JPH01304687A - High frequency heating device - Google Patents

Abstract

PURPOSE:To aim at the restraint of temperature rise at the time of empty baking by detecting a high frequency radiated into a heating chamber and controlling an inverter circuit. CONSTITUTION:A high frequency radiated from a magnetron 27 is led into a heating chamber 26. The high frequency is detected by an antenna 32 shorter than 1/4 wave length of an oscillation frequency of the magnetron 27. A signal from the antenna 32 appeares as a detected output signal Vs through an attenuator 33, a detecting circuit and an amplifying circuit. When the signal Vs is larger than the predetermined value, it is judged that a quantity of food is small or the chamber is empty so as to stop the operation of an inverter circuit 28. Consequently, an abnormal temperature rise is avoided, storming the empty baking condition for a long time. Adding to the prevention of destruction by heating to each part such as the magnetron or an transistor, a cooling assembling can be simplified.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a high-frequency heating device for performing so-called dielectric heating such as a microwave oven, and more specifically, uses an inverter as its power source and generates high-frequency power by the inverter. The present invention relates to an improvement of a high-frequency heating device configured to boost the voltage using a step-up transformer to drive a magnetron.

BACKGROUND OF THE INVENTION Various configurations of high-frequency heating devices of this type have been proposed in order to make the power transformer smaller, lighter, or less expensive.

FIG. 5 is a circuit diagram of a conventional high frequency heating device. Fifth
In the figure, the power from commercial power supply 1 is connected to diode bridge 2.
A unidirectional power source is formed. 3 is an inductor, and 4 is a capacitor, which serves as a filter for the high frequency switching operation of the inverter.

The inverter includes a resonant capacitor 5, a step-up transformer 6, a transistor 7, a diode 8, and a drive circuit 9. The transistor 7 performs a switching operation with a predetermined cycle and duty (ie, on-off time ratio) by a base current supplied from the drive circuit 9. As a result, the current Ic/Id as shown in FIG. 6(a), that is, the collector current Ic of the transistor 7 and the current Id of the diode 8
flows. On the other hand, when the transistor 7 is off, a voltage Vce as shown in FIG. 6(b) is generated between C and E of the transistor 7 due to resonance between the capacitor 5 and the primary winding 10.

Therefore, a current as shown in FIG. 6(C) flows through the primary winding 10, and high frequency power is generated at both ends of the primary winding 10.

Therefore, high frequency power and high frequency low voltage power are generated in the secondary winding 11 and the tertiary winding 12, respectively. This high frequency high voltage power is rectified by a capacitor 13 and a diode 14 and is supplied between the anode and cathode of the magnetron 15, while the high frequency low power is supplied to the cathode heater. Therefore, the magnetron 15 oscillates and can perform dielectric heating. The magnetron 15 includes a magnetron main body 15&, and a capacitor 16. which constitutes a filter.
17.18. It consists of choke coils 19 and 20. In such a configuration, the core cross-sectional area of the step-up transformer 6 becomes smaller as the frequency of the power supplied to both ends of the primary winding 10 becomes higher.
When operating at a frequency of about Hz to 100kHz, the weight and size of the step-up transformer can be reduced from a few minutes to a few minutes compared to when boosting at the commercial power frequency, making it possible to reduce the cost of the power supply section. It has the following characteristics.

Problems to be Solved by the Invention These conventional high-frequency heating devices had the following problems.

As shown in FIG. 7, in the method of heating the food 22 etc. in the heating chamber 21 using high frequency waves from the magnetron 15,
The cooling fan 23 includes a magnetron 15 and a high voltage transformer 6.
, a high voltage diode 14, an inverter power supply section 24 including a transistor 7, a diode 8, a rectifier bridge 2, etc.
However, it was designed to have the ability to cool down without thermal damage even under conditions of maximum temperature rise.

On the other hand, the temperature rise of the magnetron 15 reaches its maximum when there is no food or other object to be heated in the heating chamber 21, that is, during dry baking. This is because the radio waves emitted from the magnetron are not consumed within the heating chamber 21 but are reflected back to the magnetron 15. This caused the magnetron to generate abnormal heat, which shortened its lifespan. Furthermore, the heat generated by the magnetron causes the surrounding components to rise in temperature, lowering reliability, and requiring higher cooling capacity, which increases costs, such as increasing the size of the fan 23. In particular, the transistor 7 included in the inverter power supply is sensitive to heat, so a high cooling capacity is important.

