Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2002-01-04
2004-05-25
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S131000
Reexamination Certificate
active
06741484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to power modulators, and more specifically to power modulators having pulse generating modules utilizing primary and secondary windings.
2. Description of Related Art
a. Modulators, General Description and Definitions of Terminology
A modulator is a device which controls the flow of electrical power. When one turns on an electric lamp and turns it off again, one could be said to be modulating the current that feeds the lamp. In its most common form, a modulator delivers a train of high power electrical pulses to a specialized load like a microwave generator. Most of the world's high power radar sets use modulators to deliver power pulses to a microwave source, which, in turn, feeds the power, in the form of periodic bursts of microwaves, to an antenna structure. Other possible applications of such power modulator are listed in the text below.
In the decades since World War II, the basic structure of power modulators has not changed significantly. A conventional modulator consists of a power supply, which receives power from an AC power line, steps up the voltage, rectifies the power to produce direct current DC power, and is used to deliver energy to a reservoir, usually formed by an energetic capacitor bank. This is necessary because the input power line cannot deliver the peak power that is required, so the reservoir is used to deliver the peak power in small bites of energy, and is replenished or refilled by the DC power supply at a reasonably constant rate with much lower average power.
Part of the energy in this reservoir is then transferred to a second smaller reservoir, usually a pulse-forming network (PFN). The PFN is a network of capacitors and inductors designed to deliver power to a load in the form of a rectangular (flat-topped) pulse with a fast rise and fall-time as compared to the pulse width or duration.
The pulse-forming network (an artificial transmission line or delay line) is then switched to connect it to the primary side of a pulse transformer, usually but not always a voltage step-up transformer. The PFN voltage before switching is V, and the voltage applied to the pulse transformer primary is V/2 or a bit less. This is one disadvantage of the PFN-technology. The pulse transformer turns ratio (voltage step-up ratio) must be twice as large with a PFN as with the present invention.
The PFN discharges completely in a time T (typically a few to a few tens of microseconds), holding a reasonably constant voltage on the pulse transformer primary and producing a reasonably flat output pulse on the transformer secondary. But if a pulse flatness of 0.1 percent or so is required, then the PFN must have a very large number of inductor-capacitor (LC) sections and it will be difficult to adjust. Also, if any component in the PFN should fail, the PFN will require a new adjustment when the new part is installed, as all the parts values and positions are very critical in a PFN.
Having delivered the pulse, the PFN must be recharged completely to voltage V for the next pulse. To maintain a pulse-to-pulse repeatability of a few tenths of one percent, this large charging voltage “swing” must occur with great precision. Also, fully charging and fully discharging all the PFN capacitors for each pulse, several hundred to several thousand times per second, puts a heavy strain on the dielectric material in these capacitors, and this forces the capacitors to be designed with very low stress and hence a very low energy density. This makes the PFN a large structure in comparison to the new invention concept, where the capacitors do not discharge and recharge for each pulse and so can have much higher energy density.
To summarize, the disadvantages of prior art modulators are:
The voltages on the primary side of the pulse transformer are high, typically 10 kV or more.
The PFN must be fully charged to the 10-20 kV range for each pulse, and is fully discharged during the pulse, placing high stress on its capacitors.
The PFN capacitors have low energy density for the above reason, so they are quite large in comparison to the lower-stress capacitors used in the new concept.
If a short circuit occurs at the load (as happens frequently with magnetron tubes), there is no way to interrupt the flow of current, as the high voltage PFN switch (a gas-filled thyratron) cannot be turned off until its current falls to zero.
If a component in the PFN fails, it is necessary to re-tune the PFN for optimal pulse shape after the component is replaced. This is laborious and dangerous work, as it must be done with high voltage applied to the PFN.
If a different pulse width is needed, it is necessary to replace and re-tune the PFN structure.
b. Pulse Transformers
The story of the so-called fractional-turn pulse transformer begins with an invention of Nicholas Christofilos which was assigned to the U.S. Government's Lawrence Livermore National Laboratory (LLNL) in the early 1960s. At that time, the laboratory was named Lawrence Livermore Laboratory or LLL. This invention disclosed a way to use a large number of toroidal (doughnut-shaped) magnetic cores, each core driven by a high voltage pulse generator at several tens of kilovolts (kV) (using a spark-gap switch and a pulse-forming network or PFN) to generate an accelerating potential of several hundred kV to several megavolts (MV) to accelerate a beam of charged particles. The basic idea of this so-called Linear Magnetic Induction (LMI) Accelerator is shown below in
FIGS. 1 and 2
.
FIG. 1
illustrates a set of toroidal magnetic cores arranged so their central holes surround a straight line, along which the particle beam is to be accelerated.
FIG. 2
shows the LMI structure with more details added; one high voltage (HV) driver system is shown (each core has one) and the particle beam path is indicated.
The key feature of this type of accelerator is that it has an outer surface which is at ground potential. The voltage which drive the individual cores all appear to add in series down the central axis, but do not appear anywhere else. This means the accelerator does not radiate energy to the outside world and is easy to install in a laboratory as it needs no insulation from its surrounding. An 800 kV LMI accelerator was built at LLL in the 1960s (The ASTRON accelerator), and was used for electron-beam acceleration in fusion experiments. A larger LMI machine (FXR, for flash x-ray) was built at that laboratory in the 1970s, and accelerated an electron beam pulse into an x-ray conversion target. FXR was used for freeze-frame radiography of explosions.
The operating principle of the LMI accelerator can be illustrated with the aid of
FIG. 3
, which shows a cross-section of the machine in a plane that includes the beam axis.
Some rules of the game are needed to discuss the behavior of the multiple-core structure shown in FIG.
3
. First, the right-hand rule is needed. This (arbitrary) rule states that if you grasp a conductor with your right hand, with your thumb pointing in the direction of positive current flow, then your fingers will curl around the conductor in the direction of the magnetic flux lines that encircle the conductor. Applying that rule to
FIG. 3
, the magnetic flux induced in the toroidal magnetic cores will circulate as shown. A “dot” is used to indicate flux vectors pointing toward the reader, and an X is used to represent flux vectors pointing away from the reader.
Applying this rule to the particle beam flowing toward the right along the axis of the structure in
FIG. 3
, one find that the magnetic flux generated by this beam circulates in the direction opposite to the flux induced by the primary current, which is correct. If we think of this as a transformer, and the beam as a short circuit across the secondary winding, then the current in this secondary will flow in a direction to cancel the flux induced by the primary, causing no net flux to be induced in the magnetic cores and thus presenting a short circuit to the primary power source. No flux change
Crewson Walter
Woodburn David
Bacon & Thomas PLLC
ScandiNova AB
Sterrett Jeffrey
LandOfFree
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