PCIM’2000 CONFERENCE PAPER
1.6 MW / 150 KHZ INVERTER FOR WELDING
APPLICATIONS
Heinz Rüedi (1) and Hans G. Matthes (2)
(1) CT-Concept Technologie AG
CH-2504 Biel (Switzerland)
E-Mail Heinz.Rueedi@CT-Concept.com
Internet www.IGBT-Driver.com
(2) Elotherm GmbH
D-42855 Remscheid (Germany)
E-Mail h-g.matthes@elotherm.de
Internet www.Elotherm.de
Abstract
High frequencies are required for the inductive welding of steel tubes, an application
in which the geometry and material properties determine the necessary powers and
frequencies. Welding thick-walled steel pipes – such as those used for pipelines – with
a high material throughput requires welding powers in the megawatt range and
technologically optimized frequencies between 100 and 150 kHz. An installation
implemented in IGBT technology with a welding power of 1600 kW at a rated
frequency of 150 kHz is described. The technological obstacles and their solutions are
presented.
Introduction
Circuits designed to generate high frequencies
have been known for many years. In the period
of reconstruction after the Second World War,
high-frequency generators in the medium power
range up to approximately 300 kW
implemented in tube technology came into
industrial use. Electron tubes are components of
high dynamic range and thus particularly well
suited for the generation of high frequencies. For
physical reasons, however, only a relatively low
anode current can be cost-effectively produced.
In order to increase the tube power and thus the
generator output, anode voltages of up to 15 kV
are common. For this reason, the energy must be
decoupled by complex methods via coupling
capacitors and transformers. The optimum
operating frequencies for a vacuum-tube
generator are between 400 and 600 kHz.
Lower frequencies can only be achieved with
additional expenditures and at the cost of higher
losses.
The serious drawbacks of tube generators are
their relatively low efficiency and the limited
operating life of the electron tube due to aging.
An overall efficiency of 60% is rarely exceeded
between the high-frequency output and the
power terminal, especially at high generator
powers. Even after the initially used glass tube
was replaced by a ceramic tube, an operating
life of 5,000 to 6,000 hours could not be
significantly increased. A further disadvantage is
that the problems occurring in parallel operation
of electron tubes and tube generators make it
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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f1
f2
inverter
power bus
resonant circuit
f1
f2
Fig. 1 Block diagram of the installation with two frequency converters each of 800 kW
almost impossible to generate the powers
required for larger installations.
A concept has now been developed for the first
time to replace an existing high-frequency feed
from a tube generator with a simultaneous rise in
welding power. It makes use of two resonantcircuit frequency converters each of 800 kW
operated in parallel at 150 kHz. IGBTs are used
in the inverters of these frequency converters.
producing a total output of 1600 kW. The
frequency converters feed the RF energy into a
series-compensated resonant circuit.
Figure 2 shows the works photo of one of the
two
parallel-operated
800-kW/150-kHz
frequency converters. Each of the three inverter
cabinets contains four parallel-connected full
bridges. An 800-kW converter thus consists of
twelve parallel-connected H-bridges.
Figure 1 shows the block diagram of the
installation. It can be operated with one 800kW frequency converter or with both converters
Fig. 2. Works photo of an 800-kW frequency converter
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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The resonant converter
Technological obstacles
The link circuit capacitors are charged to a DC
voltage of U,+ = 900 V via a fully driven thyristor
rectifier and a charging choke. A square-wave
voltage is generated at the output terminals by
alternate driving of the IGBTs (see Fig. 4). Only
sinusoidal currents and voltages occur in the
series-compensated load resonant circuit.
The implementation of such a state-of-the-art
system represents significant challenges. Thus
ELOTHERM has developed a new power bus
which conducts the high-frequency energy over
a distance of 35m from the generator to the load
resonant circuit as well as a new series resonant
circuit. Details are described in /1/ and /2/.
Last
L
L
Fig. 3 Circuit diagram of an 800-kW
frequency converter (principle)
Fig. 4 Generator output voltage and current
Particular attention must be paid to the driving of
the IGBTs. The drivers used for this purpose must
not only be able to switch on and off with
sufficient speed in normal operation. They must
also be able to detect dangerous switching
processes in order to protect the IGBT module
reliably from damage by quickly selecting
suitable turn-on and turn-off variants.
