9020 Datasheet, PDF (17/25 Pages) Fairchild Semiconductor – IGBT Basic II

C. Short Circuit Protection Scheme
There are certain conditions to be met in constructing a protection circuit. These conditions
maximize the efficiency of the protection circuit and minimize the effect on other circuits. In
order to obtain the protection function desired by the designer without sacrificing other func-
tions, protection circuit design must satisfy the following conditions as much as possible. The
conditions are as follows:
First, the protection circuit must shut down the IGBT before device failure. This is applicable at
all times regardless of the conditions of operation for the IGBT. In addition, the protection cir-
cuit must limit the maximum fault current, and it must reduce stress on the device and the sys-
tem, where high current flows. The device would have to be shut down more quickly if the
maximum fault current is not limited.
Protection circuit must be responsive to both “fault under load” and “hard switch fault.” In addi-
tion, it must not degrade switching or conduction characteristics, because this causes temper-
ature to rise, and it is reflected in the efficiency and fidelity of components. Trip point, which is
the minimum current recognized as short-circuit, must be easy to manipulate. Production cost
must be minimized as it has a significant impact on the viability of the product.
The above conditions for protection circuits could be easy or difficult to meet depending on the
fault detection method. The following are some typical fault detection methods and the effects
on the conditions mentioned above.
a. Detection through resistance
This is fault detection method is the easiest to understand. Resistance is introduced at the
passage of the load current, and it is made to produce voltage that can be monitored by the
protection circuit. Sense resistor can measure current precise enough for over-current and
fault detection. In addition, it can be used in analog feedback. However, it takes up a lot of
space and requires a low inductance resistor, while self-inductance and wiring inductance
within the sense resistor makes transient response characteristics get worse. When resis-
tance is inserted into the DC loop, there is an adverse effect of inductance that deteriorates
the system’s performance. In addition, sense resistor is not insulated from the main power cir-
cuit, so the protection circuit must be insulated from the logic circuit, which processes the
sense signals. This can cause the system to become complex. In other cases, sense IGBT,
which as a built-in sensing resistor, in the current sense path. This would be easier than install-
ing it in the main current path. However, there are problems with high cost IGBT, limited uses,
consistency among sense ratios among devices, and their trade-offs must be considered.
Load
Gate
Drive
Vsense
Fig. 6. Short-circuit sensing circuit with a sensing resistor
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Rev. A, April 2002