PCIM Europe 2014, 20 – 22 May 2014, Nuremberg, Germany
Switching characteristics of modern 6.5kV IGBT/Diode
Daniel Heer, Infineon Technologies AG, Germany, Daniel.Heer@Infineon.com
Amr Khairy Bayoumi, Infineon Technologies AG, Germany, Amr.Bayoumi@Infineon.com
Abstract
New IGBT generations, i.e. Trench-Field-Stop devices, show significant differences
regarding their control characteristics in comparison to power MOSFETs [1]. The reason for
that is the stored charge effect. For a better understanding of the switching behavior of
modern IGBTs, a detailed characterization upon the variation of the decisive switching
parameters is needed. The detailed characterization, with focus on the stored charge effects,
is continued with state of the art 6.5kV IGBT and diode in a special setup [2]. Further, the
forward recovery effect of the diodes is discussed and the influence of the dI/dt and
temperature is presented. This effect results in a high blocking-voltage in reverse direction
across the IGBT parallel to the freewheeling diode. Additionally the charge, which is stored in
conduction mode and has to be extracted during the turn-off, is calculated for two modern
6.5kV IGBT types at different collector-currents and conduction times.
1.
Introduction
The turn-off of modern IGBTs is different to standard power MOSFETs. The reason behind
that is the stored charge effect. The stored charge effect determines the turn-off of today’s
IGBTs. A completely detailed turn-off characterization of the 6.5kV IGBT points out the
influence of the stored charge. Model parameters can be extracted from this study. A special
test setup, which prevents package capacitance to contribute to the extracted parameters,
was used for the characterization [2].
2.
Electrical characterization
Two types of modern 6.5kV IGBT chips have been investigated. The second IGBT
generation with the planar gate technology (IGBT2) is represented by a single chip with a
nominal current of 25A. Two variations of the Trench-Field-Stop IGBT (IGBT3) have been
also studied. These two chips have the same technology, however different active areas
resulting in chips with nominal currents of 25A and 30A. Thanks to the trench-technology, the
carrier density is increased, which leads to VCE,sat-values of the DUTs of 3.4V (IGBT2) and
2.8V (IGBT3). The first step in the characterization is the variation of the gate-resistance with
the target of finding the minimum value RG,off at which dVCE/dt becomes controllable during
turn-off. Generally, up to a certain gate-resistance value RG,controllable, IGBTs can not to be
controlled by the gate. The value of RG,controllable is only dependent on the operation conditions
(IC, ton, VCC). The typical dependence of the dVCE/dt and dIC/dt on the gate resistance during
IGBT turn-off is given in Figure 1. This means, that for a certain interval of RG,off (<
RG,controllable) no influence on the dVCE/dt and dIC/dt and therefore on the over-voltage peak is
possible. If the RG,off is further increased, the semi-controllable region with influence on the
dVCE/dt is entered. This means that the IGBT behaves not like expected from MOSFET
devices; with increased RG,off the value of dIC/dt becomes higher. The reason for that is the
longer duration of the miller phase resulting in more desaturation of the bipolar device. For
even higher values of RG,off, the device is fully controllable. With further increasing RG,off, the
dIC/dt becomes lower like known from MOSFET devices.
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PCIM Europe 2014, 20 – 22 May 2014, Nuremberg, Germany
Figure 1: Typical behaviour of dIC/dt and dVCE/dt in IGBT turn-off upon increasing RGoff.
G
Figure 2 and Figure 3 illustrate the rise of the VCE voltage during the turn-off for IGBT2 and
IGBT3, respectively. The operation conditions are: IC=25A, VCC=2kV and ton=250μs. The
comparison shows the effect of different storage charge due to the different chip
technologies. For the IGBT2, the intrinsic limitation of the collector emitter voltage rise-up
corresponds to a gate resistance value of 150Ω. On the other hand, the IGBT3 shows this
effect up to 75Ω. Consequently, the IGBT3 provides enhanced controllability compared to the
IGBT2 under the same driving and operation conditions.
Figure 4 shows the results for the 6.5kV IGBT3 chip with a nominal current of Inom=30A. The
intrinsically defined value of RG,controllable is reduced in comparison to the IGBT3 chip with
Inom=25A. This is caused by the higher level of the collector current which results in a faster
and more intensive stored charge extraction. The range of the intrinsic limitation of the
dVCE/dt correlates inversely with the operation current.
Figure 2: Turn-off of 6.5kV-IGBT2 Inom=25A, single chip upon variation of the gate-resistor. VCC=2kV,
IC=25A, ton=250μs.Up to 150Ω gate resistance, the collector-emitter voltage rise is intrinsically limited.
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Figure 3: Turn-off of 6.5kV-IGBT3 Inom=25A, single chip upon variation of the gate-resistor. VCC=2kV,
IC=25A, ton=250μs.Up to 75Ω gate resistance the collector emitter voltage rise is intrinsically limited.
Figure 4: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip upon variation of the gate-resistor. VCC=2kV,
IC=30A, ton=200μs.Up to 62Ω gate resistance the collector emitter voltage rise is intrinsically limited.
Figure 5 shows the measurements done on the 6.5kV IGBT3 with a conduction time of
630μs, the output current and DC-link voltage remain the same: 30A and 2kV. Comparing
Figure 5 to Figure 4, the obviously increased conduction time has no significant effect on the
range of the intrinsic limitation and therefore the value of RG,controllable remains unchanged. The
reason behind this is that the quantity of stored-charge at a conduction time of 630μs is not
proportionally increased to that at a conduction time of 200μs.
In Figure 6 the turn-off of the 6.5kV-IGBT3 at Inom=30A under different load currents is
presented. With gate-resistance of 1Ω the gate voltage drops before collector voltage has
risen to VCC=2kV. The dV/dt of the collector voltage varies with the load current in a way
similar to charging a capacitor with a constant current.