This paper will examine the problems and
possible solutions associated with the design of
an inverter for such a system:
Power semiconductors
Although power MOSFETs are in principle
suitable for high frequencies, they are only
available in relatively small packages so that the
output power of 1.6 MW required in this case
could only be achieved via an extreme parallel
circuit
comprising
hundreds
of
these
components. In addition, a favorable priceperformance ratio can be achieved with power
MOSFETs only at low dc-link voltages. For this
application, this would mean extremely high
inverter
output
currents
and
matching
transformers at the output. A power MOSFET
solution is thus extremely complex in design
terms and consequently expensive.
As against this, IGBT modules are available for
the desired output voltage. A large number of
high-power modules is also on the market,
which in principle reduce the number of
components needed to be connected in parallel.
However, IGBTs also have drawbacks in such an
application which should not be underestimated. Because of the switching losses,
IGBTs in hard-driven applications in the
megawatt range are as a rule switched only at
frequencies of a few kHz (see /3/, /4/, /5/
and /6/). In resonance applications, the
switching losses can be reduced, but the design
and layout of IGBT modules available as
standard limit the usable switching frequency to
several 10 kHz.
The gate resistors they contain represent an
obstacle to using standard IGBT modules for
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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high-frequency applications. The module
manufacturers build them in to symmetrize the
internal parallel-connected IGBT chips. In
practice, the switching speeds of such modules
are then in the region of several microseconds,
sufficient for most drive applications at a few
kHz. However, this solution is no longer
sustainable in an application with a periodic
duration of 6 µs. In addition, such a high power
dissipation occurs in gate resistors driven at 150
kHz that they are thermally overstressed and
simply burn out.
An IGBT module was thus developed in
conjunction with a semiconductor manufacturer
which no longer contains any gate resistors in
the module and has an improved thermal
resistance
modules.
(RthJC)
compared
with
standard
Although this means that an IGBT module
suitable for high-frequency applications is now
available, entirely new problems occur which
are quite unknown in standard modules:
no overshoot
0V
Vge = 5V/Div
Ig = 31A
no overshoot
Ig = 5A/Div
Gate inductance
The module used has a gate inductance of about
50 nH. An oscillating circuit is produced
because of the input capacitance in conjunction
with IGBT gate resistances reduced to values
close to zero. Measurements and simulations
have shown that the gate voltage suffers from
overshoots with impermissible values of 30V to
40V. To solve this problem, a carefully matched
driver output stage was developed which
exhibits hardly any overshoots at a gate current
of 30 A per IGBT module despite the high
switching speed (see Fig. 5)
0A
Fig. 5 Gate voltage and current
Reactances in the power path
S2
Load
• Gate inductance
• Reactances in the power path
• Short-circuit characteristic
These problems will be explained in detail
below:
x=100 ns/Div
OSC3444
LL
RL
CL
S1
.
Fig. 6 Test circuit of half-bridge with
resonance load
Figure 6 shows the test circuit with a half-bridge
and a resonance load with the effective
reactances in the half-bridge. Together with the
line
inductances,
the
collector-emitter
capacitances of the IGBTs form a series resonant
circuit which becomes a problem when the
voltage is very “hard-switched”. This will be
illustrated in the following example: let us
assume that the link circuit is charged to 900V
and the voltage across IGBT S2 is close to zero.
A voltage of 900 V thus exists across IGBT S1. If
S1 is now turned on quickly, a sinusoidal current
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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Fig. 7 Simulation of the turn-on characteristic
begins to flow through the load and a highfrequency current additionally flows within the
half-bridge itself (in this case approximately 8
MHz). This practically doubles the voltage
across S1 at periodic intervals (see simulation in
Fig. 7).
As 1200-V IGBTs are used in this application,
this operating case is impermissible and leads to
the immediate destruction of the IGBT. The same
problem is not actually encountered in normal
resonance operation because switching is
always performed at zero voltage, but it may
certainly occur as a result of load phase jumps,
triggering of the drive circuit at the wrong time
or when the installation is started up. A
watchdog circuit in the driver electronics thus
continuously monitors the collector voltage and
only allows “hard” turn-on when the collector
voltage is less than half the nominal voltage. At
higher collector voltages, the IGBT is turned on
more slowly and with losses – but reliably.
