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6

Figure 7. Pin−to−pin Capacitances of the IGBT

The input capacitance is made up of the parallel

combination

of

gate−emitter

and

gate−collector

capacitances, when the collector and emitter are tied

together. The gate−emitter capacitance is constant, as it

consists mainly of the metal−oxide−semiconductor

capacitance. The gate−collector capacitance is a

combination of a fixed oxide capacitor and a p−n junction

capacitor. This results in a voltage dependence that is

slightly more complex than that of a p−n junction.

The output capacitance is formed by the parallel

combination of the gate−collector and collector−emitter

capacitances. As mentioned above, the gate−collector

capacitance is voltage dependant. This is also true for the

collector−emitter capacitance. The voltage dependence of

the collector−emitter junction is that of a p−n junction.

The transfer capacitance is composed only of the

gate−collector capacitance. Its role in the device operation

is critical, as it provides negative feedback between the

collector and the gate. This capacitance is responsible for the

plateau on the gate charge curve. The change in

which reduces the gate drive current while the collector

voltage is changing.

Input capacitance is useful, but in terms of gate drive

design, the more important figure of merit is the gate charge.

It is used to size the gate drive components and predict

switching losses in the driver. To measure gate charge the

IGBT gate is driven with a current and the gate voltage

change is monitored versus time. The resulting gate voltage

versus gate charge curve is shown in Figure 8 for a constant

current gate drive signal.

a specified gate voltage. ON Semiconductor devices are

Figure 8. Theoretical Gate Charge Curve showing

channel, at which time the collector−emitter voltage begins

dependent on the load current being switched and can be

the switching current level from the transconductance

curves in Figure 5.

capacitor while the voltage from collector to emitter is

decreasing in the transition between the off−state and

on−state. This plateau corresponds to the charging of what

is also known as the Miller capacitance.

Switching Characteristics

The IGBT switching characteristics are of great

importance because they relate directly to the switching

energy losses of the device. Switching losses can be

substantial, especially at higher frequencies and increasing

temperature, where the switching losses increase.

When voltage is applied to the gate, the input capacitance

begins to flow. Once the collector current begins to flow, the

depletion layer that blocks the voltage during the off−state

begins to collapse. The voltage drops to the on−state voltage

During turn−off, the gate voltage is reduced to zero and

the opposite occurs. The channel for the MOSFET current

is closed and the current begins to drop abruptly. The voltage