Electric heating – Microwave heating – With control system
Patent
1997-03-03
1999-11-09
Walberg, Teresa
Electric heating
Microwave heating
With control system
363128, H05B 666
Patent
active
059819251
DESCRIPTION:
BRIEF SUMMARY
The subject of this invention is a power supply for a CW magnetron and it is devoted for applications in cases where a precise control and stabilisation of microwave power level is required, partcularly in constructions of professional microwave power equipment. The power supply according to the present invention also be applied in construction of microwave ovens while it is simple and cheap in manufacturing.
The well known and widely used power supply for CW magnetrons has a form of ferroresonant current stabiliser with high voltage Villard-type doubler. This kind of power supply is characterised by the high reliability, high energy efficiency and simplicity which implies low costs of manufacturing what, in turn, resulted in its common employment in microwave ovens. Variations in this kind of circuits are also known e.g. from U.S. Pat. No. 3,873,883.
The disadvantage of the known and used typical power supplies is, that when connected to the AC line they can deliver power at single rated levels and adjustment of the average power is realised by switching the AC voltage at the primary winding of high voltage transformer on and off (the so-called duty cycle control, pulsating). The high voltage transformer is normally used for simultaneously energising of the filament circuit, therefore the switching of the primary voltage causes the so-called cold starts of the magnetron and this phenomenon implies limitation of the shortest allowable on/off cycle duration, which practically can not be shorter than a few seconds. This degrades both the continuity and the precision of power control. Moreover, the on/off timing is always accompanied with the current and voltage transients caused by the high inductance of the transformer.
The application of the triacs as switching elements at the transformer primary is limited to the group control (on/off switching), that is because traces of the DC current components which usually occur when the phase-type triac control is used may lead to the severe saturation of the transformer core.
The use of silicone controlled rectifiers in the Villard-type doubler circuits is known from technical literature of the late 60's. Different aspects of the gate firing of triacs or thyristors in magnetron power supplies along with current sensing circuitry have been given in U.S. Pat. Nos. 4,001,537 and 4,012,617. In both the mentioned patents triacs or thyristors or different equivalents of those are supposed as phase -or group-type controllers of the Villard's doubler output voltage. Series connections of diode and triac or diode and thyristor are shown as possible thyristor quivalents but no means have been provided or discussed on how to facilitate the switching-on of the thyristor in such a high voltage circuit comprising a very high stray inductance of the transformer secondary winding. Moreover application of the mentioned patents would require the use of special transformer with taps at the secondary side or it would require isolation of the magnetron anode block to implement anode current sensing. Anyhow, the proposed circuits can not fulfill the role of a reliable power controller for the magnetron and a power supply built according to the claims and descriptions given therein does not bring solution of a wide range magnetron power adjustment and regulation. These limitations and shortcommings find their confirmation in the fact that after almost 20 years that passed between filing of this application and the dates of filing the above mentioned U.S. Patents nobody has used them to realize a really full range continuously adjustable and well regulated magnetron power controller. The improvement can ether not be expected from implementation of yet another U.S. Pat. No. 3,794,908 in which a high voltage device has been proposed in a form of stack of regulate low voltage thyristors equipped with dynamic RC voltage divider, however, used rather for monitoring of faults than for enhancement of switching with inductive loads.
Power supply for a magnetron according to the present invention
REFERENCES:
patent: 3794908 (1974-02-01), Lindblom et al.
patent: 4717861 (1988-01-01), Yuasa et al.
patent: 4835353 (1989-05-01), Smith et al.
patent: 4949233 (1990-08-01), Braunisch et al.
patent: 5661394 (1997-08-01), Nakao et al.
Parosa Ryszard
Reszke Edward
Pwu Jeffrey
Walberg Teresa
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