8

Aeroflex Circuit Technology

SCD5101-1 REV D 5/14/01 Plainview NY (516) 694-6700

where:

Commutation diode losses

Commutation diode forward voltage from Figure 5

Commutation diode current

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:

where:

IGBT Junction Temperature

Commutation Diode Junction Temperature

IGBT Thermal resistance from Table 2

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.

(V)

T

80

70

60

50

40

30

20

10

1.2

1.0

0.8

0.6

1.8

1.6

1.4

0

T

jIGBT

P

c

P

s

+

[] θ

jcIGBT

T

C

+

⋅

=

T

jDIODE

P

d

θ

jcDIODE

T

C

+

⋅

=