9020 Datasheet, PDF (3/25 Pages) Fairchild Semiconductor – IGBT Basic II

Section I. Gate drive considerations
1. Introduction – IGBT Structure
EMITTER
.
R_S
NPN
PNP
G. ATE
N-CHANNEL
COLL^.ECTOR
EMITTER
GATE
N-CH.
NPN
R_S
PNP
P^-
N⁺
J3
RMODULATION
P⁺
J2
N^- epi. (N^- drift)
N⁺
J1
P⁺ substrate
COLLECTOR
Fig. 1. Structure of IGBT
The structure of the IGBT is the combination of the P+ layer added to the MOSFET structure
as shown in Fig. 1. The IGBT, constructed by adding the P+ layer, has the characteristics of
the power transistor (BJT), which has high conductivity in its n-layer by injecting hole into the
n-layer with high resistance. As such, IGBT is easier to drive, and it combines the advantages
of MOSFET’s faster switching speed and power BJT’s lower conduction loss. IGBT is a useful
device in that it overcomes the shortfall of MOSFET in that it is not suitable for high voltage,
high current applications due to its high conduction loss, while IGBT has the advantage over
power BJT, which has limitations in high frequency applications due to its switching speed.
Demand for IGBT is increasing in mid to low power applications, and the applications are
becoming more varied, as the capacity of IGBT continues to increase. In order to obtain the
optimum performance from the IGBT with such characteristics, it is of foremost importance to
design a gate drive that is suited for the application. As such, this paper intends to discuss the
characteristics of IGBT and some issues to consider in designing a gate drive as well as pro-
viding necessary information in designing an application system to help engineers who design
systems using IGBT.
2. Gate Drive Considerations
The IGBT can change its switching properties through the gate drive, so designing a proper
gate drive is extremely important to the performance of the IGBT. So-called the “best perfor-
mance” of the IGBT is different by application, which means the design of the gate drive must
be different depending on the application of each IGBT. For example, hard-switching applica-
tions such as motor drives or UPS, the switching waveform must ensure that the IGBT’s loci of
operation do not exceed SOA, and the gate drive parameters must be set accordingly. This
means that it may be necessary to sacrifice switching speed from the loss of switching as well
as V_CE(sat) from the loss of conduction. On the other hand, in soft-switching applications, there
is less burden from SOA, so it is possible to select a device with V_CE(sat) and t_f with good char-
acteristics, and it is possible to choose the trade-off between V_CE(sat) and t_f with gate drive
parameter depending on whether switching loss or on-state loss is greater. In this chapter, we
would like to examine characteristics of IGBT and gate drive parameter, discuss the relation-
ships between the two, and some issues to consider in designing a gate drive.
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Rev. A, April 2002