9020 Datasheet, PDF (12/25 Pages) Fairchild Semiconductor – IGBT Basic II

e. Common emitter problems
Gate
Drive
Lgs
Les
Gate
Drive
Les
Lgs
Fig. 7. Common emitter
At the time of switching, voltage is induced across the stray inductance of the power circuit
because of di/dt from the main current. When control signals from the gate drive and the same
path as the main current are used, gate voltage decreases during turn-on, and voltage is
added to the gate voltage during turn-off to slow the turn-on/turn-off. As such, it is better not to
share stray inductance between control emitter terminal and power emitter terminal. As such,
control emitter terminal and power emitter terminal should be separate. If the two terminals are
together, common emitter inductance increases to slow the switching speed and switching
loss.
Voltage oscillation, slow gate voltage rise, noise immunity deterioration, gate voltage reduc-
tion, falling gate protection circuit efficiency are some of the effects of the layout. These can be
solved with designs to reduce stray inductance and stray resistance such as making patterns
short and thick. In addition, attention must be paid to the power circuit layout to minimize stray
inductance. For example, the area of the closed loop must be minimized with DC link capaci-
tor, load, power output, half-bridge leg and snubbers in the case of inverter, and in the case of
resistive load, line to the load should be twisted to reduce the stray inductance of the power
circuit, while snubber should be strengthened depending on the amount of over-voltage for
inductive load. As the frequency increases, voltage could change due to slowing response of
the dc link capacitor, so high-speed electrolyte cap for inverter should be used, and capacitor
with better characteristics such as film capacitor should be inserted in the main cap in parallel.
5. Conclusion
We have examined some issues to consider in the gate drive of the IGBT. The gate drive,
IGBT’s operating circuits, are simple and lends it self easily to miniaturization, so the system
designer can design the gate drive, but it is not an easy task to design an optimum gate drive
for the system. IC type gate drive solutions are designed to fit the needs of the user’s system
to customize some significant parameters by linking simple passive devices. Furthermore,
they have built-in OCP (over current protection) and SCP (short circuit protection) functions to
easily build a more stable system. It is becoming even more popular with the introduction of
the IPM (intelligent power module), which puts inverter with gate drive, which is used often in
the industrial application, in the same package.
12
Rev. A, April 2002