Hitachi IGBT Module Application Manual

S

when the snubber circuit

functions.

Figure 24. Snubber Diode Current and Voltage Waveforms

Values associated with the labeled portions of Figures 23 and 24 can be determined using Equations 3,

Equation 3:

T

S

= 2

π

!

(

LstxCs

)/ 4

∆

V = I

C

!

(

Lst

)

------------

Cs

Equation 4:

Equation 5:

∆

V

f

= L

sn

!

di

c

/ dt + V

fr

Definitions:

I

C

L

sn

IGBT turn-OFF current

Snubber circuit inductance as viewed from IGBT’s collector

and emitter terminals

Forward recovery voltage (typically about 50V)

V

fr

di

c

/ dt

Current change ratio in IGBT’s fall period.

In addition, the value for

T

n

which is defined as the time required to discharge 95% of the overcharge

voltage of

C

s

is approximated using Equation 6:

Equation 6:

T

n

= 3

!

C

s

!

R

s

Here,

R

s

must be set to a value such that

T

S

+

T

n

<

1 / f

c

.

Note:

In the above equation, for the case of a three-phase circuit, the value

for

T

S

in Equation 3 must be multiplied by

3

, and the

∆

V

in

3

.

5.2.4 Collector Current Class and Snubber Capacitor Values

The snubber circuit capacitor value can be calculated using Equation 7:

Equation 7:

C = L

st

( I /

∆

V )

2

21