where:
P
d
=
V
f
=
I
f
=
Commutation diode losses
Commutation diode forward voltage from Figure 5
Commutation diode current
1.8
1.6
1.4
T
J
= 25
°
C
V
f
(V)
T
J
= 150
°
C
1.2
1.0
0.8
0.6
0
10
20
30
40
50
60
70
80
I
f
(A)
FIGURE 5 - DIODE VOLTAGE DROP VS. FORWARD CURRENT
Once the dissipations are calculated the junction temperatures may then be computed by summing the
conduction losses and switching losses for the IGBT and the commutation diode loss and multiplying these
dissipations by the respective junction to case thermal resistance as shown below:
T
jIGBT
=
[
P
c
+
P
s
] ⋅ θ
jcIGBT
+
T
C
T
jDIODE
=
P
d
⋅ θ
jcDIODE
+
T
C
where:
T
jIGBT
=
T
jDIODE
=
θ
jcIGBT
=
θ
jcDIODE
=
IGBT Junction Temperature
Commutation Diode Junction Temperature
IGBT Thermal resistance from Table 2
Diode Thermal resistance from Table 2
Case temperature
T
case
=
It is important that the user calculate junction temperatures over the full range of operating conditions,
including maximum load and stall conditions. Typically, hybrid losses peak at just maximum load with duty
cycles approaching, but just under, unity. Single transistor and diode losses peak under stall conditions since
power is dissipated in just one channel.
EXAMPLE
As a typical application consider a 10 HP brushless DC motor operating off a 270V line in a pump
application. The drive is trapezoidal, and the nominal load current will be 30 A. The system current limit is set
to roll back the PWM to maintain a maximum load current of 25 amperes in the event of a stall. Maximum
case temperature will be 85 C. Duty cycles may approach unity, but at stall it will be approximately 0.1.
PWM frequency is 20 kHz.
Aeroflex Circuit Technology
8
SCD5101-1 REV D 5/14/01 Plainview NY (516) 694-6700
AEROFLEX [ AEROFLEX CIRCUIT TECHNOLOGY ]
