U.S. patent application number 09/816251 was filed with the patent office on 2001-11-29 for active overvoltage protection apparatus for a bidirectional power switch in common collector mode.
This patent application is currently assigned to SIEMENS AG. Invention is credited to Bruckmann, Manfred, Springmann, Walter.
Application Number | 20010046143 09/816251 |
Document ID | / |
Family ID | 7636181 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046143 |
Kind Code |
A1 |
Bruckmann, Manfred ; et
al. |
November 29, 2001 |
Active overvoltage protection apparatus for a bidirectional power
switch in common collector mode
Abstract
The present invention relates to an active overvoltage
protection apparatus for a bidirectional power switch which has two
back-to-back in series connected semiconductor switches in the
"common collector mode" topology. The overvoltage protection
apparatus has a diode network which is linked to gate and emitter
connections of the bidirectional power switch in the in such a
manner as to provide a voltage clamping circuit for each of the
semiconductor switches in the bidirectional power switch.
Inventors: |
Bruckmann, Manfred;
(Nuernberg, DE) ; Springmann, Walter; (Adelsdorf,
DE) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
44TH FLOOR
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112-4498
US
|
Assignee: |
SIEMENS AG
|
Family ID: |
7636181 |
Appl. No.: |
09/816251 |
Filed: |
March 23, 2001 |
Current U.S.
Class: |
363/56.05 |
Current CPC
Class: |
H03K 17/0828
20130101 |
Class at
Publication: |
363/56.05 |
International
Class: |
H02H 007/122 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2000 |
DE |
100 146 41.4 |
Claims
1. An active overvoltage protection apparatus for a bidirectional
power switch having two back-to-back in series connected
semiconductor switches and gate and emitter connections to provide
a "common collector mode" topology, comprising a diode network
which is linked to the gate and emitter connections to provide a
voltage clamping circuit for each of the two semiconductor switches
.
2. The apparatus according to claim 1 wherein the diode network has
two zener diodes and two decoupling diodes, the zener diodes each
being linked on the cathode side to an emitter connection, and the
decoupling diodes each being linked on the cathode side to a
corresponding one of the gate connections and wherein the zener
diodes and decoupling diodes are linked on the anode side in such a
manner that one of the emitter connections of one of the
semiconductor switches is in each case connected to the gate
connection of the other semiconductor switch.
3. The apparatus according to claim 1 wherein the diode network has
two zener diodes and two decoupling diodes, the zener diodes being
connected to one another on the anode side and each being connected
on the cathode side to a corresponding one of the gate connections
of the bidirectional power switch, the decoupling diodes each being
linked on the cathode side to a corresponding one of the gate
connections of the bidirectional power switch and each being linked
on the anode side to an emitter connection of the bidirectional
power switch.
4. The apparatus according to claim 3, comprising a driver stage
connected between the diode network and the gate connections of the
bidirectional power switch in such a manner that the junction on
the cathode side of each of the zener diodes and the decoupling
diodes in the diode network is linked to an input of the driver
stage.
5. The apparatus according to claim 1, wherein the diode network
has two decoupling diodes and a zener diode connected in a diode
bridge, and each decoupling diode is linked on the anode side to
one of the emitter connections, and on the cathode side to one of
the gate connections of the bidirectional power switch and further
wherein the zener diode is connected on the anode side to anode
connections of two diodes in the diode bridge, and on the cathode
side to cathode connections on two additional diodes in the diode
bridge and wherein the free ends of the diode bridge are linked to
a corresponding one of the gate connections of the bidirectional
power switch (2).
6. The apparatus according to claim 1 wherein the diode network has
a zener diode and four diodes, the zener diode being linked on the
anode side to the anode connections a first two of the diodes which
are each connected on the cathode side to one of the gate
connections of the bidirectional power switch and wherein the zener
diode is linked on the cathode side with the cathode connections of
a second two of the diodes which are each connected on the anode
side to one of the emitter connections of the bidirectional power
switch.
7. The apparatus according to claim 2 comprising two bidirectional
transit diodes and decoupling diodes wherein each bidirectional
transit diode is linked to the emitter connection of one of the
semiconductor switches and to the gate connection of the other of
the semiconductor switches in the bidirectional power switch.
8. The apparatus according to claim 1 wherein the diode network
comprises two zener diodes having a low breakdown voltage.
