Figure 12.
SD failure due to undervoltage (UVLO above 70 μs)
4.3.3
Shunt resistor selection
The value of the shunt resistor is calculated by the following equation:
(1)
R
SH
=
Vref
IOC
Where V
ref
is the internal comparator (CIN) (0.51 V typ.) and I
OC
is the current of overcurrent detection level. The
maximum value of overcurrent protection level should be set less than the pulsed collector current in the
datasheet. In this design the over current threshold level is fixed @ I
OC
= 9.1 A.
(2)
R
SH
=
Vref ⋅
R23 + R53
R53
+ VF
IOC
=
0.51 ⋅ 1000 + 4700
4700
+ 0.18
9.1
= 0.0877Ω
Where V
F
is the voltage drop across diodes D3, D4 and D5.
The commercial value chosen was 0.08 Ω to which corresponds a level of 9.8 A.
The power rating of the shunt resistor is calculated by the following equation:
(3)
P
SH
=
1
2
⋅
Iload max
2
⋅ RSH ⋅ margin
Deratingratio
Where:
•
I
load(max)
: Maximum load current of inverter
•
R
SH
: Shunt resistor value at Tc=25°C
•
Derating ratio: Power derating ratio of shunt resistor @ 100°C
•
Margin: Safety margin of 30%
I
load(max)
is calculated considering the RMS value of the IPM nominal current including a safety margin.
(4)
I
load max
=
Inom @80°C
2
* 0.85 = 4.2A
rms
Power shunt value is:
(5)
P
SH
=
1
2
⋅
4.22 ⋅ 0.08 ⋅ 1.3
0.8
= 1.157W
Considering the commercial value, 2W shunt resistor was selected.
UM2702
Overcurrent protection
UM2702
-
Rev 1
page 13/34