Magnetron drive step-up transformer and transformer of...

Details Magnetron drive step-up transformer and transformer of... Magnetron drive step-up transformer and transformer of...

2001-07-09

2002-09-10

Berhane, Adolf Deneke (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

Reexamination Certificate

active

06449178

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a high-frequency heater using a magnetron to execute dielectric heating, such as a microwave oven, and in particular to the configuration of a step-up transformer for driving a magnetron by using a switching power supply, and more particularly to a transformer comprising an inverter for converting large power from a commercial power supply into high-frequency, high-voltage power for driving a magnetron.
Further, this invention relates to a magnetron drive step-up transformer unit of a high-frequency heater using a magnetron to execute dielectric heating, such as a microwave oven, and in particular to a transformer structure to enhance the stability of a heater current for heating a magnetron filament and prevention performance of a short circuit of PS.
BACKGROUND OF THE INVENTION
Hitherto, as this kind of magnetron drive step-up transformer using a switching power supply, a step-up transformer comprising a primary winding
1
, a secondary winding
2
, and a heater winding
3
placed in parallel on the same axis as U-shaped magnetic substances
4
and
5
as shown in
FIG. 22
has been generally used for the following reason.
For a magnetron drive power supply often handling large power, use of a zero-volt switching technique (ZVS technique) based on voltage resonance is mainstream to lighten the load of a power semiconductor; in the ZVS technique, a step-up transformer coupling coefficient needs to be set in the range of about 0.6 to 0.85 to provide a resonance voltage. Thus, it is difficult to use a transformer of concentric multilayer winding (the coupling coefficient is 0.9 or more because the coupling between windings is strong).
However, in the described configuration in the related art, an attempt to produce higher output of the magnetron falls easily into a situation in which the peak current flowing into the primary side of the step-up transformer is more increased and thus the magnetic substances used with the step-up transformer are easily saturated. To solve this problem, it is necessary to upsize the magnetic substance, namely, the transformer, causing a bottleneck in pursuing miniaturization of the power supply.
FIG. 23
is an external view of a step-up transformer in a related art. In the figure, numeral
201
denotes a bobbin made of a resin around which a primary winding
202
, a secondary winding
203
, a short proof winding
204
for preventing the primary and secondary windings from being shorted, and a heater winding
205
for supplying power for heating a cathode of a magnetron are wound. The portion of the bobbin
201
around which the secondary winding
203
is wound is divided into four winding grooves by partitions
206
. First, the secondary winding is wound around the first winding groove and upon completion of a predetermined amount of the winding, then the remaining winding is wound around the second winding groove. Upon completion of a predetermined mount of the winding, then the remaining winding is wound around the third winding groove. Then, the remaining winding is wound around the fourth winding groove, so that the division winding of the secondary winding is completed. Thus, if aligned winding is not accomplished in each winding groove and partial winding disorder occurs, the division grooves around which the secondary winding is wound are insulated with resin and thus the risk of producing an electrical breakdown between the parts of the secondary winding can be excluded. Since the secondary winding consists of the winding divisions, heat generated by a copper loss on the secondary winding is dispersed into the winding grooves and is radiated, so that an excellent radiation characteristic, namely, an advantage of suppressing a temperature rise can be provided. Numeral
207
denotes a core made of ferrite, etc. The core
207
serves a magnetic circuit for transferring magnetic energy generated by an electric current of the primary winding
202
to the secondary winding
203
. A resin core cover
208
for insulating the core
207
and the windings is attached. The description of the step-up transformer in the related art is now complete.
However, such a step-up transformer comprises the primary and secondary windings placed in parallel, thus a method of widening the winding width of the primary winding
202
for enlarging the exposure area for cooling or widening the winding width of the secondary winding
203
or increasing the number of divisions of the secondary winding
203
for enlarging the exposure area for providing a good heat radiation property is available to provide a good radiation property and temperature performance.
To use the step-up transformer with a microwave oven, etc., increasing output is an indispensable factor for speedy heating; to increase output, the energy transferred with the transformer also grows and a temperature rise needs to be suppressed for avoiding degradation of insulation performance. Thus, to provide a good heat radiation property for lowering the temperature, the transformer must be widened and upsized; this is a problem.
Further, hitherto, for this kind of inverter power unit, high-voltage power supply for driving a magnetron, the Unexamined Japanese Patent Application Publication No. Hei 5-121159 discloses a monolithic voltage resonance inverter of a single-terminal type. The inverter power unit converts power converted into a high frequency by the inverter into a high voltage through a step-up transformer and generates a high DC voltage appropriate for driving the magnetron by a voltage doubler rectifier circuit. According to the configuration, the step-up transformer can be miniaturized by converting power into a high frequency by the inverter and the circuitry is formed on a single board, so that a compact and lightweight magnetron drive power supply (inverter power supply) can be provided.
In the described configuration in the related art, an attempt to produce higher output of the magnetron falls easily into a situation in which the peak current flowing into the primary side of the step-up transformer is more increased and thus the magnetic substances used with the step-up transformer are easily saturated. To solve this problem, it is necessary to upsize the magnetic substance, namely, the transformer, causing a bottleneck in pursuing miniaturization of the power supply.
Still further, hitherto, for an inverter power unit for converting a commercial power supply into a high-frequency, high-voltage power supply for driving a magnetron, the Unexamined Japanese Patent Application Publication No. Hei 5-121159 discloses a monolithic voltage resonance inverter of a single-terminal type. The inverter power unit converts power converted into a high frequency by the inverter into a high voltage through a step-up transformer and generates a high DC voltage appropriate for driving the magnetron by a high-voltage circuit using multiplication voltage rectification or a rectifier circuit, whereby the step-up transformer can be miniaturized by converting power into a high frequency by the inverter and the circuitry is formed on a single board, so that a compact and lightweight magnetron drive power supply (inverter power supply) can be provided.
FIG. 24
is a side view of a step-up transformer
408
of a magnetron drive step-up transformer unit in a related art. In the figure, numeral
401
denotes a bobbin made of a resin around which a primary winding
402
, a secondary winding
403
, and a heater winding
404
for supplying power for heating a cathode of a magnetron are wound. Numeral
405
denotes a core made of a magnetic substance of a ferrite, etc. The core
407
serves the function of a magnetic circuit for transferring magnetic energy generated by an electric current of the primary winding
402
to the secondary winding
403
. The windings are bound on terminal pins
406
and are dip-soldered. The step-up transformer
408
has the described configuration. The terminal pins
406
are inserted into holes made in a printed circuit board
407
and are dip

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