9. The apparatus according to as claim 1 wherein the diode network
comprises a bidirectional transit diode.
10. An active overvoltage protection apparatus for a bidirectional
power switch having two back-to-back in series connected
semiconductor switches in the "common collector mode" topology,
comprising a voltage measurement device, voltage comparison devices
and two decoupling diodes wherein the voltage measurement device is
connected upstream of, and electrically in parallel with, the
bidirectional power switch and wherein each voltage comparison
device has an input side with a voltage actual value and a
predetermined voltage reference value, wherein each decoupling
diode is linked on the anode side to an output of one of the
voltage comparison devices and on the cathode side to an input of a
driver stage of the semiconductor switches.
Description
BACKGROUND OF THE INVENTION
[0001] A bidirectional power switch of the type described above is
known from the publication "Novel Solutions for Protection of
Matrix Converter to Three Phase Induction Machine", printed in the
Proceedings of the "IEEE Industry Applications Society Annual
Meeting", New Orleans, La., Oct. 5-9, 1997, pages 1447 to 1454. The
circuit diagram of a such bidirectional switch in the "common
collector mode" topology is illustrated in more detail in FIG. 1.
For comparison, FIG. 2 shows a bidirectional power switch in the
"common emitter mode" topology, which is likewise known from the
aforementione mentioned publication. These two bidirectional power
switches each have two semiconductor switches 4 and 6, which are
connected back-toback in series. In FIG. 1, these two semiconductor
switches 4 and 6 are connected back-to-back in series in such a
manner that the two collector connections are electrically
conductively connected to one another. In FIG. 2, the circuit
formed by the two semiconductor switches 4 and 6 are connected
back-to-back in series in such a manner that their emitter
connections are electrically conductively connected. Since the
emitter connections are linked, this circuitry is referred to as
the common emitter mode. Insulated gate bipolar transistors (IGBT)
are used as the semiconductor switches 4 and 6, and each have a
reverse diode. The internal topology can be seen at the accessible
connections of the bidirectional power switch 2. In the
bidirectional power switch with the "common collector mode"
topology shown in FIG. 1, the connections E1, E2, G1 and G2 are
accessible on the power switch 2. In contrast to this, in the
bidirectional power switch 2 with the "common emitter mode"
topology shown in FIG. 2, the connections C1, C2, G1 and G2 are
accessible.
[0002] According to the aforementioned publication, bidirectional
power switches are used in a matrix converter. A matrix converter
is a self-commutating direct converter. This self-commutating
direct converter is a voltage intermediate-circuit converter
without an intermediate circuit. The bidirectional power switches
are arranged in a 3.times.3 switch matrix in the matrix converter.
This arrangement of the bidirectional power switches results in
three input phases being connected to three output phases. The
actuation of the semiconductor switches or of the semiconductor
switch in the power switches of the 3.times.3 switch matrix in each
case results in a current path being connected in a bidirectional
manner, that is to say from the input to the output and vice versa.
One phase of the matrix converter is an arrangement of three
bidirectional power switches, which produces a connection from the
three mains system phases to one output phase. An LC filter is also
connected to the input connections of the matrix converter, and is
linked on the input side to a three-phase mains system. This LC
filter, which is also referred to as an input filter, isolates
pulse-frequency oscillations from the mains system. The size of
this filter depends on the pulse frequency of the matrix converter.
This self-commutating direct converter offers the advantage that,
by virtue of the topology, it can feed back into the mains system,
and produces sinusoidal mains system currents by means of an
appropriately designed control system.
[0003] In addition to the already mentioned embodiments of the
bidirectional power switch, there is also a further embodiment,
which can likewise be found in the aforementioned cited
publication. This embodiment is a semiconductor switch which is
integrated in a diode bridge.
[0004] Since the matrix converter has no passive freewheeling
circuits such as a voltage intermediate-circuit converter, high
reverse voltages occur across the semiconductor switches owing to
the inductances which are present in the circuit, particularly in
the case of pulse blocking generated on the basis of an
EMERGENCY-OFF (switching off the actuation pulses of all the
semiconductor switches). These overvoltages can also occur as a
consequence of an incorrectly initiated commutation sequence or due
to failure of the actuation of bidirectional power switches. The
output circuit is always interrupted in these situations. The
interruption in the output circuit in conjunction with the
inductances which are present in the circuit causes the
overvoltages, which may result in destruction of the semiconductor
switches.
