US3584288A

US3584288A - Transformer power supply for microwave generators

Info

Publication number: US3584288A
Application number: US810671A
Authority: US
Inventors: Helmut Boehm
Original assignee: Technology Instrument Corp
Current assignee: Technology Instrument Corp
Priority date: 1969-03-26
Filing date: 1969-03-26
Publication date: 1971-06-08
Anticipated expiration: 1988-06-08
Also published as: GB1217469A

Abstract

Two ferroresonant transformers are provided to apply power to a magnetron, and each is adapted to resonate to provide half the power to the magnetron. The primary windings are connected in parallel across power lines, and a switch is provided to selectively connect one primary to the lines. In respective embodiments, both secondaries are adapted to operate through full-wave and half-wave rectifier means coupled to the anode of the magnetron, and in both the current drawn by the magnetron when the primary switch is closed is double that when the switch is open.

Description

D United States Patent Inventor Helmut Boehm Thousand Oaks, Calif.

Appl, No, 810,671

Filed Mar. 26, 1969 Patented June 8,1971

Assignee Technology Instrument Corporation Newbury Park, Calif.

TRANSFORMER POWER SUPPLY FOR MICROWAVE GENERATORS 5 Claims, 4 Drawing Figs.

U.S. Cl 321/16, 219/1055, 321/20, 321/27, 323/61, 328/262 Int. Cl H02m 7/04, 1105b 9/06, G05f3/06 Field 01 Search 321/16, 20, 27, 25; 323/60, 61; 315/172; 328/258, 262; 219/1055 [56] References Cited UNITED STATES PATENTS 3,217,233 11/1965 Drusch 321/27 3,247,394 4/1966 Lowrance..... 321/27X 3,371,263 2/1968 Waltz et al 321/25 3,470,942 10/1969 Fukada et a1. 2l9/10.55X

Primary Examinerwilliam H. Beha, Jr. AuorneyPerry E. Turner ABSTRACT: Two ferroresonant transformers are provided to apply power to a magnetron, and each is adapted to resonate to provide half the power to the magnetron. The primary windings are connected in parallel across power lines, and a switch is provided to selectively connect one primary to the lines. in respective embodiments, both secondaries are adapted to operate through full-wave and half-wave rectifier means coupled to the anode of the magnetron, and in both the current drawn by the magnetron when the primary switch is closed is double that when the switch is open.

TRANSFORMER POWER SUPPLY FOR'MICROWAVE GENERATORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to transformer power supplies for microwave heating apparatus.

2. Description of the Prior Art In microwave heating apparatus such as ovens for cooking foodstuffs, it is desirable to very the level of energy developed by the magnetron and injected into the oven cavity. Such an approach is desirable to permit one to selectively subject a given foodstuff to different cooking speeds and energy levels appropriate for optimum results. However, there is not presently knownsimple and reliable means for achieving this desired end.

SUMMARY OF THE INVENTION My invention embraces transformer power supply means with two power sections using constant current transformers, wherein a simple, conventional switch in one primary circuit is used to couple one or both sections to a load which is in circuit with the secondaries of both.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a power supply of my invention using full-wave rectifier means in circuit with the secondaries of constant current transformers;

FIG. 2 is a schematic diagram of a power supply of my invention using half-wave rectifier means in circuit with the secondaries of such transformers;

FIG. 3 illustrates current waveforms to aid in explaining the operation of the circuit of FIG. I; and

FIG. 4 illustrates current waveforms to aid in explaining the operation of the circuit of FIG. 2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Referring to FIG. I, a pair of constant current transformers l0, 12 have their primary windings l4, 16 connected in parallel across power lines 18, the terminals 20 of which are adapted for connection to an AC source. As shown, a switch 22 is connected in series with the primary winding 16. The switch 22 may be a simple single-pole, singleor double-throw switch of conventional construction. As will be seen, this switch is all that is needed to selectively control the level of power applied to a magnetron, illustrated at 24, and thereby control the level or intensity of microwave energy radiated from its output probe.

The

secondary windings

30, 32 of transformers l0, 12 are coupled through full-wave rectifier means to the anode 34 of the magnetron. In the particular arrangement shown, the upper end of the secondary winding is shown connected to the junction of two capacitor legs 36, 38 of a full-wave rectifier or bridge, and the lower end of such secondary is shown with a connection between diode legs 40, 42. The diode legs 40, 42 in this example also serve as diode legs of a full-wave bridge connected to the secondary winding 32, the lower end of which is shown connected to the junction of

capacitor legs

44, 46, and the upper end of which is shown with a connection between the diode legs 40, 42. An additional diode 48 is' connected between the diodes 40, 42, i.e., between the lower end of the secondary 30 and the upper end of the secondary 32. This latter diode 48 serves to isolate DC components of voltage and current of each transformer from those of the other.

