Electric lamp and discharge devices: systems – Combined load device or load device temperature modifying... – Distributed parameter resonator-type magnetron
Reexamination Certificate
1999-04-12
2001-03-20
Bettendorf, Justin P. (Department: 2817)
Electric lamp and discharge devices: systems
Combined load device or load device temperature modifying...
Distributed parameter resonator-type magnetron
C315S094000, C315S105000
Reexamination Certificate
active
06204601
ABSTRACT:
TECHNICAL FIELD OF INVENTION
The present invention refers to a method and a device for controlling the filament current of a magnetron.
TECHNICAL BACKGROUND AND PRIOR ART
Magnetrons are used within the field of microwave technology for converting electrical energy into microwaves. For example, magnetrons are used as microwave sources in radar equipment, microwave ovens, and plasma lamps.
The power supply to a magnetron essentially consists of the voltage U that is applied between the anode and cathode (filament) of the magnetron, see
FIG. 1
, said voltage generally being called the anode voltage. This anode voltage may be of the order of a few kilovolts. The current I passing through the magnetron as a result of said voltage is generally called the anode current.
In order for the magnetron to operate properly, the emission of electrons from the filament of the magnetron needs to be sufficiently large to provide a stable generation of microwaves. Generally, it is therefore necessary to provide a filament current I
f
, at least during magnetron start-up, through the filament for heating thereof. The filament typically consists of a filament body having a surface coating which increases the capacity of the filament to emit electrons. The filament current is typically provided by a low voltage power source of about of a few volts.
When the magnetron emits output power, the filament is indirectly heated as a result of some electrons returning to the filament. If these electrons have sufficiently high energy, more free electrons will be generated by secondary emission. As a result, the filament current may often be removed once the magnetron is operating at full power.
Hence, it is suitable to adjust the filament current according to the current power level if the magnetron is to be operated at varying power.
Conventionally, such adjustment has been provided by experimentally determining which filament currents are suitable at different power levels and by storing such power-filament current-relations as a table or function in a filament current control unit (denoted FCC in FIG.
1
). Using such a table or function, the filament current control unit adjusts the filament current based upon the current power level of the magnetron.
OBJECTS OF THE INVENTION
An object of the invention is to provide more optimal and stable magnetron operation.
Another object of the invention is to provide more optimal control of the filament current of a magnetron.
A further object of the invention is to increase the life of a magnetron.
Yet another object of the invention is to minimize the total power supplied to a magnetron at a certain output power.
SUMMARY OF THE INVENTION
The above mentioned objects is achieved by methods or devices according to the accompanying claims.
According to the invention, control of the filament current passing through a filament of a magnetron is accomplished by detecting, during operation, a parameter which is related to the actual emission capacity of the filament and by controlling the filament current of the magnetron depending on the detected emission capacity.
Hence, the invention is based upon the insight as to the advantage of controlling the filament current based upon information relating to characteristics of the magnetron reflecting the emission capacity of the filament, specifically whether or not the emission capacity of the filament is sufficient to provide a desired magnetron operation, and not only based upon the current power level.
The prior art adjustment, which is described above and which is based solely upon the current power level, does not consider changes occurring as the magnetron grows older, is heated, undergoes varying load, or the like. This means that a selected filament current, which for a certain power level were considered suitable when the magnetron was new, may prove less suitable when the magnetron has been used for a longer period of time. If, for example, the table stored in the filament current control unit, which provides the relations between power and filament current, is not updated in consideration of such aging effects, the working point of the magnetron will gradually be moved away from the desired one. Similar negative effects may occur when the magnetron load is varied or when the magnetron is gradually heated during start-up.
Consequently, for optimal operation, the magnetron may require a higher filament current at a desired power level after having been used for a period of time compared to when it was new. The prior art adjustment scheme is hence insufficient in this respect.
Furthermore, nor is a too high emission desired, while the emission capacity of the filament will then decrease prematurely, thus decreasing the life of the magnetron. The use of either a too high or a too low filament current will consequently result in a premature decreasing of the filament emission capacity at a specific temperature.
The lifetime and function of the magnetron is enhanced by the invention, as the control according to the invention adjusts the filament current in consideration of the magnetron characteristics also after having been used a period of time and at varying loads, etc.
Hence, the invention is based upon an understanding that the function of the magnetron is associated with the emission capacity of the filament. A basis for a proper operation of the magnetron is that the emission of electrons from the filament is not allowed to decrease under a defined working level. By ensuring that the actual or actual emission level does not fall below a threshold level, the magnetron will continue to operate in a desired manner.
If the emission is kept sufficiently large to provide a stable oscillation at high efficiency, a longer working lifetime of the magnetron will be obtained.
Hence, the control of the filament current according to the invention is preferably performed by relating the detected emission capacity to a desired emission capacity, preferably depending on the magnetron power level, and by then controlling the filament current depending on this relation.
According to one aspect of the invention, control of the filament current is achieved by detecting the actual or current dynamic impedance of the magnetron. The detection of the actual dynamic impedance is performed in association with a desired dynamic impedance at the current power level. Control of the filament current is then performed depending on this relation.
Referring to
FIG. 2
, the dynamic impedance of a magnetron is generally defined as the ratio between a change &Dgr;U in the anode voltage of the magnetron and the corresponding change &Dgr;I in the anode current of the magnetron which, at that specific point in time, is associated which said change in the anode voltage.
When the filament current, and hence the emission of electrons, is too low, an increase in the anode voltage will only result in a small change in the anode current, as the supply of current carriers is limited, and hence the magnetron will show a large dynamic impedance. When, on the other hand, the filament current, and hence the emission of electrons, is too high, an increase in the anode voltage will give rise to a larger change in the anode current, as the supply of current carriers now is good, and the magnetron will hence show a low dynamic impedance.
According to the inventors, the value of the dynamic impedance at the required filament current for required emission will not change essentially when the emission capacity or the required emission level changes as a result of the magnetron aging or being heated or as a result of load changes, etc. Hence, control or the filament current based upon the dynamic impedance represents an essential inventive step which provides a number of advantages compared to prior art.
As is clear from the discussion above, according to a preferred embodiment, the filament current is increased when said actual dynamic impedance is larger than said desired dynamic impedance and is decreased when said actual dynamic impedance is lower than said desired
Grabs Jan
Lidstrom Kjell
Bettendorf Justin P.
Burns Doane , Swecker, Mathis LLP
Fusion Lighting Inc.
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