Means for Solving the Problems The present invention aims to solve the above problems. A unidirectional power supply and at least one semiconductor switch and its driver.

an inverter circuit for generating high-frequency power having a dynamic circuit; a high-voltage transformer that converts the output of the inverter circuit into high-voltage high-frequency waves; a magnetron that receives the output of the high-voltage transformer and radiates high-frequency waves; and the high-frequency waves that the magnetron radiates. a heating chamber into which power is supplied and stores objects to be heated such as food; an antenna shorter than a quarter wavelength of the radiation frequency of the magnetron; an attenuator for attenuating the power that can be input from the antenna; a detection means for detecting the oscillation state of the magnetron through the detection means; and a drive circuit configured to control the drive circuit so as to stop the operation of the inverter circuit if the output signal of the detection means is outside a predetermined range for a predetermined time. It is.

Function The present invention has a configuration in which the high frequency waves radiated into the heating chamber can be received by an antenna, and the amount of reception is small when there is a lot of food in the heating chamber because the high frequency waves are absorbed by the food, and increases during dry baking.

For this reason, when the amount of food is extremely small or during dry baking, the output of the detection means becomes large, so after receiving this signal, the inverter circuit that supplies the return power for a predetermined time is stopped, which is sensitive to heat. power transistor,
It can protect parts such as magnetrons from abnormal temperature rise during dry firing.

Therefore, the cooling means can be simplified and the reliability of each component can be improved.

Embodiments Hereinafter, embodiments of the present invention will be described based on the accompanying drawings.

FIG. 1 is a block diagram of the main body of a high-frequency heating device according to an embodiment of the present invention. In FIG. 1, 25 is an object to be heated such as food;
26 is a heating chamber, and 27 is a magnetron. 28 is an inverter circuit section, 29 is a high-voltage transformer, and 30 is a high-voltage rectifier section.The output of the high-voltage transformer 29 is voltage-doubled and rectified by the high-voltage rectifier section, and then applied to the magnetron 27. magnetron 2
The high frequency waves radiated from the heating chamber 2 pass through the waveguide 31.
6. An antenna 32 has a wavelength shorter than 1/4 of the oscillation frequency of the magnetron 27 (for example, 2450 MHz). (In the present invention, a to 511) 33 is an attenuator, 34 is a detection means, 35 is an amplifier for amplifying the detected signal, and 36 is a cooling fan.

The inverter circuit section 28 also has a control means including a timer circuit for controlling the transistor drive circuit based on the detected signal.

FIG. 2 shows a circuit diagram of the high frequency heating device of the present invention.

A commercial power supply 37, a diode bridge 38, an inductor 39 for filtering, and a capacitor 40 are connected to a unidirectional power supply 41.
It consists of The output of the unidirectional power supply 41 is converted into high frequency power by an inverter circuit 28 having a semiconductor switch 42 (transistor), boosted by a high voltage transformer 29, rectified by a voltage doubler rectifier circuit 30, and applied to the magnetron 27. An input detector 44 is provided to stably supply input power.The input detector 44 detects 1 inch of input current from a commercial power source, and outputs a signal rectified by an input current signal rectifier circuit 45 and a current reference signal. 46 is amplified by the current error amplification circuit 47, and the difference between the comparator 48
Enter. The comparator 48 generates an on/off pulse signal for the transistor 42 based on this input signal and the sawtooth wave from the sawtooth wave generating circuit 49. Input current I
When in decreases, the output of the current error amplifier circuit 47 increases, the on-time of the on-off pulse becomes longer, and the input current Ii
It operates in the direction of increasing n. Conversely, when the input current Iin increases, it operates to reduce the input current.

