U.S. patent application number 09/725160 was filed with the patent office on 2001-06-21 for switching arrangements.
Invention is credited to Richardson, Robert.
Application Number | 20010004323 09/725160 |
Document ID | / |
Family ID | 10865255 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004323 |
Kind Code |
A1 |
Richardson, Robert |
June 21, 2001 |
Switching arrangements
Abstract
In a switching arrangement for applying a pulse to a load, for
example a magnetron, a plurality of FET modules are stacked along
the longitudinal axis and surrounded by a housing enclosing four
capacitors. The capacitor means provides a current return path for
current applied via the FET switches of the modules the coaxial
current cancelling construction of the arrangement results in low
circuiting conductance. The capacitance may be sufficiently large
to provide electrostatic screening.
Inventors: |
Richardson, Robert;
(Chelmsford, GB) |
Correspondence
Address: |
VENABLE
P.O. Box 34385
Washington
DC
20043-9998
US
|
Family ID: |
10865255 |
Appl. No.: |
09/725160 |
Filed: |
November 29, 2000 |
Current U.S.
Class: |
363/59 |
Current CPC
Class: |
H03K 3/57 20130101 |
Class at
Publication: |
363/59 |
International
Class: |
H02M 003/18 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 1999 |
GB |
9928049.7 |
Claims
I claim:
1. A switching arrangement for applying a pulse across a load
comprising: switching means and annular capacitor means surrounding
and coaxial with the switching means, the capacitor means being
electrically connected to provide a current return path for current
applied via the switching means to the load.
2. An arrangement as claimed in claim 1 wherein the switching means
is solid state.
3. An arrangement as claimed in claim 2 wherein the switching means
comprises at least one power FET.
4. An arrangement as claimed in claim 1 wherein the switching means
comprises a plurality of switches carried by a plurality of stacked
modules.
5. An arrangement as claimed in claim 1 wherein the annular
capacitor means comprises a plurality of capacitors distributed
coaxially about the switching means.
6. An arrangement as claimed in claim 5 and including four
capacitors distributed orthogonal to one another.
7. An arrangement as claimed in claim 1 wherein the capacitor means
includes at least one capacitor having a plurality of capacitor
elements connected in series.
8. An arrangement as claimed in claim 7 and including four
capacitors distributed orthogonal to one another.
9. An arrangement as claimed in claim 1 wherein the capacitor means
is fluid filled.
10. An arrangement as claimed in claim 1 wherein the capacitor
means is contained in a housing having an outer wall and an inner
wall, and the switch means is located within the volume interior to
the inner wall.
11. An arrangement as claimed in claim 10 wherein the housing is of
a plastics material.
12. An arrangement as claimed in claim 10 wherein the housing has a
rectangular cross-section configuration.
13. An arrangement as claimed in claim 1 and including dielectric
fluid in which the switching means is immersed.
14. An arrangement as claimed in claim 1 wherein there is fluid
communication between the volume where the switching means is
located and the volume where the capacitor means is located.
15. An arrangement as claimed in claim 1 wherein a trigger driver
is located at a high voltage end of the switching means.
16. An arrangement as claimed in claim 1 including a heater
transformer for the load located at a grounded end of the switching
means.
17. An arrangement as claimed in claim 1 wherein the load is a rf
device or system.
18. A switching arrangement for applying a pulse across a load
comprising: switching means and annular capacitor means surrounding
and coaxial with the switching means, the capacitor means being
electrically connected to provide a current return path for current
applied via the switching means to the load; wherein the switching
means comprises a plurality of solid state switches carried by a
plurality of stacked modules and the annular capacitor means
includes at least one capacitor having a plurality of capacitor
elements connected in series.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a switching arrangement and more
particularly, but not exclusively, to an arrangement employing a
plurality of solid state switches.
