Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-02-21
2003-04-08
Sherry, Michael (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S284000, C363S132000
Reexamination Certificate
active
06545452
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
The present invention relates to a method and an apparatus for controlling a power converter valve having at least two turn-off, non-latching power semiconductor switches which are electrically connected in series and each have an active collector-emitter limiting circuit.
Turn-off, non-latching power semiconductor switches include, for example, Insulated Gate Bipolar Transistor (IGBT), bipolar Power Transistors (PTR), Metal Oxide Semiconductor Field Effect Transistor (MOSFFT) or Hard Driven Gate Off Thyristor (HD GTO). In contrast to the latching power semiconductor switches, for example, Gate Turn Off thyristor (GTO), MOS Control Thyristor (MCT) or thyristors, the non-latching power semiconductor switches constantly require a drive signal in order to remain reliably switched on or off. Only the IGBT will be described below as non-latching power semiconductor switch. However, the description is in no way intended to be restrictive.
Conventional IGBT components (modules) can be used to realize power converters in the MV range without modules being connected in parallel. If the voltage range and/or the power range of a power converter of this type-is intended to be increased, then it is appropriate to connect in series a plurality of IGBT modules per power converter valve. A power converter circuit of this type is referred to as a power converter having a series connection number of Two or more.
FIG. 1
illustrates a basic circuit diagram of a power converter having three series connections of one phase of a polyphase power converter. This bridge circuit has three upper and three lower power semiconductor switches T
1
o, T
2
o, T
3
o and T
1
u, T
2
u, T
3
u which are electrically connected in series. The junction points between the upper and lower power semiconductor switches T
1
o, T
2
o, T
3
o and T
1
u, T
2
u, T
3
u form a phase terminal
2
. On the input side, this phase power converter is linked to an intermediate circuit capacitor C which stabilizes the intermediate circuit DC voltage U
d
. An RC snubber, which includes a capacitor C
1
and a resistor R
1
and a balancing resistor R
2
, is electrically connected in parallel with each power semiconductor switch T
1
o, T
2
o, T
3
o and T
1
u, T
2
u, T
3
u. Each power semiconductor switch T
1
o, T
2
o, T
3
o and T
1
u, T
2
u, T
3
u has a control apparatus, of which only a driver stage
4
with a gate resistor Rg connected downstream is illustrated here for reasons of clarity. Each of these apparatuses also has an active collector-emitter limiting circuit. A phase power converter of this type is described in the publication entitled “High Power IGBT Converters with new Gate Drive and Protection Circuit”, printed in EPE′95, pages 1.066 to 1.070.
The RC snubber in each case minimizes the effect of nonlinear depletion-layer capacitances of the IGBT with internal inverse diode. Uniform static voltage sharing is achieved using the balancing resistors R
2
. The active collector-emitter limiting circuits of each stage limit the maximum voltage for each IGBT module T
1
o, T
2
o, T
3
o, T
1
u, T
2
u, T
3
u below the permissible blocking voltage.
In contrast to the power converter having the series connection number of One, in the case of a power converter having at least two series connections, the possible voltage potentials of the individual semiconductor switches are not fixedly predetermined a priori.
FIG. 2
illustrates, by way of example, voltage profiles of the collector-emitter voltages U
CE1
, U
CE2
, U
CE3
, for example of the power semiconductor switches T
1
o, T
2
o, T
3
o, during the phases “switch-on” P
1
, “on state” P
2
, “switch-off” P
3
and “off state” P
4
, against the time t.
During the switch-on and -off phases P
1
and P
3
, respectively (as shown in FIG.
3
and
FIG. 4
, respectively), primarily semiconductor-inherent properties such as, e.g., differences with regard to storage charge and depletion-layer capacitance, different delays and switch-on and off times determine the voltage distribution, but differences in driving as a result of tolerance, jitter- and drift-encumbered signal propagation times and also properties in the load circuit (control inductances, stray and ground capacitances and additional snubbers) also have a non-negligible influence. As shown in
FIG. 3
, an instant t
1
is marked which demonstrates the appearance of the voltage distribution on the series connected power semiconductor switches T
1
o, T
2
o, T
3
o during the switch-on phase P
1
at this instant t. This diagram shown in
FIG. 3
illustrates that the power semiconductor switch T
1
o takes up the greatest part of the reverse voltage.
During the off-state phase P
4
(shown in FIG.
4
), the voltage sharing is not stable but rather depends on the preceding switch-off operation, the turned-off current and on the magnitude, tolerance and drift of the leakage current and also on a snubber. After a period of time which depends on the depletion-layer and snubber capacitances, the leakage currents lead to a non-uniform steady-state voltage sharing in which, in the worst-case situation, a single power semiconductor switch has to take up the entire reverse voltage. The instant t
4
shown in
FIG. 4
illustrates a moment of a voltage distribution in the switched-off state (phase P
4
).
European Patent No. 0 653 830 describes a method and apparatus for driving a power converter having three series connections, with which the problems evinced are solved. With this conventional method, the collector-emitter voltages of the turn-off, non-latching power semiconductor switches which are electrically connected in series, and the total voltage present across the series circuit are measured. From an n-th part of the total voltage and a respective collector-emitter voltage, a respective differential voltage is determined, from which switch-on and switch-off times are then calculated. Using a delay circuit and these switch-on and switch-off times, the time switching points of a control signal that is provided are determined for each power semiconductor switch of the series circuit. The switch-on and switch-off times and the switching instants of a control signal are calculated in such a way that all the power semiconductor switches of the series circuit are loaded identically, in voltage terms. In the switched-off state of the power semiconductor switches which are electrically connected in series, the magnitude of the individual control signals is in each case calculated from the measured collector-emitter voltages, with the aid of an n-th part of the total voltage, in such a way that an identical voltage loading results in the off state for all of the switches. These calculated switching instants are implemented only during the subsequent switching operation.
European Patent Application No. 0 666 647 describes a method and circuit arrangement for driving semiconductor switches of a series circuit, each semiconductor switch being assigned a voltage limiting apparatus. Using this known method, the power loss of each voltage limiting apparatus is detected by a regulating device for evening out the voltage sharing over the semiconductors. The regulating device generates modified control pulses for each semiconductor switch from a common control pulse on the basis of the detected power losses of the voltage linking apparatuses. As a result, the power loss of the voltage limiting apparatuses is regulated to a minimum. This European published patent application specifies an embodiment in which an RCD snubber, balancing resistors and short-circuit elements are electrically connected in parallel with each semiconductor switch. The RCD snubber protects the IGBT module against overvoltage sparks during the turn-off of the load current. The balancing resistors provide for steady-state voltage sharing and the short-circuit elements, for example avalanche diodes or varistors, take over the current flow if the modules fail and the module voltage exceeds the response threshold of the voltage limiting apparatus and the h
Bruckmann Manfred
Marquardt Rainer
Sommer Rainer
Baker & Botts LLP
Laxton Gary L.
Sherry Michael
Siemens Aktiengesellschaft
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