[0005] An overvoltage protection apparatus is known from the
aforementioned publication "Novel Solutions for Protection of
Matrix Converter to Three Phase Induction Machine". This
overvoltage protection apparatus has two 6-pulse diode bridges
which are linked to one another on the DC-voltage side by means of
a capacitor. On the AC-voltage side, the one 6-pulse diode bridge
is connected to the input connections of the matrix converter. The
other diode bridge is connected on the AC-voltage side to the
output connections of the matrix converter. A resistor is connected
electrically in parallel with the capacitor, and it discharges said
capacitor. An LC filter is also connected to the input connections
of the matrix converter, and is linked on the input side to a
3-phase mains system. This LC filter, which is also referred to as
an input filter, isolates pulse-frequency oscillations from the
mains system. The size of this filter depends on the pulse
frequency of the matrix converter.
[0006] Any overvoltages which occur are rectified by the diode
bridges, and are passed to the capacitor. This capacitor thus
absorbs the commutation energy. This overvoltage protection
apparatus, which is also the subject matter of U.S. Pat. No.
4,697,230, requires an initial charging circuit for the capacitor.
This initial charging circuit is required to prevent overvoltages
at twice the mains voltage from occurring when the matrix converter
is switched on. Without initial charging, high peak currents
likewise occur, which must be carried by the diodes in the diode
bridge. The resistor is designed such that a predetermined amount
of energy is discharged from the capacitor. This amount of energy
depends on a predetermined difference between the mains system
voltage and the capacitor voltage.
[0007] An overvoltage protection apparatus which has two 6-pulse
diode bridges is also known from the publication "Performance of a
two Steps Commutated Matrix Converter for AC-Variable-Speed
Drives", printed in the EPE '99 Proceedings, Lausanne, September
1999, pages 1 to 9. Each of these two diode bridges has a capacitor
on the DC-voltage side. These two capacitors are electrically
connected in parallel. A zener diode and a pulse resistor are
electrically connected in parallel with these two capacitors and
are used to limit the voltage across the capacitors to a
predetermined value. Furthermore, each bidirectional power switch
has a varistor and a back-to-back series connected zener diode, by
means of which the overvoltages across the bidirectional power
switch are limited.
[0008] In the publication "A Matrix Converter without Reactive
Clamp Elements for an Induction Motor Drive System", by Axel
Schuster, printed in IEEE, 1998, pages 714 to 720, a number of
varistors are provided as the overvoltage protection apparatus. A
varistor is connected electrically in parallel with each
semiconductor switch with each bidirectional power switch in the
3.times.3 switch matrix. These 18 varistors protect the 18
semiconductor switches of the nine bidirectional power switches
against overvoltages.
[0009] When this overvoltage protection device is being used, the
connection point of the two collector connections of the two
back-to-back series connected semiconductor switches must be
connected to the exterior in the bidirectional power switches in
the common collector mode. It is also possible for the
bidirectional power switch to be formed from individual
semiconductor components. A varistor can be electrically connected
in parallel with each semiconductor switch in a bidirectional power
switch only if the collector connections, or their junction point,
are or is accessible.
[0010] In the publication "Theory and Design of a 30-hp Matrix
Converter", printed in IEEE Transactions on Industry Applications",
Volume 28, Issue 3, May/June 1992, pages 546 to 551, an RCD circuit
for the transistor which is integrated in a diode bridge is used as
the overvoltage protection apparatus for a bidirectional power
switch. The energy which is fed into the capacitor in the RCD
circuit is normally converted into heat in the resistor in the RCD
circuit. This RCD circuit is also referred to as a snubber circuit.
The stored energy can also be used to supply energy for actuation
of the semiconductor switches. This over-voltage protection
apparatus is less suitable for a bidirectional power switch as
shown in FIG. 1 or 2. Furthermore, this overvoltage protection
apparatus requires physical volume, with the magnitude of this
physical volume being dependent on the commutation energy.