BACKGROUND TO THE INVENTION
[0002] Previously, modulators used to drive rf devices, such as
magnetrons, or for providing variable pulses for test equipment
have used pulse transformers. More recently, we have previously
proposed the use of a solid state switching arrangement in which a
plurality of switch modules are connected together to give
flexibility in operating voltage, duty cycle and pulse width with
PRF's up to 750 kHz.
[0003] The present invention arose from consideration of the
implementation of a solid state switching arrangement but it is
envisaged that it will be applicable to other types of switching
arrangements also.
SUMMARY OF THE INVENTION
[0004] According to the invention, a switching arrangement for
applying a pulse to a load comprises: switching means and annular
capacitor means surrounding and coaxial with the switching means,
the capacitor means being electrically connected to provide a
current return path for current applied via the switching means to
the load.
[0005] The annular distribution of the capacitor means around the
switching means to give a coaxial assembly results in a substantial
and desirable reduction of circuit inductance because of the
coaxial current cancelling construction, with current travelling in
one direction through the switching means and in the opposite
direction through the capacitor means. The low inductance improves
switching performance compared to another arrangement in which a
capacitor is located physically remote from the switching
means.
[0006] Preferably, the energy stored in the capacitance of the
capacitor means is sufficiently large compared to the energy of the
pulse output applied to the load that the capacitor means acts as
an electrostatic screen. The charge of the capacitor means does not
deplete significantly during an output pulse applied to the load
and there is virtually no voltage change across its terminals.
Thus, pulse displacement currents on the outside of the capacitor
means are minimized and hence it is possible to provide
electrostatic shielding in a particularly low volume by the annular
configuration of the capacitor. This significantly reduces
interference to other equipment located in the vicinity of the
switching arrangement.
[0007] Preferably the capacitor means is contained in a housing
having an outer wall and an inner wall, and which defines a tank in
which the switching means is located. That is, the switching means
is located within the volume bounded by the inner wall. A
dielectric fluid may be contained within the tank, in the volume
surrounded by the inner wall of the housing. The dielectric fluid
may provide electrical insulation around the switching arrangement
to prevent electrical breakdown. Alternatively, or in addition, the
dielectric fluid may provide cooling for the switching means. In a
preferred embodiment, the dielectric fluid is oil but other fluids
may be suitable, for example, air cooling may be used.
[0008] The invention is particularly applicable where the switching
means is solid state switching means. In one preferred embodiment,
the solid state switching means includes a power FET. In
alternative embodiment it could include an IGBT. The invention may
be applied to a switching arrangement having a single solid state
switch but is particularly advantageous where the switching means
comprises a plurality of switches. These may be carried by a
plurality of stacked modules with the switches being connected in
series or in parallel. However, a single switch of any alternative
technology may also be employed in the invention, such as a high
power vacuum tube having a controllable input.
[0009] Where a plurality of stacked switching modules are used, it
enables high voltage output pulses to be achieved, for example, of
the order of tens of kilovolts, the number of modules may be of the
order of 60 or more.
[0010] Preferably, the annular capacitor means comprises a
plurality of capacitors distributed coaxially about the switch
means. However, it could be embodied as a single annular capacitor
in some arrangements. In one convenient arrangement, the annular
capacitor means has a rectangular transverse cross-section, but
other configurations could be used, for example, the annular
capacitor could have a circular transverse cross-section.
[0011] Preferably, where the capacitor means comprises a plurality
of capacitors, each capacitor comprises a plurality of capacitor
elements connected in series. Thus, the capacitor elements are
arranged along the longitudinal length of the switching means in a
direction parallel to the current flow through the switching means.
Preferably, the capacitor elements are arranged such that there is
a nominally linear voltage gradient from one end of the capacitor
means to the other. Thus, dc and low frequency electric field
appear across the length of the device whilst pulse stress appears
transversely to this across the smaller coaxial structure in a
radial direction.
