< Dual-In-Line Package Intelligent Power Module >
Series Application note
Publication Date: September 2016
15
(3) Calculation of shunt resistance
The value of current sensing shunt resistance for current sensing is calculated by the following formulation:
R
Shunt
= V
SC(ref)
/SC where V
SC(ref)
is the SC trip voltage.
The maximum SC trip level SC(max) should be set less than the IGBT minimum saturation current which is 1.7
times as large as the rated current. For example, the SC(max) of PSS25MC1FT should be set to 25x1.7=42.5A.
The parameters (V
SC(ref)
, R
Shunt
) dispersion should be considered when designing the SC trip level. The dispersion
of series is +/-0.025V in the specification of V
SC(ref)
as shown in Table 2-2-1.
Table 2-2-1 Specification for V
SC(ref)
Symbol
Condition
Min
Typ
Max
Unit
V
SC(ref)
T
ｊ
=25
°
C, V
D
=15V
0.455
0.480
0.505
V
Therefore, the range of SC trip level can be calculated by the following descriptions with +/-5% dispersion of
shunt resistor :
R
Shunt(min)
=V
SC(ref) max
/SC(max)
where SC(max) is 1.7 times of rated current, and so 0.95 is due to -5% dispersion of shunt resistor that
R
Shunt(typ)
= R
Shunt(min)
/ 0.95
Therefore, SC(typ) = V
SC(ref) typ
/ R
Shunt(typ)
.
R
Shunt(max)
= R
Shunt(typ)
x 1.05* *1.05 is due to +5% dispersion of shunt resistor
Therefore, SC(min)= V
SC(ref) min
/ R
Shunt(max)
In this case, SC trip level is 42.5A,
R
Shunt(min)
= 0.505V / 42.5A =
11.9 mΩ, R
Shunt(typ)
=
11.9mΩ / 0.95 = 12.5 mΩ, R
Shunt(max)
= 12.5 x 1.05 =
13.1mΩ
When the both of SC trip level and shunt resistor will be maximum, typical and minimum, these will be described
as follows;
SC(max)= 42.5 A (setting), SC (typ) = 0.480 / 12.5 = 38.4 A, SC(min) = 0.455 / 13.1 = 34.7 A
From the above, the SC trip level range is described as Table 2-2-2.
Table 2-2-2 Operative SC Range
Condition
min.
typ.
max.
Unit
T
ｊ
=25
°
C, V
D
=15V
34.7
38.4
42.5
A
There is the possibility that the actual SC protection level becomes less than the calculated value. This is
considered due to the resonant signals caused mainly by parasitic inductance and parasitic capacitance. It is
recommended to make a confirmation of the resistance by prototype experiment.
(4) RC filter time constant
It is necessary to set an RC filter in the SC sensing circuit in order to prevent malfunction of SC protection due
to noise interference. The RC time constant is determined depending on the applying time of noise interference
and the SCSOA of the DIPIPM.
When the voltage drop on the external shunt resistor exceeds the SC trip level, The time (t1) that the CIN
terminal voltage rises to the referenced SC trip level can be calculated by the following expression:
)
1
ln(
1
)
1
(
1
c
shunt
SC
t
c
shunt
SC
I
R
V
t
I
R
V
⋅
−
⋅
−
=
−
⋅
⋅
=
−
τ
ε
τ
Where Vsc is the CIN terminal input voltage, Ic is the peak current,
τ
is the RC time constant.
On the other hand, the typical time delay t2 (from Vsc voltage reaches Vsc(ref) to IGBT gate shutdown) of IC
is shown in Table 2-2-3.
Table 2-2-3 Internal time delay of IC
Item
Min
typ
max
Unit
IC transfer delay time
-
-
1.0
μs
Therefore, the total delay time from an SC level current happened to the IGBT gate shutdown becomes:
t
TOTAL
=t1
＋
t2
