6.5kV IGBT and FWD with Trench and VLD Technology for reduced Losses and high dynamic Ruggedness

6.5kV IGBT and FWD with Trench and VLD Technology for
reduced Losses and high dynamic Ruggedness
1)
2)
2)
3)
3)
Thomas Duetemeyer , Josef-Georg Bauer , Elmar Falck , Carsten Schaeffer , G. Schmidt ,
1)
Burkhard Stemmer ,
1
Infineon Technologies AG, Max-Planck-Straße 5, D-59581 Warstein Germany
2
Infineon Technologies AG, Am Campeon 1-12, D-85579 Neubiberg, Germany
3
Infineon Technologies AG, Siemensstr. 2, A-9500 Villach, Austria
Abstract
Potentials offered by introducing trench technology into the 6.5kV voltage regime are investigated. In
addition a VLD edge termination is presented with significantly reduced termination width compared to
a field plate termination design. Both technologies result in significantly reduced on state losses and
dynamic losses. At the same time improved robustness and a soft switching behaviour of the IGBT and
diode are achieved. The surge current integral I²t of the diode can be extended more than a factor of
two and excellent rugged short circuit behaviour of the IGBT is proven.
The availability of 6.5kV IGBTs makes it possible
to reduce the complexity of inverter designs that
had to be built with 3.3kV modules or GTOs in
former times, thus resulting in lower system costs
and better reliability. The market has an increased demand for 6.5kV modules with higher
current rating to avoid or reduce the necessity of
paralleling modules in the application.
Inverters in industrial and traction applications in
the upper voltage regime are typically characterized by comparatively low switching frequencies
in the range below 1 kHz. Therefore the contribution of on state losses plays the dominant role.
To reduce the on state and switching losses of
IGBTs in traction applications the trench concept
has successfully been introduced for 1.7kV and
3.3kV modules. This is now supplemented by a
6.5kV module with trench concept for the IGBTs.
The implementation of VLD edge termination
1
structure with low reverse current behaviour allows an additional increase of the active chip
area for IGBT and diode chips.
Further important aspects concerning application
in the high power regime are very strong requirements as far as device ruggedness is concerned. Special care in device optimisation is
taken with respect to surge current and switching
ruggedness.
2
Static Characteristics IGBT
& Diode
2.1
Static Characteristic Diode
In Fig. 1 it is shown that due to improvement of
the vertical design and the smaller edge termina-
tion very low on state losses of the optimized diode have been achieved.
2,0
RT
125C
1,5
1,0
0,5
5
4
3
2
1
04
@600A
@750A
VF[V]
Introduction
I/INom
1
0,0
0
1
2
3
VF [V]
EC2
5
EC3
6
rd
Fig. 1 On state voltages of optimized 3 generation diode for RT and +125°C. The inset
nd
shows VF of emitter controlled (EC) diodes of 2
rd
versus 3 generation at Tvj=125°C
The forward voltage of the diode shows a positive
temperature coefficient for currents higher than
1.2 times the rated current. Compared to the
state of the art diode the forward voltage at rated
current is reduced by more than 25% while at the
same time the rated current is increased by 25%
without changing the outer chip dimensions.
2.2
Static Characteristic IGBT
The combination of the smaller VLD edge termination system with larger active area and trench
concept in the new IGBT result in significantly reduced on state losses. As shown in Fig. 2 the
forward voltage is reduced more than 30% at
125°C temperature in comparison to the planar
6.5kV IGBT. The achieved positive temperature
coefficient over the whole current range eases
the paralleling of chips and modules.
2,0
RT
125C
1,0
@600A
6
VCE(sat)[V]
0,5
0,0
0
1
2
3
@750A
4
2
4 0
VCE(sat) [V]
5
6
IGBT2
7
100
IGBT3
IR (mA/module)
Fig. 2 Forward characteristic of the planar IGBT
vs trench IGBT3 for RT and +125°C. The inset
shows VCE(sat) at Tvj=125°C
3
VLD Termination System
for IGBT and Diode
As a further improvement a narrow VLD (variation of lateral doping) edge structure is implemented in place of the current p ring field plate
termination system (pFP). A sketch of the structure is presented in Fig. 3.
passivation layer
+
p
VLD
n
n
channel
stopper
10
1
0.1
pFP
0.01
2.25
2.50
2.75
3.00
VLD
3.25
3.50
1000/T (K)
Fig. 5 Module reverse-blocking current versus
1/T for pFP (p ring field plate termination system)
and VLD termination system (24 IGBT and 12 diode chips in parallel)
The reliability of the termination structure is successfully proven with passed 1000h high temperature reverse bias (HTRB) and high temperature, humidity and voltage tests (H3TRB).