[0011] A voltage clamping circuit is disclosed in the publication
"Beschaltung von SIPMOS-Transistoren" [Circuit of SIPMOS
Transistors], printed in "Siemens Components", Volume 22, Issue 4,
1984, pages 157 to 159. This voltage clamping circuit is shown in
more detail in FIG. 3 for a semiconductor switch 4, and is denoted
by 8. This voltage clamping circuit 8 comprises a zener diode 10,
in particular a high-voltage zener diode, which is also referred to
as a transit diode, and a decoupling diode 12. This voltage
clamping circuit 8 is connected between the collector connection C
and the gate connection G of the semiconductor switch 4. The
semiconductor switch 4 is an insulated gate bipolar transistor
(IGBT) with a reverse diode. The decoupling diode 12 disconnects
the voltage clamping circuit 8 from the gate connection G of the
semiconductor switch 4 when the semiconductor switch 4 is switched
on. When the semiconductor switch 4 is in the switched-off state,
the semiconductor switch 4 is actuated automatically as soon as its
collector-emitter voltage exceeds the sum of the zener voltage of
the transit diode 10, the threshold voltage of the decoupling diode
12 and the gate-emitter threshold voltage. Any overvoltage which
occurs across the semiconductor switch 4 is thus actively limited
by this semiconductor switch 4, although losses occur in the
semiconductor switch it and in the transit diode 10.
[0012] This active overvoltage protection apparatus can be used
directly for a bidirectional power switch in the "common emitter
mode" topology (FIG. 2). This means that a voltage clamping circuit
8 is electrically connected in parallel with the collector-gate
path in each of the two semiconductor switches 4 and 6 in the
bidirectional power switch 2 in the common emitter mode. This can
also be achieved without any major complexity since the required
connections, the collector connection C and the gate connection G,
are accessible.
[0013] This known voltage clamping circuit 8 cannot be used
directly in a bidirectional power switch in the common collector
mode. For circuit 8 to be used, the common collector connection
must be connected to the exterior from the bidirectional power
switch 2. This demands a special form of the bidirectional power
switch, which may need to be produced on a customer-specific basis.
Any deviation from standard components increases the price of a
product on the market.
SUMMARY OF THE INVENTION
[0014] The present invention is based on the object of specifying
an active overvoltage protection apparatus for a bidirectional
power switch in the common collector mode. Since the bidirectional
power switch in the common collector mode has an associated diode
network which is linked to the accessible gate and emitter
connections of the bidirectional power switch in such a manner that
the function of a voltage clamping circuit is provided for each of
its two semiconductor switches, any overvoltage which occurs is no
longer absorbed by the overvoltage protection apparatus, but is
actively limited by one of the two semiconductor switches in the
bidirectional power switch. The polarity of the reverse voltage
applied to the bidirectional power switch determines which of the
two semiconductor switches in the bidirectional power switch will
be automatically activated. The diode network as a circuit in a
bidirectional power switch in the common collector mode results in
an active overvoltage protection apparatus for such a power
switch.
[0015] In another embodiment of the present invention, the
overvoltage protection apparatus has a voltage measurement
apparatus, two voltage comparison devices and two decoupling
diodes. The voltage measurement apparatus is electrically connected
to the emitter connections of the bidirectional power switch in the
common collector mode, and its two outputs are each linked to an
actual input of the two voltage comparison devices. On the output
side, each voltage comparison device is linked by means of the
decoupling diode and by means of a driver stage to a gate
connection of the bidirectional power switch. A predetermined
reference value is applied to the reference value input of each
voltage comparison device. If the actual value which is determined
is greater than or equal to the predetermined reference value, the
associated semiconductor switch in the bidirectional power switch
in the common collector mode is actuated, so that this
semiconductor switch can actively limit the overvoltage which
occurs. In a preferred embodiment , a driver stage is connected
between each of the junction points of the series circuits and the
gate connections of the bidirectional power switch in the common
collector mode. This greatly reduces the current load on each of
the zener diodes.
[0016] One embodiment of the diode network, has two zener diodes
and two decoupling diodes. one zener diode and one decoupling diode
are in each case electrically conductively connected to one another
on the anode side. Each of these series circuits is linked to a
gate connection and to an emitter connection of in each case one
semiconductor switch in the bidirectional power switch. Each series
circuit thus forms a known voltage clamping circuit, although this
is not electrically connected to the collector-gate path of a
semiconductor switch. These two voltage clamping circuits are
connected, crossed over, to the accessible connections of the
bidirectional power switch n the common collector mode.
[0017] In another embodiment of the diode network, the network
likewise has two zener diodes and two decoupling diodes, with one
zener diode and one decoupling diode each being linked to one
another on the cathode side. These junction points are each
connected to a gate connection of the bidirectional power switch.
