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As shown in Fig. 1.3a, the turn-on and the
turn-off states of the MOSFET are controlled by the gate
voltage. If the gate voltage is less than the threshold
voltage with respect to the emitter, no MOSFET inversion
layer is created and the device is turned off. Here any
applied forward voltage will drop across the reversed biased
junction J2. The forward breakdown voltage is therefore
given by the breakdown voltage of this junction at the turn-off
time. This is an important factor, particularly for power
devices where high voltages and currents are being dealt
with. The breakdown voltage of junction J2 depends on the
doping of the lower doped side of the junction, i.e., the
n-epi region. This is because the lower doping results in
a wide depletion region and thus a lower maximum electric
field in the depletion region. This is a reason the n- drift
region is doped much lighter than the p+ body region.
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Until recently,
the limitation of the IGBTs to serve many applications has
been a result from slow turn-off speed when compared to the
power MOSFETs. While turn-on is fairly rapid, the turn-off
time of the IGBT is slow because minority carriers are stored
in the n-epi region. Figure 1.5 illustrates the turn-off switching
waveform, the tail time and contributing factors of the fast
IGBT designed for a Pulse Width Modulation (PWM) motor control
service [Mot95]. When the gate is initially brought below
the threshold voltage, the n-epi contains a very large concentration
of electrons. There will be significant injection into the
p+ substrate and a corresponding hole injection into the n-epi
region. As the electron concentration in the n-epi decreases,
the electron injection decreases, leaving the rest of electrons
to recombine. Therefore, the turn-off state of the IGBT has
two phases: An injection phase where the collector current
falls very quickly, and a recombination phase which the collector
current decreases more slowly. This kind of turn-off mechanism
was originally suggested in a reference paper by Baliga [Bal85].
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| Turning the device on is achieved
by increasing the gate voltage that is greater than the threshold
voltage. This state forms an inversion layer under the gate,
which provides a channel linking between the source and the
drift region of the device. Electrons are then injected from
the source into the drift region, while junction J3 is in the
forward bias state and injects holes into the n-epi drift region.
The injection causes the conductivity modulation of the drift
region where both the electron and hole densities are several
orders of magnitude higher than the original n-doping. It is
the conductivity modulation that gives the IGBT its low on-state
voltage because of the reduced resistance of the drift region.
Some of the injected holes will recombine in the drift region,
while others will cross this region via drift and diffusion
mechanism and reach the junction of the p-type region where
they will be collected. The operation of the IGBT can therefore
be considered like a wide base pnp-transistor and base drive
current is supplied by the MOSFET current through the channel.
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| For high performance systems, electronic
package design has been moved towards larger chips, higher number
of I/O ports, increased circuit density and improved reliability.
Greater circuit density means increased power density (W/m2).
Power density has increased exponentially over the past fifteen
years and it appears that it will continue to do so in the future.
As the power density is high, thermal management should be considered
carefully . |
| There are many practical engineering
problems that require the analysis of a heat transfer equation.
The solution of heat conduction is usually sufficient for general
engineering problems. If the equations governing coupled heat
and mass transfer and thermal convection are considered, the
application areas expand considerably. Combining the thermal
analysis and the mechanical stress can provide an answer to
questions in power electronics [Lew96]. |
| Analytical solutions of the heat
transfer equation can only be obtained by simplifying assumptions
for the geometry, material properties and boundary conditions.
For the analysis of practical problems, such simplifications
are not generally possible. Therefore, numerical methods with
the flexibility in dealing with complex geometries are an ideal
approach for the solution of such problems. |
| In this chapter, a basic differential
equation and a concept of the numerical formulation governing
the heat conduction are introduced prior to the use of Computer
Aided Design (CAD) tool. In addition, a paper review of the
various methodologies useful in the thermal analysis and design
of the power electronics is achieved. |
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