Electric lamp and discharge devices: systems – Discharge device load – Electromagnetic influenced discharge device
Reexamination Certificate
2001-06-19
2004-09-07
Vo, Tuyet T. (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load
Electromagnetic influenced discharge device
C315S355000, C315S356000, C315S285000, C361S250000, C361S249000
Reexamination Certificate
active
06788012
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for generating a high voltage pulse having an extremely high voltage and a large content with the aid of semiconductor switches.
2. Related Art Statements
In order to generate plasma, an abruptly raising high voltage pulse of several kV to several tens kV having a very short duration (sometimes 50 nano seconds is required) has to be applied to a load, i.e. a discharge gap provided in a plasma generating reactor.
FIG. 1
is a circuit diagram showing a principal structure of a known high voltage generating circuit. A DC supply source
1
having a high output voltage which is equal to a voltage of an output high voltage pulse is connected across a pulse energy supplying capacitor
3
via a charging resistor
2
. The capacitor
3
is connected across a load (discharging site)
5
via a switch
4
. After charging the capacitor
3
, when the switch
5
is made conductive, energy is transferred from the capacitor
3
to the load
5
.
An inductance existing in a path of a current flowing from the capacitor
3
to the load
5
through the conducting switch
4
is denoted by an inductance
6
in FIG.
1
. The load
5
is formed by the discharge gap and it generally consisting of a capacitive element. In
FIG. 1
, for the sake of explanation, this capacitive element of the load
5
is denoted by a capacitor
7
connected in parallel with the discharge gap
5
. When the switch
4
is made conductive, a current flows to the capacitor
7
and the capacitor is charged. The larger and steeper this current is, the steeper the output pulse generated across the capacitor
7
becomes. In this manner, a preferable pulse for the plasma discharge can be attained. However, in practice, the switch
4
has a finite switching time and could not be made conductive instantaneously, and the relatively large inductance
6
is always existent in the circuit. Therefore, a raising edge of the output pulse could not be steep and an output pulse having a short duration could not be generated.
In order to solve the above explained problem of the known pulse generating circuit, there has been proposed a magnetic compression circuit utilizing a saturable iron core.
FIG. 2
illustrates such a magnetic compression circuit. In
FIG. 2
, elements similar to those shown
FIG. 1
are denoted by same reference numerals used in FIG.
1
and their detailed explanation is dispensed with. A series circuit of saturable iron or magnetic cores
8
-
1
,
8
-
2
and
8
-
3
is connected across the switch
4
and the load
5
, capacitors
3
-
1
,
3
-
2
and
3
-
3
are connected between terminals of these saturable iron cores and a negative terminal of the DC supply source
1
, and a saturable iron core
8
is connected across the load
5
.
An inductance of the saturable iron core is very high until the core is saturated, and when a product of a voltage and time reaches a predetermined value, the inductance of the saturable iron core decreases abruptly. For the sake of explanation, it is assumed that inductance values of the saturable iron cores
8
-
1
,
8
-
2
,
8
-
3
and
8
are decreased in this order and the capacitors
3
-
1
,
3
-
2
and
3
-
3
have a same capacitance value. After the switch
4
has been made conductive and the saturable iron core
8
-
1
has been saturated at an instance T
0
, voltage pulses v
1
, v
2
and v
3
appearing across the capacitors
3
-
2
,
3
-
3
and
7
are successively compressed on a time axis as depicted in FIG.
3
. That is to say, the voltage pulse v
1
appearing across the capacitor
3
-
2
begins to increase from the Instance T
0
and becomes maximum after a time duration T
1
. Since the circuit Is designed such that the saturable iron core
8
-
2
is saturated at such a time instance, the voltage pulse v
2
appearing across the capacitor
3
-
3
begins to raise and becomes maximum after a time period T
2
. At this time, the saturable iron core
8
-
3
is saturated and the voltage pulse v
3
begins to raise. After a time period T
3
which is shorter than the time period T
2
, the voltage pulse v
3
reaches a maximum value. In this manner, the voltage pulse v
3
having a sharp raising edge as well as a relatively short pulse width can be applied across the load
5
.
As illustrated in
FIG. 2
, the known high voltage pulse generating circuit including the saturable reactors has a complicated construction. Since a high voltage is applied to all the elements in the circuit, it is required to use special parts, and it is also required to provide a longer insulation distance. Moreover, the DC supply source
1
has to generate a high voltage. In this manner, the known circuit is liable to be large in size and expensive in cost.
In the known high voltage pulse generating circuit, the switch
4
is generally formed by a thyratron which is a kind of vacuum tube. Since the thyratron has a very high switching speed and can be used under a high voltage, the switch
4
can be formed by a single thyratron, and therefore an inductance of the switch
4
is small. However, the thyratron has the following demerits:
(1) The thyratron cannot operate at a high repetition frequency.
(2) The thyratron cannot be self-turned off and thus a limitation is imposed upon designing the circuit.
(3) The thyratron has a short lifetime and maintenance is cumbersome and expensive.
(4) The thyratron requires a heater circuit as well as a gas control, and therefore the overall circuit is liable to be complicated.
(5) The thyratron malfunctions due to jitter and miss-ignition.
Recently semiconductor switches have been developed in accordance with the progress of power electronics, and there have been designed semiconductor switches which can turn-on and turn-off a large current under a high voltage. However, a semiconductor switch has a lower withstand voltage and could not be substituted for the thyratron. A switch is composed of a series circuit of a number of semiconductor switches and a necessary circuit voltage is sheared by these semiconductor switches. In order to turn-on simultaneously the semiconductor switches connected in series, it is necessary to provide special gate driving circuits. Furthermore, a high voltage is applied between the gate driving circuits, and therefore gate power sources and gate control signals have to be isolated from each other. In general, a remarkable advantage could not be attained by only replacing the thyratron by a series circuit of semiconductor switches.
Recently semiconductor switches have been developed in accordance with the progress of power electronics, and there have been designed semiconductor switches which can turn-on and turn-off a large current under a high voltage. However, a semiconductor switch has a lower withstand voltage and could not be substituted for the thyratron. A switch is composed of a series circuit of a number of semiconductor switches and a necessary circuit voltage is shared by these semiconductor switches. In order to turn-on simultaneously the semiconductor switches connected in series, it is necessary to provide special gate driving circuits. Furthermore, a high voltage is applied between the gate driving circuits, and therefore gate power sources and gate control signals have to be isolated from each other. In general, a remarkable advantage could not be attained by only replacing the thyratron by a series circuit of semiconductor switches.
As explained above, in the known high voltage pulse generating circuit, a high DC voltage source is required and all the circuit components are subjected to a high voltage. Moreover, a pulse having a short width could not be produced due to a limitation in switching speed and a circuit inductance, and therefore the magnetic compression circuit has to be used. Then, the circuit becomes large and expensive.
SUMMARY OF THE INVENTION
The present invention has for its object to provide a simple and low cost high voltage generating circuit which can generate directly a narrow high voltage pulse raising sharply without u
Iida Katsuji
Sakuma Takeshi
Burr & Brown
NGK Insulators Ltd.
Vo Tuyet T.
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