Page 124 - Rashid, Power Electronics Handbook

7 Insulated Gate Bipolar Transistor 111

depending on the initial condition of the capacitor and its 7.7 Circuit Models
value, the IGBT current may reduce to zero and then turn on
again. Another method is to softly turn off the IGBT after the
fault and to reduce the overvoltage due to di =dt. Therefore, A high-quality IGBT model for circuit simulation is essential
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the overvoltage on IGBT caused by the parasitic inductance is for improving the ef®ciency and reliability in the design of
limited while turning off large currents. The most common power electronic circuits. Conventional models for power
method of IGBT protection is collector-voltage monitoring or semiconductor devices simply described an abrupt or linear
desaturation detection. The monitored parameter is the collec- switching behavior and a ®xed resistance during the conduc-
tor-emitter voltage, which makes fault detection easier tion state. Low switching frequencies of power circuits made it
compared to measuring the device current. However, voltage possible to use these approximate models. However, moving
detection can be activated only after the complete turn on of to higher switching frequencies to reduce the size of a power
IGBT. If the fault current increases slowly due to large fault electronic system requires high-quality power semiconductor
inductance, the fault detection is dif®cult because the collec- device models for circuit simulation.
tor-emitter voltage will not change signi®cantly. In order to The n-channel IGBT consists of a pnp-bipolar transistor
determine whether the current that is being turned off is whose base current is provided by an n-channel MOSFET, as is
overcurrent or nominal current, the Miller voltage plateau shown in Fig. 7.1. Therefore, the IGBT behavior is determined
level can be used. This method can be used to initiate soft by physics of the bipolar and MOSFET devices. Several effects
turn-off and to reduce the overvoltage during overcurrents. dominate the static and dynamic device characteristics. The
Special sense IGBTs have been introduced at low-power in¯uence of these effects on a low-power semiconductor
levels with a sense terminal to provide a current signal device is negligible and therefore they cannot be described
proportional to the IGBT collector current. A few active by standard device models. The conventional circuit models
device cells are used to mirror the current carried by the were developed to describe the behavior of low-power devices,
other cells. Unfortunately, however, sense IGBTs are not and therefore were not adequate to be modi®ed for IGBT. The
available at high-power levels and there are problems related reason is that the bipolar transistor and MOSFET in the IGBT
to the higher conduction losses in the sense device. The most have a different behavior compared to their low-power coun-
reliable method to detect an overcurrent fault condition is to terparts; they also have different structures.
introduce a current sensor in series with the IGBT. The The currently available models have different levels of
additional current sensor makes the power circuit more accuracy at the expense of speed. Circuit issues such as
complex and may lead to parasitic bus inductance, which switching losses and reliability are strongly dependent on the
results in higher overvoltages during turn off. device and require accurate device models. However, simpler
After the fault occurs the IGBT has to be safely turned off. models are only adequate for system-oriented issues such as
Due to large di =dt during turn-off, the overvoltage can be the behavior of an electric motor driven by a pulsewidth
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very large. Therefore, many techniques have been investigated modulation (PWM) converter. Finite-element models have
to obtain soft turn-off. The most common method is to use a high accuracy, but are slow and require internal device
large turn-off gate resistor when the fault occurs. Another structure details. Macromodels are fast but have low accuracy,
method to reduce the turn-off overvoltage is to lower the which depends on the operating point. Commercial circuit
fault-current level by reducing the gate voltage before initiat- simulators have introduced one-dimensional (1D) physics-
ing the turn-off. A resistive voltage divider can be used to based models, which offer a compromise between the ®nite-
reduce the gate voltage during fault turn-off. For example, the element models and macromodels. The Hefner model and the
gate-voltage reduction can be obtained by turning on simul- Kraus model are such examples that have been implemented
taneously R goff and R gon in the circuit of Fig. 7.12. Another in Saber and there has been some effort to implement them in
method is to switch a capacitor into the gate and rapidly PSPICE. The Hefner model depends on the redistribution of
discharge the gate during the occurrence of a fault. To prevent charge in the drift region during transients. The Kraus model
the capacitor from charging back up to the nominal on-state depends on the extraction of charge from the drift region by
gate voltage, a large capacitor should be used, which may cause the electric ®eld and emitter back-injection.
a rapid gate discharge. Also, a Zener diode can be used in the The internal BJT of the IGBT has a wide base, which is
gate to reduce the gate voltage after a fault occurs, but the slow lightly doped to support the depletion region to have high
transient behavior of the Zener diode leads to large initial peak blocking voltages. The excess carrier lifetime in the base region
fault current. The power dissipation during a fault determines is low to have fast turn-off. However, low-power bipolar
the time duration that the fault current can ¯ow in the IGBT transistors have high excess carrier lifetime in the base,
without damaging it. Therefore, the IGBT fault-endurance narrow base and high current gain. A ®nite base transit time
capability is improved by the use of fault-current limiting is required for a change in the injected base charge to change
circuits to reduce the power dissipation in the IGBT under the collector current. Therefore, quasi-static approximation
fault conditions. cannot be used at high speeds and the transport of carriers