As shown, the junction 50 of capacitor 38 and diode 42 are connected to the anode 34, which is connected to a point of reference or ground potential. The junction 52 of the capacitor 36 and diode is connected to one leg of the filament 54 of the magnetron, such filament having its legs adapted for connection in conventional fashion to the secondary of a filament transformer (not shown).

The circuit of FIG. I will first be described with the switch 22 open as shown, and then with switch 22 closed. With such switch open, only the transformer 10 is energized. Accordingly, the bridge constituted of capacitors 36, 38 and diodes 40, 42 functions as a full-wave rectifier, the positive and

negative terminals

50, 52 of which are connected between the anode 34 and the cathode 56 of the magnetron. Referring to FIG. 3 along with FIG. I, such rectifier develops current pulses 60 have a repetition rate double that of the frequency of the line voltage, e.g., 120 pulses per second for a 60 cycle line voltage. The magnitude of current pulses 60 is determined by the values of the capacitors 36, 38 and the turns ratio of the windings of the transformer 10. Such capacitor values and turns ratio preferably are designed so that the power applied to the magnetron 24 via transformer 10 is one-half the maximum power to be applied thereto.

In similar fashion, the turns ration of the windings of the transformer 12 and the capacitances of

capacitors

44, 46 are designed so that when the transformer 12 is energized, current pulses developed by the bridge that includes

capacitors

44, 46 are of the same magnitude as current pulses 60. Accordingly, when the switch 22 is closed to energize the transformer 12, current pulses 62 are developed which are twice the magnitude of the current pulses 60. Therefore, the power applied to the magnetron when the switch 22 is closed is double that applied to it when the switch is open.

Referring to FIG. 2, the

secondary windings

30, 32 are connected to respective half-wave rectifiers. As shown, a capacitor 70 and diode 72 are connected in series between the ends of the secondary 30, and a capacitor 74 and diode 76 are connected in series between the ends of the secondary 32. The junction 78 of the capacitor 70 anddiode 72 are connected to the anode 34 of the magnetron, and the junction 80 of the capacitor 74 and diode 76 are connected to one leg of the filament 54. Also, the lower and upper ends of the

secondaries

30, 32 are connected together between the diodes 72, 76. Still further, the diodes 72, 76 are connected in the forward direction between the

junctions

80, 78.

Referring to FIG. 4 along with FIG. 2, let it first be assumed that the switch 22 is open as shown, in which case only the transformer I0 is energized. Due to half-wave rectification, current pulses 84 are developed which have the same repetition rate as the line frequency. Due to the value of capacitor 70 and the turns ratio of the windings of transformer 10, the magnitude of current pulses 84 is such that the power supplied to the magnetron via transformer 10 is one-half the maximum power to be applied thereto.

When the switch 22 is closed to energize the transformer I2, current pulses 86 are developed which are of the same magnitude and repetition rate as current pulses 84. However, current pulses 86 fall intermediate current pulses 84. Further, the windings are so poled as to insure the same polarities of the

junctions

78, 80 for both current pulses 84, '86. When switch 22 is closed, therefore, the power applied to the magnetron is double that applied to it when the switch is open.

It is preferable that iron core transformers be used for the transformers 10, I2, and that they be provided with means which during operation cause them to be driven to saturation in the vicinity of their secondary windings. An example of such a transformer is a pair of C-shaped cores having their ends butted together, and having a magnetic shunt extending between opposed portionsof the resultant structure on which the primary and secondary windings are placed. Magnetic shunts are indicated at 88, 90 for transformers l0, 12.

It will be apparent that my invention embraces the use of more than'two power sections. For example, there may be three transformers with inputs of separate full-wave rectifiers connected to respective secondaries, and with the respective rectifier outputs tied together and to appropriate elements of the magnetron. In such case, each transformer is adapted to mers, depending on whether a third, two-thirds, or full power is to be applied. Also, of course, ganging of such switches may be employed to apply either two-thirds or full power as desired.

lclaim:

1. A power supply for a microwave generator comprising;

a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings AC lines,

said means including a switch in circuit with the primary winding of one transformer and operably to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers;

and means coupling both pulse developing means to the generator, thereby to cause current pulses from both pulse developing means to be applied to the generator when said switch is closed and to cause current pulses from the pulse developing means in circuit with said other transformer to be applied to the generator when said switch is open.

