Electric heating – Microwave heating – Fluid heater
Patent
1992-03-20
1994-04-05
Reynolds, Bruce A.
Electric heating
Microwave heating
Fluid heater
219725, 219760, 219715, 323325, 323356, 361 88, 363 56, 363 97, H05B 668
Patent
active
053007441
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a high-frequency heating device for such use as an electronic oven for heating food, fluid, or the like, a heat treatment apparatus for waste processing, or a humidification device for medical treatment, and more particularly to a high-frequency heating device having a construction where a magnetron is driven by means of a switching power source employing a semiconductor switching element.
BACKGROUND OF THE INVENTION
Conventionally, a high-frequency heating device of the above type as represented by an electronic oven employs a high-frequency energy generating system which consists of a magnetron and a power source unit for driving the magnetron. A permanent magnet type magnetron and a ferroresonance type power transformer have been practically used for a long time. In recent years, with improvements in semiconductor technology, a switching type power source was proposed and put to practical use in place of the conventional ferroresonance type transformer achieve improved functional development, compact and light weight design, and cost reduction of the high-frequency heating device. In other words, use of a switching type power source enables a continuous proportional control of the microwave output and permits compact and light weight designing of the high-voltage transformer, which also leads to achieving high-performance heating with high-grade control, compact and light weight designing, and cost reduction of the high-frequency heating device.
FIG. 11 shows the circuit construction of a conventional high-frequency heating device. In FIG. 11, a commercial AC power source 1 is rectified by a rectifier 2 into a DC power and then converted into a high-frequency AC power by an inverter circuit 3 consisting of capacitors and a semiconductor switching element. The high-frequency AC power obtained by the inverter circuit 3 is applied to a transformer 4. The transformer 4 comprises a primary winding W1 to which the high-frequency AC power output produced in the inverter circuit 3 is applied, a secondary winding W2 for yielding a high voltage through a voltage step-up, and a tertiary winding W3 for yielding a low voltage.
The high AC voltage yielded in the secondary winding W2 is converted into a high DC voltage by a voltage doubler rectifier circuit 5 and is applied across the anode and the cathode of a magnetron 6 to thereby activate the magnetron.
The low voltage yielded in the tertiary winding is applied to the cathode of the magnetron 6 to heat the filament.
The magnetron 6 exhibits a non-linear characteristic as shown in FIG. 12. In a condition where the filament of the magnetron 6 is sufficiently heated, the magnetron starts to oscillate when the voltage VAK applied across the anode and the cathode thereof reaches about -4 kV, with which the voltage VAK across the anode and the cathode is clipped at -4 kV to reduce the impedance of the magnetron 6 to about several kiloohms. On the contrary, in a condition where the filament is not heated, or the voltage VAK across the anode and the cathode is lower than about -4 kV, the magnetron 6 is set in a non-oscillating condition and the impedance thereof remains virtually infinite. FIGS. 13 (a), (b), and (c) show the mutual relationship between the voltage VAK applied across the anode and the cathode, a current I.sub.f flowing through the filament, and the temperature of the filament in a period from the activation time of the inverter 3 to the oscillation time of the magnetron 6. At the activation time, a greater filament current I.sub.f1 flows in order to rapidly increase the filament temperature T.sub.f to a rating temperature T.sub.f2. The tertiary winding W3 of the transformer 4 for supplying a current to the filament and the secondary winding W2 for yielding a high voltage to be applied across the anode and the cathode are provided in the same transformer. Therefore, the secondary winding W2 is compelled to yield such a high voltage as to form a voltage VAKS, which is higher than the oscillation vol
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Bessyo Daisuke
Maehara Naoyoshi
Matsumoto Takahiro
Nakabayashi Yuji
Hoang Tu
Matsushita Electric - Industrial Co., Ltd.
Reynolds Bruce A.
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