Application Note
39 of 53
V 1.0
2019-04-01
IM393 Application note
IM393 IPM Technical Description
Thermal design
7.2.2
Switching losses
Different devices have different switching characteristics, and vary according to the handled voltage/current
and operating temperature/frequency. However, the turn-on/off energy loss (joule) can be experimentally
measured, indirectly, by integrating power over time where power is obtained by multiplying the current and
voltage, under a given circumstance. Therefore, the linear dependency of the switching energy loss on the
switched current is expressed during one switching period as follows.
Switching energy loss = ( +
) × [
]
(20)
=
.
+
.
(21)
=
.
+
.
(22)
Where, E
I
is a unique constant of IGBT related to the switching energy, and different IGBTs have different E
I
values. E
D
is for the diode. These should be derived by experimental measurement. From the equation (15), it
should be noted that the switching losses are a linear function of current and directly proportional to the
switching frequency.
As mentioned before, the output current can be considered a sinusoidal waveform, and the switching loss
occurs every PWM period for the continuous PWM schemes. Therefore, depending on the switching frequency
f
SW
, the switching loss of one device is:
=
1
2
( + )
=
( +
)
(23)
7.3
Thermal impedance
During operation, power losses generate heat which elevates the temperature in the semiconductor junctions.
This limits the performance and the lifetime of the device. As junction temperature increases, the operation
characteristics of a device are altered from the normal state, and the failure rate increases exponentially. This
makes the thermal design of the package a very important factor in the device development stage and also in
the application field. The generated heat must be properly conducted away from the power chips and into the
environment using an adequate cooling system.
Thermal impedance qualifies the capability of a given thermal path to transfer heat in the steady state.
( ) =
∆
( )
∆
The thermal impedance is typically represented by an RC equivalent circuit as shown in Figure 31.
Figure 31
Thermal impedance RC equivalent circuits (Foster model)
Figure 32 shows thermal impedance from junction-to-case curves of IM393-M6F. The thermal resistance goes
into saturation in about 10 seconds. Other types of IM393-XX also show similar characteristics.
Cth4
Rth4
Cth3
Rth3
Cth2
Rth2
Rth1
Cth1
