Semiconductor switching devices

Details Semiconductor switching devices Semiconductor switching devices

2001-11-08

2003-10-28

Toatley, Jr., Gregory J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Voltage regulator protective circuits

C361S018000, C361S141000

Reexamination Certificate

active

06639767

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a semiconductor switching device used, for example, for a converter in a power system to convert AC to DC and vise versa.
BACKGROUND OF THE INVENTION
A conventional semiconductor switching device, such as described in Japanese Unexamined Patent Publication No. 33,001/1999, is shown in FIG.
26
. Although a semiconductor switching device typically comprises semiconductor switching elements connected in series and parallel, only one of them is shown in FIG.
26
. As shown in
FIG. 26
, a reverse-conducting diode
3
is connected to a semiconductor switching element
2
to produce a reverse-conducting switching element
1
. Moreover, a snubber circuit
10
is connected to the reverse-conducting switching element
1
. In a semiconductor switching device in which reverse-conducting switching elements
1
are connected in series and parallel, each reverse-conducting switching element
1
is called a series valve and each reverse-conducting switching element
1
with the snubber circuit
10
is called a stage.
In a snubber circuit of
FIG. 26
,
4
denotes the first diode,
5
denotes the first capacitor,
6
denotes the first impedance element,
7
(
7
a
,
7
b
) denotes the first Zener diode,
8
(
8
a
,
8
b
) denotes the first resistor, and
9
(
9
a
,
9
b
) denotes the first controlling semiconductor element. The first impedance element
6
, the first controlling semiconductor elements
9
, the first resistors
8
, and the first Zener diode
7
comprise a non-linear circuit
16
. In
FIG. 6
, moreover,
12
denotes a gate drive circuit to drive the semiconductor switching element
2
,
13
denotes an input terminal to apply a control signal for the gate drive circuit
12
, and
33
denotes a power supply for the gate drive circuit.
In the semiconductor switching device in which the stages as shown in
FIG. 26
are connected in series, overvoltage may be applied to a semiconductor switching element
2
mainly by following three causes.
Firstly, overvoltage is caused by asynchronous turn on/off operation among the semiconductor switching elements. If a semiconductor switching element
2
turns on later than the others, overvoltage is applied to this semiconductor switching element, turning on late. If a semiconductor switching element
2
turns off earlier than the others, an overvoltage is applied to this semiconductor switching element, turning off early.
Secondly, overvoltage is also caused by inductance of circuits connected thereto. When the semiconductor switching elements turns off, variation in current generates an electromotive force across the inductance of the circuits. This electromotive force is added to the semiconductor switching elements, whereby each semiconductor switching element is charged with the overvoltage at the same time.
Thirdly, in a semiconductor switching device in which semiconductor switching elements are connected in series, leakage current of each semiconductor switching element probably varies so that divided voltage for each semiconductor switching element also varies. Therefore, some semiconductor switching elements are charged with higher voltage than the other semiconductor switching elements, that is, overvoltage.
When an overvoltage occurs and is applied to the semiconductor switching element
2
in the semiconductor switching device as shown in
FIG. 26
, electrical charge from this overvoltage flows through the first diode
4
into the first capacitor
5
. If, thereby, the voltage across the first capacitor
5
exceeds a Zener voltage determined by the first Zener diode
7
(
7
a
,
7
b
), current is drawn through he first impedance element
6
and the first controlling semiconductor element
9
(
9
a
,
9
b
) so that the voltage on the first capacitor
5
is decreased to the Zener voltage and the semiconductor switching element
2
is protected from the overvoltage.
By the way, if a voltage equal to or just above the normal voltage for each stage is chosen as the Zener voltage of the Zener diode
9
(
9
a
,
9
b
), a slight overvoltage easily exceeds the Zener voltage and current continuously flows through the first controlling semiconductor element
9
(
9
a, b
) so that the first controlling semiconductor element
9
(
9
a
,
9
b
) may be thermally destroyed. While, if voltage considerably higher than the normal voltage is chosen as the Zener voltage, unevenness in applied voltage among the stages, which is caused by uneven leakage current among the semiconductor switching elements
2
in the off state, is not compensated until the highest voltage among semiconductor switching elements
2
reaches this considerably high Zener voltage. Therefore, compensation to apply equal voltage for each semiconductor element
2
cannot be achieved.
Moreover, in case a semiconductor switching device is constructed from a large number of semiconductor switching elements
2
connected in series and parallel, a power supply
33
for a gate drive circuit
12
is required for each semiconductor element
2
so that the semiconductor switching device as a whole becomes complicated and manufacturing cost thereof rises.
As described above, electromotive force across the inductance of the circuits is generated by turn off of the semiconductor switching elements
2
and, thereby, current flows into the first capacitor
5
. During the current flows into the first capacitor
5
, power is also supplied from the power source to the first capacitor
5
. Therefore, the current caused by the electromotive force of inductance and the current from the power source flowing together with the current are stored in the first capacitor
5
and lost at the snubber circuit
10
.
By setting an upper limit voltage of the first capacitor
5
, i.e., the Zener voltage of the Zener diode
7
(
7
a
,
7
b
), above the normally applied voltage of each semiconductor switching element
2
, the current from the power source toward the first capacitor
5
, which originates from the electromotive force at the inductance, hardly flows. Thus, the term in which the current flows is shortened so that energy from the power source supplied to the first capacitor
5
is reduced.
As described above, by s fling the voltage of the first capacitor
5
higher than the normally applied voltage of ea h semiconductor switching element
2
, loss in the snubber circuit
10
is reduced. However, if the voltage of the first capacitor
5
is set too high, protection for the semiconductor elements
2
becomes insufficient so that breakdown of the semiconductor elements
2
may be caused. In contrast, if voltage of the first capacitor
5
, i.e., the Zener voltage of the Zener diode
7
(
7
a
,
7
b
), is set too small, the voltage applied to each semiconductor element
2
easily exceeds the Zener voltage by a slight increase thereof due to an accident or the like in the power system and, thereby, current continuously flows through the Zener diode
7
(
7
a
,
7
b
) so that the Zener diode
7
(
7
a
,
7
b
) may be thermally destroyed.
Moreover, when a higher energy voltage is chosen to reduce loss in the snubber circuit
10
, unevenly divided voltages among the semiconductor switching elements
2
in their off state hardly exceed this higher Zener voltage so that compensation to apply equal voltage for each semiconductor element
2
in off state cannot be achieved.
Furthermore, especially in a semiconductor switching device in which a large number of the semiconductor switching elements
2
are connected in series to convert very high voltage, a power supply
33
to supply adequate voltage for a gate drive circuit
12
of each semiconductor switching element
2
becomes complicated and costly.
SUMMARY OF THE INVENTION
Therefore, in the first aspect of the present invention, switching elements of self-quenching function are connected in series to constitute a bridge arm, and at least two bridge arms are connected in parallel to constitute a high voltage semiconductor switching device. Moreover, a snubber circuit comprising a first diode, a first capacitor and

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