9020 Datasheet, PDF (18/25 Pages) Fairchild Semiconductor – IGBT Basic II

b. Current Transformer
This is also an excellent fault detection method. Current transformer is placed on the power
output needed to be monitored or on the conductor where short circuit current is expected to
flow. This method has an advantage in that it allows the selection of the transformer for precise
AC sensing. Since the transformer itself provides insulation between the power circuit and the
protection circuit, there need not be additional considerations for insulation. It also provides
high-level signal output with noise immunity since the protection circuit is driven by current.
However, DC level cannot be detected without the use of the much more complex DC trans-
formers. These are Hall Effect sensors, which are generally expensive. Aside from the Hall
Effect sensors, current transformers are expensive in general, so they are not very economi-
cal. In addition, it is not an easy task to design a proper transformer since current transformers
must operate in a large bandwidth. If a quick response to a rapid rise in fault current is neces-
sary, it must operate in the MHz region as well as the system’s minimum operating frequency.
Load
Gate
Drive
Vsense
Hall Sensor
Fig. 7. Short-circuit sensing circuit with CT
c. De-saturation detection
This method detects V_CE in its operation. Short circuit has a switching device but no load, so
all of the supplied voltage appears across the device. As such, during short circuit, V_CE of the
IGBT diverges from the low on-state voltage and rise to DC loop voltage following the power
output curve. If such high voltage appears in C-E terminal, where low on-state voltage region,
then it is detected by the V_CE(sat) sensing circuit and protection circuit. This method has no
loss from components in detecting the current, which improves the overall efficiency of the cir-
cuit. In addition, there is less stray inductance due to the protection current, which enables
faster operations. Furthermore, since it uses the characteristics of the device without any addi-
tional device in the circuit, it is an effective circuit in both AC and DC circuits. It has reasonable
cost, and integration is easier as it uses simple passive devices. However, this method cannot
detect exact amount of operating current that has been set, and it provides only fault/no-fault
command. In this method, protection circuit is not insulated from the power stage, so gate sig-
nals are eliminated by IGBT, and insulation is necessary in the process of sending error mes-
sage to the logic circuit. This circuit cannot be left in sensing mode at all times. It is not
operational when IGBT off, but it becomes operational with the conduction signal. However,
there must be a blanking time in the turn-on transient period to allow turn-on switching pro-
cess. (refer to AN-9001)
18
Rev. A, April 2002