POWER SUPPLY FOR A CONTINUOUS WAVE MICROWAVE MAGNETRON

Abstract: A power supply for a CW magnetron, devoted for use in microwave heating equipment. A power supply according to the present invention has a high-voltage line transformer (1) with secondary winding (2) which is connected through a voltage-doubling rectifier circuit to the magnetron cathode (4). The voltage-doubling rectifier circuit comprises a high-voltage diode (7) and a capacitor (3). The distinctive feature of the present invention is that the diode (7) of voltage doubler is connected in-series with at least one thyristor-capacitor network (8) which is composed of a high-voltage thyristor which has its anode and cathode connected together with an additional capacitor (Cd) or with series connection of said capacitor with a resistor (Rs).

Description: Power supply for a continuous wave microwave magnetron 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, particularly in constructions of professional microwave power equipment. The power supply according to the present invention can 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 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. Patent 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. Power supply for a magnetron according to the present invention has such a distinctive feature that in series with the diode in the voltage doubler it comprises at least one thyristor-capacitor network created by a high-voltage thyristor whose anode and cathode are connected together through an additional capacitor or through a series connection of that capacitor and a resistor. In the present invention the thyristor together with the diode play the role of controllable element in voltage doubler which enables both the group or the phase control of the power at the secondary, well from the AC mains isolated side of the transformer. Therefore, the transformer can instantly be connected to the mains and all the microwave power on/off controls may be provided as group-type control at elevated frequencies without noticeable current transients at the primary side. The present invention also enables a precise phase-type control fn the magnetron current stabilisation feedback loop featuring also possible modulation of the power with different timeshapes. Further advantage of the present invention is that it can be realised with the use of typical elements such as transformer, voltage doubler capacitor and high voltage diode that are available as replacement parts for microwave ovens. The subject of the present invention is shown in Fig. 1 which shows a schematic circuit diagram of one embodiment in which the thyristor-capacitor network represented by a single thyristor jumpered with a single capacitor. Fig. 2 is a circuit diagram of another embodiment in which the thyristor-capacitor network comprises at least two series-connections of separate thyristors, each of those jumpered with an accompanying parallel capacitor. According to the present invention a power supply for a continuous wave magnetron has a transformer , the secondary winding 2 of the transformer 1 is connected through the capacitor to the cathode 4 of the magnetron having its filament energised from winding 5 wound round the core 6 of the transformer 1_. The high voltage diode 7 has its anode connected to the common point of the connections between the capacitor 3 and cathode 4 of the 5 magnetron, while the cathode of diode 7 is connected to the common ground of the whole circuit by means of the thyristor-capacitor network 8 consisting of thyristor T with its cathode and anode connected together with an additional capacitor Cd which insures an initial current of thyristor Ty at the moment of firing. Additional o capacitor Cd is charged via the high voltage diode 7. High-voltage diode 7 along with the thyristor-capacitor network 8 and with capacitor 3, all together form a controllable voltage doubler. In the second embodiment shown in Fig. 2 the thyristor- capacitor network 8a is accomplished by series connections of at 5 least two thyristors Ty . - Tvn fired simultaneously by the control circuit 9. In this embodiment the thyristors TyJ. - Tvn are fired by means of the separation circuit 10 comprising gate transformers Tb1 - Tbn with their primaries connected in series and energised from a pulse- forming circuit 11. which acts in the closed feedback loop o stabiliser consisting of current-sensing resistor Ro .integrator 12 , and differential amplifier 3 The one side if the sensing resistor Ro is connected to the thyristor Tvn and its other side is connected to the common ground of the whole circuit. Each thyristor Tyl - Tvn is jumpered by a respective additional capacitor Cd1 - Cdn , respective 5 resistor Rd1 - Rdn and a respective series snubber Ru1Cu1 - RunCun The elements Rd1 - Rdn , Ru1 - Run , Cu1 - Cun enhance the dynamics and reliability of the power controller. By changing the set reference voltage one can obtain different power settings from zero level up to the rated value which depends on the o set of the base components, namely the transformer 1 and capacitor 3 that comes into resonance in the near of line frequency with a stray inductance of the secondary, high voltage winding 2 .In the practice a continuous adjustment of power has been achieved in the range of 10 - 600 W with stability better than 5 % at line variations from 190 5 to 250 V AC. The operation of the power supply according to the present invention is the following: _After switching on the mains to the primary winding of transformer the positive half-waves from the secondary winding 2 charge the capacitor Cd via the diode 7 and through the capacitor 3 . After firing the gate of the thyristor Ty the capacitor Cd discharge through the thyristor and thus providing current necessary to start the thyristor to conduct and than maintain its conduction in the circuit consisting of the secondary winding 2 of the transformer 1., the capacitor 3 , and the diode 7 . The current flows as long as the capacitor 3 is fully charged. The level of the capacitor 3 charging depends on firing angle .During the negative half waves across the secondary winding 2 the diode 7 is blocked and the voltage of the winding 2 adds to the voltage of the capacitor 3 and the sum of both voltages is sent to the magnetron cathode 4 .At the design dedicated for very high powers the capacitors Cd should be connected in series with additional resistors Rs which must limit starting currents of the thyristors Ty below the level allowable by the manufacturer. Instead of the thyristors Ty , Tyl. - Tvn one can with the same result apply opto-thyristors. It is also obvious that a power supply claimed in this disclosure can be accomplished in a multiplicated form leading to a full- wave voltage multiplying rectifier circuit with the help of one or two transformers , however, in such cases the cathode of magnetron must be connected with voltage multipliers with the use of additional separation diodes.

Claims: Patent Claims 1. Power supply for a continuous-wave magnetron, said supply comprising a line transformer, which has its secondary, anode winding connected through the voltage doubler to the cathode of said magnetron and having the distinctive feature that in series with diode (7) of the voltage doubler there is connected at least one thyristor- capacitor network (8) composed of the high voltage thyristor (Ty) having its cathode and anode connected together by means of additional capacitor (Cd) 2. A power supply as set forth in claim 1 wherein the anode of said thyristor (Ty) is connected with its cathode by means of a series connection of an additional capacitor (Cd) with a resistor (Rs).

Inventors: PAROSA RYSZARD, RESZKE EDWARD, PAROSA, RYSZARD, RESZKE, EDWARD
Applicants: PAROSA RYSZARD, RESZKE EDWARD, PAROSA, RYSZARD, RESZKE, EDWARD
Application year: 1995
Main classification: H05B6
IPC codes: H02M7, H05B6