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Figure 5: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip upon variation of the Gate-resistor. VCC=2kV,
IC=30A, ton=630μs. Up to 62Ω gate resistance, the collector-emitter voltage rise is intrinsically limited.
Figure 6: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip under different load currents. The variation in
dVCE/dt with changing collector/load current is illustrated.
In further investigations, the variation of the on-time, the time period in which the IGBT is
conducting prior to turn-off, illustrates the influence of the amount of stored charge on the
switching behavior. By increasing the on-time of the IGBT, higher charge will be stored in the
device; this has a direct influence on the dV/dt of the collector emitter voltage. The quantity of
the stored charge rises with the on-time until the full saturation of carrier injection in the onstate is reached.
The measurements done with the 6.5kV-IGBT2 Inom=25A as depicted in Figure 7, point out
that on-times longer than 256μs have no influence on the dV/dt of the collector emitter
voltage. The analysis for the 6.5kV-IGBT3 Inom=30A in Figure 8 shows that on-times longer
than 494μs have no influence on the dVCE/dt. Figure 9 illustrates the same investigation
based on 6.5kV-IGBT at twice the rated current. The analysis shows that on-times exceeding
399μs have no influence on the dVCE/dt of the collector emitter voltage. This indicates that
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the stored charge saturation of the IGBT is defined by the conduction-time and the collector
current.
Figure 7: Turn-off of 6.5kV-IGBT2 Inom=25A, single chip upon variation of the on-time at an operation
current IC=25A, VCC=2kV.
Figure 8: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip upon variation of the on-time at an operation
current IC=30A. VCC=2kV.
Figure 9: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip upon variation of the on-time at an operation
current IC=60A. VCC=2kV.
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In Figure 10 the calculated charge, which is stored during the conduction mode and has to
be extracted during the turn-off for the investigated IGBT chips, is presented. Details about
the charge calculation method can be found in [2]. The charge is normalized to the operation
current. The effects seen in Figure 7 to 9 can be explained by the graphs from Figure 10.
Due to the trench-gate technology, IGBT3 shows a significant increase of the stored charge
in comparison to IGBT2.
Figure 10: The charge which is stored in conduction mode and has to be extracted during the turn-off,
is calculated for three modern 6.5kV IGBT types at different collector-currents and conduction times.
The turn-off of the 6.5kV-IGBT3 at a collector current of 30A and under different DC-link
voltages is presented in Figure 11. The variation in dIC/dt and the tail-trend with increasing
DC-link voltage is noticed. For high current levels, the rate of the current decay (dIC/dt) is
reduced by increasing the DC-link voltage because a higher DC-link voltage results in a
faster extraction of the stored charge. The turn-off behavior of the IGBT3 under different DClink voltages and a collector current of 60A is presented in Figure 12. At current levels above
50% of the rated current, no impact of the DC-link voltage on dIC/dt can be seen.
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Figure 11: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip upon variation of the DC-link voltage at an
operation current IC=30A.
Figure 12: Turn-off of 6.5kV-IGBT3 Inom=30A, single chip upon variation of the DC-link voltage at an
operation current 2•Inom=60A. For higher collector-current values, no impact by the DC-link voltage is
noticed.
Figure 13 illustrates the forward-recovery effect of the Emitter-Controlled-2 diode in
dependence of the dIC/dt and the temperature. The voltage over the diode VAK is shown at
the turn-off of the IGBT. During the commutation, the current’s rate of change (dIC/dt) is
defined by the IGBT. At the beginning of the commutation, the diode possesses a low carrier
charge density. This results in a high resistance and a large voltage drop upon the diode in
forward-direction. This behavior can be modelled by a transient resistance, with an initial
value that depends on the current level and the temperature. In the application, this behavior
has to be considered, as the forward voltage-drop across the freewheeling diode has to be
blocked by the IGBT in the reverse direction. At an operating temperature of 125°C and
different dIC/dt, a forward recovery voltage of more than 500V has been measured.
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Figure 13: Forward-recovery effect of the 6,5kV Emitter-Controlled-2 diode Inom=50A in dependence of
dIC/dt and temperature.
3.
Conclusion
The presented analysis allows a better understanding of the switching behavior of IGBTs and
diodes in dependence of the stored charge.
The results of a detailed characterization of 6.5kV IGBT2/3 & Emitter-Controlled diode chips
are presented. At lower values of RGoff, the dVCE/dt and dIC/dt are intrinsically defined. A
semi-controllability is achieved for values above RG,controllable. Increasing the RG,off results in a
lower dVCE/dt but a higher dIC/dt and hence a higher over-voltage. A further increase of RG,off
guarantees a full-controllability over the IGBT. It was shown that the 6.5kV IGBT3 with trench
gate technology has more stored charge compared to the 6.5kV IGBT2 with planar gate
technology, however reaches controllability at significantly lower values of RG,off. For the
IGBT3, the dIC/dt depends not only on the operation current but on the DC-link voltage and
RG,off as well. Furthermore, the forward-recovery effect of the 6.5kV Emitter-Controlled-2 was
discussed. For all test conditions the forward-recovery voltage was at least 500V. This has to
be considered as this voltage has to be blocked by the IGBT in the reverse direction.
4.
References
[1]
F. Pfirsch, R. Bayerer: MOS-gesteuerte Leistungsschalter: Konzepte und Schaltverhalten, ETG 2006, Bad Nauheim, Germany
[2]
D. Heer, R. Bayerer: Simple turn-off description of Trench- Field-stop IGBT–
IGBT3/3.3kV; PCIM 2012, Nuremberg, Germany
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