19-07-2014, 02:24 PM
IGBT/MOSFET Gate Drive Optocoupler
IGBTMOSFET.pdf (Size: 169.27 KB / Downloads: 124)
INTRODUCTION TO IGBT
The Insulated Gate Bipolar transistor (IGBT) is a cross
between a MOSFET (metal oxide semiconductor field effect
transistor) and a BJT (bipolar junction transistor) since it
combines the positive aspects of MOSFETs and BJTs.
The IGBT has the fast switching capability of the MOSFET
and is capable of handling the high current values typical of
a BJT. In addition, IGBTs have a lower on-state voltage drop
and are capable of blocking higher voltages.
The IGBT, as a first approximation, can be modeled as a
PNP transistor driven by a power MOSFET, as shown in
figure 1.
Fig. 1 - Simplified IGBT Equivalent Circuit
Figure 2 shows the most common symbols used for IGBT.
Fig. 2 - IGBT Symbols
The equivalent circuit for the input of IGBT is the same as a
MOSFET and is purely capacitive. This allows the use of a
voltage drive, which means that it is possible to have a less
complex circuit with lower power consumption compared to
a BJT. IGBTs are used for high current, high voltage
applications when switching speed is importan
IGBT GATE DRIVE REQUIREMENTS AND
FEATURES
Minimum Output Current
A very important requirement for an IGBT gate driver
optocoupler is to supply the minimum output or gate current
(IOL or Igate) to switch the IGBT to the low impedance state.
This is illustrated in figure 11.
Power Supply By-Passing
Since turning an IGBT/MOSFET on and off amounts to
charging and discharging large capacitive loads, the peak
charge current need to be within the capability of drive
circuit. At the same time the driver will have to draw this
current from its power supply in a short period of time. This
means that using of proper by-pass capacitors for the
power supply becomes very important. A pair of by-pass
capacitors of at least 10 times the load capacitance with
complementary impedance, used in parallel and very close
to the VCC pin, can take care of this issue. These by-pass
capacitors should have the lowest possible equivalent
series resistance (ESR) and equivalent series inductance
(ESL) and the capacitor lead lengths should be as short as
possible.