2. A power supply for a microwave generator comprising:

a pair of constant current transformers each having a primary winding and a secondary winding;

means connecting said primary windings to AC lines,

said means including a switch in circuit with the primary winding of one transformer and operable to selectively connect it to and disconnect it from the lines,

whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open;

means in circuit with each secondary winding for developing unidirectional current pulses from said transformers,

said pulse developing means including half-wave rectifier means,

each current pulse from each transformer when said switch is closed occurring intermediate current pulses from the other transformer;

and means coupling both pulse-developing means to the generator, thereby to cause current pulses from both pulse developing means to beapplied to the generator when said switch is closed and to cause current pulses from the pulse developing means in circuit with said other transformer to be applied to the generator when said switch is open.

3. The combination of claim 2, wherein said half-wave rectifier means includes respective rectifiers each having a capacitor and a diode in series with a respective secondary winding,

said diodes being directly connected in the same direction;

and a respective connection from the junction of each series-connected capacitor and diode, said connections being adapted for connection across the generator. 4. A power supply for a microwave generator comprising: a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings to AC lines,

said means including a switch in circuit with the primary winding of one transformer and operable to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers, including full-wave rectifier means, the current pulses applied to the generator from both transformers when said switch is closed being coincident; said full-wave rectifier means including a pair of bridges each having a pair of capacitor legs, the remaining legs of each bridge being two diode legs that form common legs for the bridges for both pair of capacitor legs; respective connections from each side of said isolation diode to one end of each secondary winding, the other ends of said secondary windings being connected respectively to the functions of the capacitor legs; and respective connections from the ends of said diode legs for connection across the generator. 5. A power supply for a microwave generator comprising: a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings to AC lines,

said means including a switch in circuit with the primary winding of one transformer and operable to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers, including a pair of half-wave rectifiers each having input terminals connected to the ends ofa respective secondary winding, respective input terminals of said rectifiers being connected together; and means coupling both pulse developing means to the generator, thereby to cause current pulses from both pulse developing means to be applied to the generator when said switch is closed and to cause current pulses from the pulse developing means in circuit with said other transformer to be applied to the generator when said switch is open, including output connections from both rectifiers connected across the generator.

Claims

1. A power supply for a microwave generator comprising; a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings AC lines, said means including a switch in circuit with the primary winding of one transformer and operably to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers; and means coupling both pulse developing means to the generator, thereby to cause current pulses from both pulse developing means to be applied to the generator when said switch is closed and to cause current pulses from the pulse developing means in circuit with said other transformer to be applied to the generator when said switch is open.

2. A power supply for a microwave generator comprising: a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings to AC lines, said means including a switch in circuit with the primary winding of one transformer and operable to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers, said pulse developing means including half-wave rectifier means, each current pulse from each transformer when said switch is closed occurring intermediate current pulses from the other transformer; and means coupling both pulse developing means to the generator, thereby to cause current pulses from both pulse developing means to be applied to the generator when said switch is closed and to cause current pulses from the pulse developing means in circuit with said other transformer to be applied to the generator when said switch is open.

3. The combination of claim 2, wherein said half-wave rectifier means includes respective rectifiers each having a capacitor and a diode in series with a respective secondary winding, said diodes being directly connected in the same direction; and a respective connection from the junction of each series-connected capacitor and diode, said connections being adapted for connection across the generator.

4. A power supply for a microwave generator comprising: a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings to AC lines, said means including a switch in circuit with the primary winding of one transformer and operable to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers, including full-wave rectifier means, the current pulses applied to the generator from both transformers when said switch is closed being coincident; said full-wave rectifier means including a pair of bridges each having a pair of capacitor legs, the remaining legs of each bridge being two diode legs that form common legs for the bridges for both pair of capacitor legs; respective connections from each side of said isolation diode to one end of each secondary winding, the other ends of said secondary windings being connected respectively to the functions of the capacitor legs; and respective connections from the ends of said diode legs for connection across the generator.

5. A power supply for a microwave generator comprising: a pair of constant current transformers each having a primary winding and a secondary winding; means connecting said primary windings to AC lines, said means including a switch in circuit with the primary winding of one transformer and operable to selectively connect it to and disconnect it from the lines, whereby both said one transformer and the other transformer are energized when said switch is closed and only said other transformer is energized when said switch is open; means in circuit with each secondary winding for developing unidirectional current pulses from said transformers, including a pair of half-wave rectifiers each having input terminals connected to the ends of a respective secondary winding, respective input terminals of said rectifiers being connected together; and means coupling both pulse developing means to the generator, thereby to cause current pulses from both pulse developing means to be applied to the generator when said switch is closed and to cause current pulses from the pulse developing means in circuit with said other transformer to be applied to the generator when said switch is open, including output connections from both rectifiers connected across the generator.