These two series circuits are electrically linked to one another in
such a manner that the two zener diodes are electrically
conductively connected on the anode side. The associated decoupling
diodes are each connected on the anode side to an emitter
connection of the bidirectional power switch. In contrast to the
first embodiment, the decoupling diodes need not be high blocking
capability diodes.
[0018] In a further embodiment of the diode network, the network
has two decoupling diodes and one zener diode, which is integrated
in a diode bridge. The free connections of this diode bridge are
electrically conductively connected to the gate connections of the
bidirectional power switch in the common collector mode. The
decoupling diodes are each connected electrically in parallel, but
in the reverse direction, to a gate-emitter path of a semiconductor
switch in the bidirectional power switch in the common collector
mode. This refinement of the diode network now requires only one
zener diode, in particular a high blocking capability zener diode,
thus significantly reducing the complexity and the costs of the
active overvoltage protection apparatus.
[0019] In yet another embodiment of the diode network, the network
has four diodes and one zener diode. On the anode side, this zener
diode is linked to the anodes of the two diodes which, for their
part, are connected to the gate connections of the bidirectional
power switch. On the cathode side, the zener diode is connected to
the cathodes of two diodes which, for their part, are linked to the
emitter connections of the bidirectional power switch. In this
embodiment of the diode network not only is one high blocking
capability zener diode saved, but also two decoupling diodes. The
complexity for an active overvoltage protection apparatus for a
bidirectional power switch in the common collector mode has thus
been further simplified.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention is further disclosed with reference to
the drawings, which show a number of embodiments of an active
overvoltage protection apparatus according to the invention,
schematically.
[0021] FIG. 1 illustrates a circuit diagram of a known
bidirectional power switch in the common collector mode.
[0022] FIG. 2 illustrates a circuit diagram of a known
bidirectional power switch in the common emiter mode.
[0023] FIG. 3 illustrates a known voltage clamping circuit for a
semiconductor switch.
[0024] FIG. 4 illustrates a first embodiment of the diode network
of the active overvoltage protection apparatus according to the
present invention.
[0025] FIG. 5 illustrates an advantageous variant of the diode
network as shown in FIG. 4.
[0026] FIG. 6 illustrates a second embodiment of the diode network
of the active overvoltage protection apparatus according to the
present invention.
[0027] FIGS. 7-9 illustrates advantageous variants of the
embodiment shown in FIG. 6.
[0028] FIG. 10 illustrates a third embodiment of the (diode network
of the active overvoltage protection apparatus according to the
present invention.
[0029] FIG. 11 illustrates a fourth embodiment of the diode network
of the active overvoltage protection apparatus according to the
invention.
[0030] FIG. 12 illustrates yet a further embodiment of the
overvoltage protection apparatus according to the invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0031] FIG. 4 shows a first embodiment of the diode network 14 of
the active overvoltage protection apparatus according to the
invention. This diode network 14 has two zener diodes 16 and 18 and
two decoupling diodes 20 and 22. The zener diode 16 is electrically
conductively connected on the anode side to the anode of the
decoupling diode 20. The zener diode 18 and the decoupling diode 22
are likewise electrically conductively connected to one another on
the anode side. The two series circuits 24 and 26 are electrically
conductively linked, crossed over, to the accessible connections
E1, E2, G1, G2 of the bidirectional power switch 2. This means that
the series circuit 24 connects the connection E1 to the connection
G2 of the bidirectional power switch 2, with the series circuit 26
connecting the connection E2 to the connection G1.
[0032] If an overvoltage occurs when this bidirectional power
switch 2 in the common collector mode is in the switched-off state,
that is to say the two back-to-back series connected semiconductor
switches 4 and 6 are switched off, either of the semiconductor
switches 4 or 6 in the bidirectional power switch 2 is actuated as
soon as the value of the overvoltage exceeds the sum value of the
zener voltage of the zener diode 16 or 18, the threshold voltage of
the decoupling diode 20 or 22 and the gate-emitter threshold
voltage of the semiconductor switch 6 or 4. The polarity of the
reverse voltage which is present across the bidirectional power
switch 2 determines which of the semiconductor switches in the
bidirectional power switch 2 is actuated. Any reverse voltage which
is present across the bidirectional power switch 2 in the common
collector mode is thus actively limited to a predetermined value.