+
Fig. 3 Schematic cross section of diode with the
VLD termination system
4
Dynamic Characteristics
under nominal conditions
4.1
Softness and Switching Energies
-
Linked to the p-emitter of the anode is a p zone
with decreasing doping concentration towards the
outside of the chip. With an optimized doping
gradient the edge termination is about 25%
smaller compared to a pFP termination while reducing the maximum electrical field strength.
With an improved VLD profile a high breakdown
voltage over a wide temperature range can be
realized as shown in Fig. 4.
8
7.8
7.6
7.4
7.2
7
6.8
6.6
6.4
6.2
6
Fig. 6 shows the turn off waveforms of a
6.5kV/750A module at rated current IC= 750A at
VCE= 4.5kV for -40°C, room temperature (RT)
and +125°C.
800
8
700
7
600
6
500
5
400
4
300
3
200
[email protected] -40°C
[email protected] RT
100
[email protected] +125°C
0
[email protected] -40°C
UCE[kV]
UBR [kV]
[email protected] (VLD)
[email protected] (VLD)
[email protected] (pFP)
[email protected] (pFP)
125°C
-50 -25
0
25 50 75
Temperature [°C]
100 125
Fig. 4 Typical breakdown voltages VBR(T) of
new 6.5kV/750A module
The termination is robust against avalanche
IC [A]
I/I Nom
1,5
breakdown over the whole temperature range between -50°C and +125°C. For room temperature
and +125°C a maximum collector-emitter voltage
of 6500V will be specified. With the expected
higher reverse-blocking currents of the new chip
design compared to the pFP termination system
thereby generated losses are still less than one
per cent of the maximum allowed losses of the
module and thus are negligible.
[email protected]
0 RT1 2
[email protected] +125°C
3
4 5 6
t[µs]
7
8
9 10
Fig. 6 Turn-off waveforms of a 6.5kV/750A
3
trench IGBT module for VCE=4.5kV, IC=750A,
Lσ=280nH at -40°C, 25°C and 125°C temperature
1.0
0.5
0.0
-0.5
VCE
IC
1.0
2.0
3.0
4.0
The dependency of turn-on and recovery losses
against current slope for the new 6.5kV/750A
trench IGBT3 module is plotted in Fig. 9. For a
typical dIC/dt of 3 kA/µs the losses per amp are
about 10% lower than for the current 6.5kV/600A
planar IGBT2 module.
-1.0
Eon
5.0
6.0
Erec
10000
8000
6000
time [µs]
4000
Fig. 7 Diode
recovery
waveform
of
a
6.5kV/750A trench IGBT3 module at room temperature. IC=750A, VCE=3.6kV, Lσ=280nH
4.2
Etot
14000
12000
-1.5
0.0
4.2.2 Turn-On Losses (Eon) IGBT and
Recovery Losses Diode (Erec)
E [mJ]
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
The losses per amp are 20% lower than for the
state of the art planar IGBT2. A further significant
reduction of the turn-off losses can be achieved
by increasing dVCE/dt up 5 kV/µs.
IC [kA]
VCE[kV]
It can be seen that soft switching behaviour is
achieved over the whole temperature range even
at that high voltage level. The increase of switching losses between 25° and 125°C as plotted in
Fig. 8 is mainly caused by the evolution of the tail
current. Even at lower temperatures the dIC/dt is
still low enough to ensure that VCE,max stays in the
same regime like at 125°C.
Soft switching behaviour is also observed for the
diode with improved vertical structure.
