128

I/Udiagramof adiode

In static operation the diode losses ensue according t

(

)

F F vj

T vj

)TO(

F vj

FF D

I I)T(r )T( U I)T,I(U P

⋅ ⋅

+

= ⋅

=

Usually the forward voltages of freewheeling diodes in IGBT modules are quoted in

datasheets at chip level and nominal current and for at least two different junction

temperatures (25°C and 125°C). For later chip generations even three temperature

values (25°C, 125° and 150°C) can be found. Further, the load current in a range from

0A to

nom,F

I2

⋅

is shown as a set of characteristics (similar as given in

to the

right). If U

F

is not stated at chip level reference but for the module terminals instead, it

needs to be recalculated to chip level. To do this the voltage drop across the internal

module resistance R

CC’+EE’

, which is usually quoted in the datasheet, needs to be

deducted from the valueof U

F,Module

.

F 'EE 'CC

Module ,F

Chip ,F

I

R

U U

⋅

−

=

+

The threshold voltageU

(TO)

can be derived from the characteristics by placing a tangent

at I

F,nom

and extend it to the abscissa (X-coordinate). The intersection of the tangent with

the abscissa gives U

(TO)

, as can be seen

to the left. Also the forward slope

(differential) resistance r

T

of the diode in forward direction can be derived from the

characteristics. Given these values, the forward losses of the diode at a certain current

cannowbe ascertained.

In a real application, however, very seldomwill there be static conditions with constant

current. In drive applications for example the current through the diode changes

sinusoidally due to the control algorithm. So this changing (in current) has to be

consideredwhen calculating the forward losses. The average losses per one sinewave

period are delineated b

It is assumed here that the diode conducts for 50% of

the timeandblocks for the remaining time.

dt )t( )t(i)t(u

T

1

P

2

T

0

F F

D,

cond

⋅ τ⋅

⋅

=

∫

with

)t(i)T(r )T( U)t(u

F vj

T vj

)TO(

F

⋅

+

=

and

)t

sin(

iˆ )t(i

F

F

ϖ ⋅ =

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