Turn-off at high collector current
and short circuit
Another problem occurs if the IGBT is turned off
when a high collector current flows or even
when a short circuit has to be turned off. As a
result of the high switching transients resulting
from a hard turn-off, such a high voltage occurs
at the stray inductance’s that the IGBTs are
destroyed. Moreover, it should be noted that the
IGBTs no longer have any guaranteed shortcircuit characteristic because the modules now
dispense with gate resistors. For this reason, the
driver circuits include a fast current-measuring
device which triggers a slow turn-off of the
IGBTs in the event of a fault. In normal
resonance operation, this presents no problem
either because the turn off occurs as the current
drops to zero.
In order to reliably manage all the problems
described above, CONCEPT has developed
special drivers for this application. But they also
contain a series of additional measurement and
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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Vce monitoring
24V=
DC/DC
Supply
monitoring
Ic monitoring
Driver logic
Programmable
driver
Fiber-optic
interface
Vge monitoring
Fig. 8 Block diagram of a driver
diagnostic functions. All logic functions are
contained in a logic component (FPGA). Every
individual driver has the following local
monitoring and control functions:
• Monitoring the fiber-optic links
Fig. 9
•
•
•
•
•
•
Signal integrity check
Collector voltage monitoring
Collector current monitoring
Gate voltage monitoring
Supply voltage monitoring
Operating mode (moment of switching)
Driver card (in front) and interface card for 48 driver cards (behind)
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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Because twelve parallel-connected H-bridges
are used for each 800-kW inverter, 48 IGBTs
and 48 drivers are required for a unit of this
kind. In order to distribute these signals among
this number of drivers and to evaluate the status,
diagnostic and error information relating to all
48 drivers, CONCEPT has developed a special
card which performs these functions and forms a
simple logic interface to the installation
controller. The interface card is shown in the
background and a driver card in the foreground
in Fig. 9.
The effort required merely for controlling and
protecting the IGBTs for the 1.6-MW installation
can be seen from the following summary:
are also maintenance free, whereas the tubes
had to be regularly replaced.
Further advances in IGBTs will allow the
installation concept and all subsystems
developed for it to achieve a power increase
with the same number of components or to make
the same power available at higher output
frequencies.
References
/1/
H. G. Matthes, R. Jürgens:1.6 MW 150
kHz Series Resonant Circuit
Converterincorporating IGBT Devices for
Welding Applications, IHS '98 Induction Heating Seminar, University of
Padua, May 1998 (Note 1)
/2/
H. G. Matthes, R. Jürgens: HFRohrschweißen mit IGBT
Reihenschwingkreisumrichter,
elektrowärme international, Dezember
1998, pp. B159 - B 162. (Note 1)
/3/
H. Rüedi, P. Köhli: Dynamic Gate
Controller – A new IGBT gate unit for high
current / high voltage IGBT modules,
PCIM Nürnberg 1995, pp. 241-249,
(Note 2)
/4/
H. Rüedi, P. Köhli: SCALE Driver for High
Voltage IGBTs, PCIM Nürnberg 1999,
pp. 357-363, (Note 2)
/5/
H. Rüedi, P. Köhli: New drivers feature
active clamping, Power Electronics Europe
1/2000, pp. 32-36. (Note 2)
/6/
H. Rüedi, P. Köhli: HV-IGBT driver
includes active clamping function, PCIM
Europe 3/2000, pp. 12-14. (Note 2)
• 96 drivers and two driver interface cards
• A total of 4,000W (!) drive power for the
gates
• 192 fiber optic links with total 384 fiberoptic components
• A total of 102 FPGAs with a total complexity
of 216,000 gates
The signal delay trough the whole system is very
short. There are essentially three components
which determine the transit time from installation
controller to power section: fiber-optic link (140
ns), driver end-stage (130 ns) and IGBTs (170
ns). All protection functions operate in quasireal time.
Summary and outlook
High frequencies are required for inductive and
conductive
welding.
An
resonant-circuit
frequency converter has been developed in IGBT
technology for the MW-range at 150 kHz.
Several such units have been installed at
customers’ premises and operate to the latter’s
full satisfaction. Studies have shown that the new
installations offer an efficiency advantage of
between 10 and 15 percent compared with the
previously used vacuum-tube generators. They
Note 1: For a copy please contact ELOTHERM
Note 2: Paper is also available on the internet:
www.igbt-driver.com
POWER CONVERSION CONFERENCE PCIM’2000 • JUNE 2000 • NÜRNBERG, GERMANY
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