Switching Losses
2000
1,0
2,0
3,0
4,0
5,0
6,0
dIC/dt [kA/µs]
Fig. 9 Etot, Eon and Erec as f(dI/dt) at 125°C for
3
trench IGBT at IC=750A, VCE=3.6kV, Lσ=280nH
The trade off optimization of the emitter controlled diode and the trench IGBT is already de1
scribed in an earlier publication . In the following
the switching losses in dependency of the driving
condition will be presented. All values are given
for a 6.5kV/750A module with 24 IGBT and 12
diode chips in parallel. All measurements are performed with a stray inductance Lσ of 280nH.
At the same time the level of recovery losses
rises, but is still on a low level. These increased
losses are well overcompensated by the reduced
on state losses of the diode. As will be later
shown the thermal limitation of the module is still
caused by the IGBT chip losses if a realistic inverter operation is regarded.
4.2.1 Turn-Off Losses (Eoff) IGBT
4.3
The turn-off losses of the new 6.5kV/750A module are given in Fig. 8. A dVCE/dt of 2.5kV/µs
which is common for traction applications results
in losses of about 3.8 mJ.
4500
125°C
4000
Eoff[mJ]
RT
3500
3000
2500
2000
2000
3000
4000
5000
6000
7000
dV CE/dt [kV/µs]
Fig. 8 Eoff(dV/dt) for trench IGBT3 at IC=750A,
VCE=3.6kV, Lσ=280nH
IGBT Turn Off at varying driving
conditions
Fig. 10 shows the dependency dVCE/dt=f(RGoff)
for a 750A/6.5kV IGBT3 module if a resistive
driver is applied and VGE switches from +15V to 15V. Depending on IC there is a regime in which
nd
dVCE/dt doesn’t depend on RGoff and a 2 regime
in which dVCE/dt decreases with rising RGoff. The
behaviour is also observed in other HV IGBT3
2
specimen e.g. in the 3.3kV voltage class . An explanation for the dVCE/dt self limiting behaviour,
that is observed for IGBT3 technology under certain operation conditions can be given as follows:
In order to enable VCE to rise at a defined slope,
a space charge region has to build up in the base
region. The driving force for the extraction of
charge carriers which is necessary for the formation of the depletion zone is the collector current
IC itself. Thus the reachable dVCE/dt becomes
small if a low current far below the rated current
1500A
750A
5
300A
225A
4
150A
75A
3
2
1
0
1
2
3
4
5 6 7
RGoff [Ohm]
8
9
10 11
Fig. 10 dVCE/dt(RGoff) of 6.5kV/750A IGBT3 module at IC=750A, VCE=3.6kV, Lσ=280nH, Tvj=RT
8
7
6
5
4
3
2
1
0
20
VCE
IC
VGE
15
10
5
0
VGE[V]
VCE[kV] & IC[A*10]
The presently presented 6.5kV IGBT3 is optimized in a way that for high load currents IC
above ½ IC,nom full control of dVCE/dt is possible in
the relevant RGoff regime. Therefore reduction of
switching losses is possible in the interesting current regime. Furthermore dVCE/dt or dIC/dt feedback methods can be employed to reduce overvoltage in case of overcurrent.
Turning-off low currents shows a second effect.
At the beginning of turn-off the dVCE/dt is limited
as already mentioned and about linear. After
reaching a certain value of VCE the slope of the
VCE changes abruptly to a significantly higher rate. The reason for this change is that the depletion zone is extended over the whole base region
and the field reaches the field stop area of the
chip at that moment.
-5
-10
-15
0 2 4 6 8 10 12 14 16 18 20
time [µs]
Fig. 11 Low current turn-off (75A) of a
6.5kV/750A IGBT3 module at VCE=3.6kV,
Lσ=280nH; Tvj=RT
5
Switching under extreme
conditions
5.1
Short Circuit
Fig.12 shows the short circuit waveforms at
125°C temperature of 750A IGBT3 module. The
short circuit is limited to less than 7 times the
nominal current. The short circuit is proven to be
free from VCE and IC oscillations in the full operation voltage range of 1000V to 4500V and the
temperature range of -40°C to +125°C using a
gate-emitter voltage clamping of 17V. As shown
in Fig. 12 tests are performed with a pulse time of
14µs giving an additional safety limit to the spec
limit of 10µs.
