temperature (

T

jp

) can be estimated approximately

using Equation 27 and the transient thermal impedance curve of Figure 39.

Equation 27:

T

jp

= P

1

[ R

th

(st)

!

( t

1

/ t

2

)

+ (1 - t

1

/t

2

)

!

R

th

(t

l

+ t

2

)- R

th

( t

2

) + R

th

( t

l

)] + T

c

A suitable heat sink should be selected so that T

jp

will never exceed T

j max

.

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