On the other hand, the signal from the antenna 32 is outputted as a detection output signal vII through the attenuator 3G, the detection circuit 34, and the amplifier circuit 3o. As shown in FIG. 3, this output signal Vs increases as the weight of the object to be heated, such as food, in the heating chamber decreases. That is, among the high frequencies emitted from the magnetron, a portion of the high frequencies that are reflected without being absorbed by the food etc. are received by the antenna, so the signal is at its maximum during dry baking. Therefore, when the detection output signal Vs is larger than a predetermined value, it is determined that the amount of food is small or the food is baked, and the switching operation of the inverter circuit 28 is stopped to prevent abnormal heat generation of each component such as the magnetro 727. . In FIG. 2, the detection signal output Vs is rectified by a rectification circuit 50 and input to a detection output comparison comparator 51. The comparator 51 compares the signal with a predetermined reference voltage 52, and outputs a dry firing determination signal 53 when the detection signal output vlI is larger than a predetermined value, that is, during dry firing. This signal is outputted as an L signal to an AND circuit 56 after a predetermined time by a timer circuit 54 and a buffer circuit 55. The logic &1N1i5] path 56 drives the transistor 42 with the AND signal of the on/off pulse from the comparator 48 and the dry firing determination signal. Therefore, during dry firing, the switching operation of the NiHA transistor 42 is stopped after a predetermined period of time defined by the timer circuit 54.

FIG. 4 is a circuit diagram of the detection means 34 for detecting the signal of the magnetron 27. 57 is a 50Ω resistor, 58 is a detection diode (for example, Schottky via diode) 5
9. Into, the resistor 61 is a pie-pane capacitor, and the signal from the magnetron 27 is detected as a voltage V by these. Note that 3a is an attenuator, and the power radiated by the magnetron 27 into the heating chamber 26 is several hundreds of watts, so that the detection means 34 does not receive an excessive input.

By adopting the above configuration, the dry firing state does not continue for a long period of time, so the maximum temperature rise of each component including the magnetron 27 can be suppressed to a low value. In particular, since switching transistors are components that are susceptible to thermal damage, this configuration greatly improves reliability.

Therefore, parts with low heat resistance grade can be used, and costs can be reduced. Also, since the cooling configuration can be made simpler, it is advantageous in terms of cost.

Effects of the Invention As described above, the high frequency heating device of the present invention provides the following effects.

(1) When the number of objects to be heated, such as food, is low, the detection signal determines the dry baking state and the operation is stopped after a predetermined period of time, so there is no abnormal temperature rise in components such as magnetrons and transistors. Therefore, a high-frequency heating device with long durability and high reliability can be realized.

Shun) Since the maximum temperature rise is smaller, parts with lower heat resistance grades can be used, and the cooling configuration can be simplified, resulting in lower costs for high-frequency heating equipment.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a high-frequency heating device according to an embodiment of the present invention, Fig. 2 is a circuit block diagram of the same device, Fig. 3 is a diagram of the weight of the object to be heated and the signal characteristics of the detection means, and Fig. 4 is a circuit diagram of the detection means of the same high-frequency heating device, FIG. 5 is a circuit diagram of a conventional high-frequency heating device, FIG. 6 is a waveform diagram of each part of the same circuit diagram, and FIG.
The figure is a configuration diagram of a conventional high-frequency heating device. 25...Object to be heated, 26...Heating chamber,
27... Magnetron, 28... Inverter circuit, 29... High voltage transformer, 32...
...Antenna, 33...Attenuator, 41...
... Unidirectional power supply, 42 ... Semiconductor switch (transistor), 43 ... Drive circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person25
- Nucleus, 27rJ round nose A-2-Ta 1 button 27-"- Magnetron 28- Inverter circuit 33-5A Suiseki, 3/ Fig. 2 Fig. 3 Fluid adjustment and adjustment Q Fig. 4 Fig. 6

Claims (1)

[Claims] an inverter circuit for generating high-frequency power having at least one semiconductor switch and a drive circuit thereof; a high-voltage transformer for converting the output of the inverter circuit into high-voltage high-frequency power; a magnetron that radiates, a heating chamber into which the high frequency waves radiated by the magnetron are fed, and in which an object to be heated such as food is stored; an antenna shorter than a quarter wavelength of the radiation frequency of the magnetron; an attenuator for attenuating the power input from the antenna; a detection means for detecting the oscillation state of the magnetron via the antenna; and an output of the detection means. A high-frequency heating device that controls a drive circuit so as to stop the operation of the inverter circuit if a signal is outside a predetermined range for a predetermined period of time.