It is thus no longer possible for any overvoltage to occur across
the bidirectional power switch 2 in the common collector mode. The
diode network 14, which in this embodiment has high blocking
capability diodes, is integrated in the two drive circuits. It thus
does not occupy any additional physical volume.
[0033] FIG. 5 illustrates an advantageous variant of the diode
network 14 of the active overvoltage protection apparatus. A
bidirectional high blocking capability zener diode 28 or 30,
respectively, is in each case provided instead of the diodes 16, 20
and 18, 22 in the series circuits 24 and 26. This bidirectional
high blocking capability zener diode 28 or 30 is also referred to
as a bidirectional transit diode. Since a bidirectional high
blocking capability zener diode 28 or 30, respectively, is in each
case used instead of the two diodes 16, 20 or 18, 22, respectively,
this saves installation space.
[0034] FIG. 6 shows a second embodiment of the diode network 14 of
the active overvoltage protection apparatus. This diode network 14
likewise has two high blocking capability zener diodes 16 and 18,
which are also referred to as transit diodes, and two low blocking
capability decoupling diodes 32 and 34. The two transit diodes 16
and 18 are connected back-to-back in series between the two
accessible connections G1 and G2 of the bidirectional power switch
2 in the common collector mode. The low blocking capability
decoupling diode 32 is connected electrically in parallel, in the
reverse direction, to the connections G1 and E1 while, in contrast,
the low blocking capability decoupling diode 34 is connected
electrically in parallel, in the reverse direction, to the
connections G2 and E2.
[0035] In accordance with FIG. 6, an over-voltage which is relevant
for the semiconductor switch 6 occurs at the connections E1, E2 on
the bidirectional power switch 2 in the common collector mode, that
is to say the collector of the semiconductor switch 6 is positive
with respect to the emitter. When an overvoltage such as this
occurs, the decoupling diodes 32 and the transit diode 18 are
polarized in the forward direction, and the transit diode 16 and
the decoupling diode 34 are polarized in the reverse direction. In
the event of a fault, this voltage increases to a value at which
the transit diode 16 breaks down (corresponding to its zener
voltage) and current passes through thus charging the gate of the
semiconductor switch 6. If the control voltage is sufficiently
high, the semiconductor 6 enters the active area and in consequence
limits any further voltage rise. The circuit operates in an
analogous manner in the event of an overvoltage which is relevant
for the semiconductor switch 4 (collector positive with respect to
the emitter of the semiconductor switch 4).
[0036] FIG. 7 shows a variant of the diode network 14 (as shown in
FIG. 6) of the active overvoltage protection apparatus for a
bidirectional power switch 2 in the common collector mode, in more
detail. This variant differs from the embodiment shown in FIG. 6 in
that the cathode connections 36 and 38 of the diodes 16, 32 and of
the diodes 18, 34 are not connected directly to the gate
connections Gi and G2 of the bidirectional power switch 2, but are
each connected to one input of a driver stage 40 and 42 of the
semiconductor switches 4 and 6 of the bidirectional power switch 2.
Each driver stage 40, 42 is a component of an actuation device
(which is not shown in any more detail) of the respective
semiconductor switch 4 or 6. The only change is the current load on
the diodes 16, 18, 32 and 34 in the diode network 14. Since, in
this variant of the diode network 14, the diodes 32, 16 and 34, 18
do not have to carry the gate current, components with lower design
ratings can be used, and their cost is less. The required gate
current is generated by the driver stage 40 or 42,
respectively.
[0037] FIG. 8 shows another variant of the of the diode network 14
(as shown in FIG. 6) of the active overvoltage protection apparatus
of the bidirectional power switch 2 in the common collector mode.
This variant differs from what is shown in FIG. 6 in that zener
diodes 44 and 46 having a lower zener voltage, for example 16 V,
are respectively used instead of the low blocking capability
decoupling diodes 32 and 34. Since a zener diode 44 or 46 is
respectively used as the decoupling diode 32 or 34, the decoupling
diode at the same time carries out a second function. This second
function is to limit the gate-emitter voltage of the semiconductor
switch 4 or 6 to a predetermined value.
