Hitachi IGBT Module Application Manual

Figure 31. Equalization of Unbalanced Current

5.4.3.4.2

Unbalanced Current Period caused by Wiring

during the steady-state condition after activating the IGBT, finally settling down to values I

CA

and I

CB

as

determined by V

CE

(sat).

The time required for the unbalanced current to be equalized can be calculated as the attenuation over

the L - R circuit caused by the inductance within the closed-loop forming a parallel circuit and the operating

resistance of the IGBT element. The operating resistance " r

on

" can be easily calculated from the output

characteristics curve. For example, for an MBN300GS12AW IGBT module having a single arm, an " r

on

"

of approximately 4.2 milliohms (when T

j

= 25

o

C, 1/2 rated current) will be present. If the loop inductance

is 100 nH for a parallel configuration, ( L

CA

+ L

CB

+ L

EA

+ L

EB

), the equalization in unbalanced current

occurs based on the time constant

Ï„

is approximated by Equation 15.

Ï„

= 100 nH / ( 4.2 m

â„¦

!

2)

Equation 15:

â‰…

12

Âµ

s

If the stability in the current variation is assumed to be three times

Ï„

(that is, variation is approximate,

up to 95%), the current balance cannot be determined by V

CE

(sat) within 36

Âµ

s after turning ON. This

means if the carrier frequency is high or the active time is shorter, the current balance may be determined by

the wiring (including the shape) for almost the whole period.

30