465

Using snubbers across the AC-terminals of the IGBT module changes the stray

inductance, which is reduced by the parts L

σ

,DC_ESL

and L

σ

,DC

. Therefore, with sufficiently

low stray inductanceof the snubber itself, it follows:

Module ,3

DC,2

ESL _DC,1

Module ,3

Snubber

,4

L

L

L

L

L

σ

σ

σ

σ

σ

+

+

<

+

This leads to a reducedover-voltage:

(

)

noSnubber

,

peak

Module ,3

Snubber

,4

DC

snubber

,

peak

U

dt

di

L

L U

U

< ⋅

+

− =

σ

σ

The capacitance of the snubber can be roughly calculatedwith the following equation:

2

DC

peak ,CE

peak

total

,

Snubber

)U U(

I

L

C

−

⋅

≈

σ

L

σ

,total

: Total stray inductance (module, DC-bus, capacitors) [H]

I

peak

: Maximum peak current during switching [I]

U

CE,peak

: Maximum permitted collector-emitter voltageminus

safetymargin [V]

U

DC

:

DC-bus voltage [V]

The use of snubbers creates a damped resonant circuit, which generates a current

oscillation with a frequency f

osc

according to

uring the turn-on and turn-off

switching sequences.

Snubber

Snubber

,4

DC,2

ESL _DC,1

osc

osc

C)

L

L

L(

2

1

T

1

f

⋅

+

+

⋅π⋅

= =

σ

σ

σ

Themaximum value of the oscillation (without the portion of theDC-bus voltage) can be

describedwit

Snubber

2

C DC,2

ESL _DC,1

peak , osc

C

I)

L

L(

U

⋅

+

≤

σ

σ

When selecting a suitable snubber capacitor, various aspects have to be considered.

Apart from the actual capacitance, assure a low self inductance of the capacitor (given

by the ESL value). Preferable are capacitors which have connections directly matching

the IGBT module

. This minimises the area covered by the leads and, hence, the

resulting inductance. In addition, the voltage rating of the capacitor has to match the

voltage rating of the semiconductor and theDC-bus voltage. The nominal voltage of the

capacitor should not be below 1kV if, in an application, 1.2kV IGBTmodules are used,

which are typically operated at a DC-bus voltage of less than 800V. It has to be noted

that this capacitor voltage is temperature dependent and it may lead to the selection of a

capacitor with more than 1kV when the de-rating is high due to high operating

temperatures. In order not to damage the capacitor during operation, one must not

7

Epcos, for example, manufactures capacitors with suitable connections for the IGBT modules of various

manufacturers such as Fuji Electric, InfineonTechnologies, Mitsubishi Electric, SemikronandToshiba.

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