20
7
15
6
10
5
5
4
0
3
-5
VCE
2
-10
IC
1
VGE
-15
0
-20
VGE[V]
dVCE/dt [kV/µs]
6
Once the field stop layer is reached the further
charging of the collector-emitter capacity happens almost without requiring the depletion zone
to extend deeper which leads to the observed
high dVCE/dt. This high dVCE/dt is completely
harmless because it doesn’t lead to critical field
strength in the chip.
VCE[kV] & IC[kA]
is turned off and vice versa. Consequently for
small currents a regime is observed, in which
dVCE/dt doesn’t depend on RGoff. In principle this
effect should be observable for all IGBTs with
very high on-state charge carrier concentration at
the emitter side pn junction (e.g. IGBT3) because
for these class of devices the rate by which the
depletion zone can form is effectively impeded by
the presence of the excessive carrier concentration close to the pn junction.
0
2
4
6
8 10 12 14 16 18 20
time [µs]
Fig. 12 Type I short circuit of a 750A/6.5kV IGBT
module at VCE=4500V; Tvj=125°C, tpulse=14µs
5.2
IGBT Dynamic Robustness
/RBSOA IGBT
3,4
Previous investigations addressed techniques
in order to improve the robustness of the planar
6.5kV IGBT. The extraordinary robustness of the
IGBT3 is visible in Fig. 13. It shows the turn-off
waveform for IC=2250A which corresponds to 3
times rated current at VCE=4.5kV. Fig. 13 proves
that the turn-off is passed successfully 50%
above the specified limit. RBSOA tests are performed under a variety of boundary conditions. It
can be concluded that excellent device ruggedness is observed in an extended temperature
range (-50°C...+125°C), a wide range of operation voltages (up to 4.5kV) and high stray inductance.
Fig. 13 Turn-off waveform for 3 times INom
(IC=2250A) for IGBT3. VCE=4.5kV, Lσ=280nH,
Tvj=RT
IGBT Dynamic Self Clamping
VCE [kV]
Dependent on how the IGBT is controlled by the
driver a dynamic self clamping is seen for switching off high currents with a high stray inductance.
Measurements are performed with a 500A/6.5kV
3
module with 16 IGBT chips in parallel.
pulse #1
pulse #951
pulse #2000
0
IC/INom[A]
The decisive parameter for diode robustness is
the maximum power Pmax during switch off. The
optimization of the carrier profile of the diode and
the improved ruggedness of the termination
structure leads to a substantial increase of Pmax.
Pmax values of more than 4 MW for a unit rated at
250 A (4 diodes in parallel) have been reached
without destruction.
6000
1500
1300
1100
900
700
500
300
100
-100
-300
5000
1 2
3 4 5 6 7
time [µs]
8 9 10
3000
2000
1000
0
-1000
1,0
2,0
3,0
4,0
5,0
time[µs]
Fig. 15 Diode
recovery
waveform
of
a
6.5kV/250A module. IC=250A, VCE=4.5kV,
Tvj=125°C, Pmax=4.5MW
The high safety margin allows turn-on of the
IGBT with high dIC/dt without exceeding the allowed Pmax of the diode. This is a necessary prerequisite to minimize the total switching energies
as depicted in Fig. 9.
5.5
pulse #1
pulse #951
pulse #2000
4000
IC
UCE
P
0,0
7
6
5
4
3
2
1
0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Diode Robustness/SOA Diode
Surge Current
In fault conditions of inverter applications high
surge currents can occur at the diode. The ability
to withstand these high currents is an important
criterion for the usability of the module.
9
0 1 2 3 4 5 6 7 8 9 10
time [µs]
Fig. 14 VCE and IC waveforms for turn-off showing self clamping of the IGBT. VCE=4.5kV,
Lσ=620nH, Tvj=125°C
The clamping is observed most pronounced if a
high stray inductance is inserted in the setup and
high currents (here: 3.2 times rated current) are
turned off.
As depicted in Fig. 14 even after 2000 times
repetition of this high current no change in
switching behaviour of the IGBT can be seen.
This shows that there is a large safety margin between RBSOA limit for continous operation and
IC [kA]/module
5.3
5.4
UCE[V] & P[kW]
0 2 4 6 8 10 12 14 16 18 20
time [µs]
conditions successfully passed in single pulse
laboratory examinations.