[0038] FIG. 9 shows another variant of the diode network 14 (as
shown in FIG. 6) of the active overvoltage protection apparatus in
which, in contrast to the variant shown in FIG. 8, a bidirectional
transit diode 46 is used instead of the two back-to-back series
connected transit diodes 16 and 18 between the two connections G1
and G2 of the bidirectional power switch 2. As in FIG. 7, this
diode network 14 is arranged upstream of the driver stages 40 and
42 of the semiconductor switches 4 and 6 of the bidirectional power
switch 2. However, as in FIGS. 6 and 8, this variant of the diode
network 14 can also be connected directly to the accessible
connections G1, G2, E1, E2 of the bidirectional power switch 2 in
the common collector mode.
[0039] FIG. 10 illustrates a third embodiment of the diode network
14 of the active overvoltage protection apparatus for a
bidirectional power switch 2 in the common collector mode. This
diode network 14 has two decoupling diodes 32 and 34 and a transit
diode 16 which is embedded in a diode bridge 48. The free ends of
this diode bridge 48 are electrically conductively connected to the
connections G1 and G2 of the bidirectional power switches 2. The
circuitry of the decoupling diodes 32 and 34 is identical to that
of the decoupling diodes 32 and 34 in the second embodiment of the
diode network 14 shown in FIG. 6. The difference from the
configuration shown in FIG. 6 is that one transit diode 16, which
is embedded in a diode bridge 48, is used instead of two
back-to-back series connected transit diodes 16, 18. In this case,
the anode side of the transit diode 16 is linked to the anodes of
two diodes 50, 52 in the diode bridge 48 which, for their part, are
connected on the cathode side to the connections Gi and G2 of the
bidirectional power switch 2. On the cathode side, the transit
diode 16 is connected to the cathodes of two further diodes 54, 56
in the diode bridge 48 which, for their part, are linked on the
anode side to the connections Gi and G2 of the bidirectional power
switch 2. This embodiment of the diode network 14 has the smallest
number of high-voltage zener diodes.
[0040] FIG. 11 illustrates a fourth embodiment of the diode network
14 of the active overvoltage protection apparatus for a
bidirectional power switch 2 in the common collector mode. In
contrast to the embodiment shown in FIG. 10, this embodiment no
longer has any decoupling diodes 32 and 34. To this end, the
further diodes 54 and 56 in the diode bridge 48 are now
electrically conductively connected on the anode side to the
connections E1 and E2 of the bidirectional power switch 2, rather
than to the connections G1 and G2. The number of diodes in the
diode network 14 has thus been further reduced, with only one
transit diode 16 being required for a bidirectional power switch 2
in the common collector mode.
[0041] Instead of the diode network 14, the active overvoltage
protection apparatus of the bidirectional power switch 2 in the
common collector mode may also have a voltage measurement device
58, two voltage comparison devices 60 and 62, and two decoupling
diodes 64 and 66 as shown in FIG. 12. The voltage measurement
device 58 comprises three resistors 68, 70, 72 which are
electrically connected in series, and of which only the resistor 70
is in the form of a high-voltage resistor. This voltage measurement
device 58 is connected electrically in parallel with the
connections E1 and E2 of the bidirectional power switch 2. The
connection points 74 and 76 of the resistors 68, 70 and 70, 72 each
form one output of the voltage measurement device 58, at each of
which a voltage actual value of the semiconductor switch 4 or e,
respectively, can be tapped off. These outputs 74 and 76 of the
voltage measurement device 58 are respectively connected to an
actual value input of the voltage comparison devices 60 and 62. A
predetermined voltage reference value U.sub.ref4 or U.sub.ref6,
respectively, is present at the reference value input of these
voltage comparison devices 60 and 62. On the output side, the
voltage comparison devices 60 and 62, respectively, are
electrically conductively connected by means of the respective
decoupling diode 64 or 66 to one input of the respective driver
stage 40 or 42 of the actuation circuit (which is not shown in any
more detail) of the respective semiconductor switch 4 or 6 of the
bidirectional power switch 2. As soon as the voltage actual value
which has been determined for the semiconductor switch 4 or 6 is
greater than or equal to the predetermined voltage reference value
U.sub.ref4 or U.sub.ref6, the corresponding driver stage 40 or 42
is actuated, so that a current is generated in order to charge the
gate of the semiconductor switch 4 or 6. If the control voltage is
sufficiently high, the semiconductor switch 4 or 6 changes to the
active region, and in consequence limits any further voltage rise.
This active overvoltage protection apparatus is advantageously used
in a high-power bidirectional power switch 2.
* * * * *