[0012] Where the capacitor means is fluid filled, then the housing
of the capacitor means may communicate with the interior volume
within which the switching means is located so that mixing occurs
between the fluid in the capacitor volume and fluid in the
switching means legion where identical or compatible dielectric
fluids are used. A fluid expansion system located separately from
the capacitor means housing may be included to allow for expansion
of fluid within the capacitor means.
[0013] In a preferred arrangement where the load includes a vacuum
tube, for example, a magnetron, or some other device using a
thermionic cathode, a heater supply is located at the pulse output
end of the switching means.
[0014] An advantage of an arrangement in accordance with the
invention is that impedance matching is not a design requirement or
limitation.
[0015] A trigger circuit for the switching means may be included at
the high voltage end of the switching means remote from the load to
minimize displacement capacitance which occurs when the switching
means is operated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Some ways in which the invention may be performed are now
described by way of example with reference to the accompanying
drawings in which:
[0017] FIG. 1 is a schematic longitudinal view of a switching
arrangement in accordance with the invention;
[0018] FIG. 2 is a schematic cross-sectional view along the line
II-II of FIG. 1;
[0019] FIG. 3 is a schematic circuit diagram of the arrangement
shown in FIG. 1; and
[0020] FIG. 4 shows an alternative arrangement in transverse
view.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] With reference to FIGS. 1 and 2, a switching arrangement
includes switching means comprising a plurality of FET modules 1,
2, 3, 4 . . . n which are stacked along a longitudinal axis X-X.
The modules 1, 2 . . . n are nominally identical and in this
arrangement there are 64 of them. Each module carries an FET switch
5 and a secondary winding 6 of a transformer. The primary winding 7
of the transformer is common to all FET modules, interconnecting
them and is used to supply power to the FET switches. Power into
the switching arrangement is applied at 8 via a trigger driver 9
located at the high voltage end of the switching arrangement which
is maintained at -55 kV. The load 10, which in this case is a
magnetron, is connected to the output 11 of the switching
arrangement to receive the pulses which are output from the
switching means. Power is applied to a heater transformer 12 via
line 13 for connection to the magnetron 10.
[0022] The FET modules 1,2,3 . . . n are surrounded by a housing 14
of a plastics material which has an outer wall 15 and an inner wall
16, the housing being annular and rectangular in transverse
section. Four capacitors 17, 18, 19 and 20 are arranged within the
housing 14 around the periphery of the switching means, each
capacitor being connected at the high voltage end to the first
switch module and at the low voltage end, in this embodiment at
ground, to the load 10. Each capacitor 17 to 20 comprises a
plurality of rectangular plates forming capacitor elements 21, 22
shown in FIG. 3, which are interconnected such that a nominally
linear voltage gradient appears from the power supply end to the
zero volt end. In this embodiment, the capacitors are each 0.15
micro Farads.
[0023] The interior volume 23 defined by the housing 14 which
contains the FET modules 1, 2 . . . n is filled with oil for
voltage insulation and cooling. Similarly, in this arrangement, the
capacitors 17 to 20 are also oil filled. Openings 24 and 25 are
included between the interior 23 defined by the housing 14 and the
capacitor region to allow fluid communication between them. A
conduit connects with a separate reservoir 26 to allow for
expansion of fluid within the capacitors.
[0024] The axial length of the stack and the surrounding capacitors
is approximately 1 meter and the outer diameter is approximately 40
cms.
[0025] In operation, power is supplied to the modules via the
primary winding 7 and on application of a trigger signal, the FET
switches are closed. A drive pulse is applied to the load 10
current travels in opposite directions through the load 10
switching means and the capacitors 17,18, 19 and 20 to give current
cancelling. The bulk of the capacitors acts to provide
electrostatic shielding.
[0026] FIG. 4 shows a slightly different arrangement of capacitors
27 to 30 around FET modules 31. In an alternative embodiment not
shown, a single elongate capacitor is included located around the
switching modules.
* * * * *