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8

and the falling collector current. Because the IGBT is a

minority carrier device, the collector current continues to

flow after the time where the collector voltage has fully

risen. This residual current, called tail current, eventually

decays to zero. It is customary to add a fixed length of time

to the end of the turn−off time to capture the energy lost

during the entire tail current. This added time is denoted as

x

ms in Figure 10.

Figure 10. Turn−off Switching Illustration Showing

the Definitions of the Turn−off Switching

Characteristics

The total switching losses comprise the sum of the

turn−on and turn−off switching losses.

Typical switching time and switching energy loss graphs

are given that describe the dependence of the switching

characteristics on a variety of system variables. The

dependence on junction temperature, collector current,

collector−emitter voltage, and gate resistance are all

provided to aid in the design process.

Diode Characteristics

Figure 11. Copackaged IGBT and Freewheeling

Diode

IGBTs are frequently used in applications where the load

is inductive, such as motor control. These applications are

hard switching and require that the IGBT be in parallel with

a freewheeling diode. ON Semiconductor

offers

copackaged IGBT and diode devices. The diode cathode and

IGBT collector are connected together and the diode anode

and IGBT emitter are also connected, as shown in Figure 11.

The freewheeling diode takes the place of the body diode

that otherwise exists in a power MOSFET. For IGBTs that

are copackaged with a freewheeling rectifier diode, the

datasheet will also include electrical specifications for the

diode, as shown in Table 6.

Table 6. ELECTRICAL CHARACTERISTICS OF THE DIODE

Parameter

Test Conditions

Symbol

Min

Typ

Max

Unit

DIODE CHARACTERISTIC

Forward voltage

1.6

1.6

1.85

V

Reverse recovery time

270

ns

Reverse recovery charge

350

nc

Reverse recovery current

5

A

Reverse recovery time

350

ns

Reverse recovery charge

1000

nc

Reverse recovery current

7.5

A

The forward voltage of the rectifier is measured while the

IGBT gate and emitter terminals are tied together, ensuring

the IGBT is in its off−state. A forcing current enters the

emitter terminal and the emitter−collector (anode−cathode)

voltage is measured.

Forward voltage is an important parameter in hard