IC[A]
20
15
10
5
0
-5
-10
-15
-20
VCE
IC
VGE
VGE[V]
VCE[kV] & IC[kA]
8
7
6
5
4
3
2
1
0
8
7
EC3
6
EC2
5
4
3
2
1
0
2 3 4 5 6 7 8 9 10 11 12 13 14 15
VF [V]
Fig. 16 On state losses of state of the art diode
compared to optimized diode up to 10 times
nominal current for same chip size in an ensemble of 12 parallel chips at Tvj=125°C
160
EC3 System
150
EC3 Module
IFSM (%)
140
EC2 Module
130
100
90
2,0
2,3
2,6
2,9
3,2
3,5
VF (V) 125°C
3,8
4,1
4,4
Fig. 17 comparison IFSM values current diode
(EC2) versus new diode (EC3) at Tvj=125°C
Simulation of Inverter Output Current
Fig. 18 shows the calculated maximum achievable output current using thermal calculations.
The simulations were performed with maximum
allowed Tvj of 125°C, Ta=40°C and a watercooled heat sink. The RMS output current is
given as a function of the switching frequency for
the conventional 600A module with planar IGBT
in comparison to the new 750A module with
trench IGBT. Both modules are of the same
housing size and footprint.
As can be seen the inverter output current is limited by the IGBT chip losses in both case. Therefore the improvement of the maximum output
current for the whole module is given by the ratio
of the respective maximum output currents for
the IGBT chips.
IGBT3
IGBT2
1000
900
new Diode
current Diode
800
700
600
500
400
300
200
400
600
800
switching frequency f [Hz]
1000
Fig. 18 Inverter output current RMS versus PWM
frequency by a thermal calculation for 6.5kV
modules using planar and trench IGBT. cos
φ=+0.85 (IGBT) and -0.85 (diode), VCE=3.6kV,
fo=50Hz, Rth(H-A)=6°K/kW
The improvement achieved is bigger than 20%
for very high switching frequencies and more
than 30% in the lower frequency regime. The usability at low frequencies is of major importance
in traction applications.
7
120
110
6
1100
max RMS current [A]
In Fig.16 the forward characteristic of the new diode with improved vertical design is compared to
the state of the art diode to about 10 times the
rated current of a module. That current regime is
relevant for surge current limitation. It can be
seen that the voltage drop is reduced by more
than 30% over the presented regime. This reduction directly corresponds to a better surge current
capability.
Furthermore the surge capability benefits from
the small VLD edge termination which enlarges
the active chip area and at the same time reduces the Rth value without changing the outer
chip dimension.
Fig. 17 proves the dramatic improvement in
surge current capability that can be reached by
an optimized vertical design and application of
the VLD concept. The strongly reduced on state
voltage leads to beyond 40% higher IFSM (forward
surge current maximum) values corresponding to
2
more than a two times increased I t value.
Conclusion
It is shown that by using trench technology and
VLD termination structure the switching robustness can be increased substantially whereas dynamic and on state losses are considerably reduced. The reliability is furthermore enhanced
due to the improvements in short circuit behaviour and the higher surge current capability.
8
[1]
[2]
[3]
[4]
Literature
J.G. Bauer, T. Duetemeyer, E. Falck, C.
Schaeffer, G. Schmidt, H. Schulze: Investigations
on 6.5kV Trench IGBT and adapted EmCon Diode, Proc. ISPSD, 2007, p. 2
J. Biermann, T. Schuetze, O. Schilling, M. Pfaffenlehner, C. Schaeffer: New 3300V Trench
IGBT Module for Highest Converter Efficiency,
Proc. PCIM, 2004, Nürnberg
J.G. Bauer, O. Schilling, C. Schaeffer, F. Hille:
Investigations on the Ruggedness Limit of 6.5kV
IGBT, Proc. ISPSD, 2005, pp. 71-74
M. Rahimo, A. Kopta, S. Linder, Novel Enhanced-Planar IGBT Technology Rated up to
6.5kV for Lower Loss and Higher SOA Capability,
Proc. ISPSD 2006, pp. 69-72