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Unformatted text preview: ronment for implementing soft
turn-off. There are two ways of doing this. Just
by connecting a resistor of appropriate value
from Gate to source/emitter, the CGS gets discharged through this resistor and, depending on
the value of the connected resistor, soft turn-off of
any duration can be achieved. Another way, as
shown in the Fig. (2), is to use a signal MOSFET Q1
to pull down the Gate, when short circuit is detected.
The resistor in series with this signal MOSFET
determines the time duration of this soft turn-off. Soft
turn-off helps protect IGBT/MOSFET from any
voltage transients generated due to L*dIC/dt (or
L*dID/ dt) that could otherwise bring about avalanche
breakdown. The PC board layout and Bill of
Materials for this circuit are shown in Fig. (3).
For an IGBT, Desat detection (Desat = Desaturation
of forward voltage drop) is a method used for short
circuit/overload occurs, the forward voltage drop of
the IGBT (V CE) rises to disproportionately high
values. One must ignore the initial turn-on rise in
VCE, when output from Driver has still not risen to
high enough value. Nevertheless, when V CE rises to a level of, say, 7 Volts, in presence of sufficient Gate
Drive voltage, it means the collector current IC has risen to a disproportionately high value, signalling
overload. When a voltage level higher than 6.5 Volts
is detected, Gate signal can be softly turned off,
resulting into soft turn-off of the IGBT. Fig. (12)
shows how Desat feature can be wired into a total
Driver Circuit, using also other features, such as
Opto-isolation and –ve turn-off bias.
3. HIGH SIDE DRIVING TECHNIQUES
BOOTSTRAP METHODS AND For driving the upper MOSFET/IGBT in a phase leg
employed in a bridge topology, a buck converter or a
2-transitor forward converter, low side drivers cannot
be used directly. This is because the source/emitter
of upper MOSFET/IGBT is not sitting at ground
Fig. (5) shows how a charge pump creates a higher
Vcc to be used for the...
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This note was uploaded on 01/15/2014 for the course ECE 624 taught by Professor Staff during the Winter '08 term at Ohio State.
- Winter '08