JP2000116146A - Inverter power supply for high-frequency heating apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an inverter power supply, in which the loss of a switching element is reduced and which can use a small switching element. SOLUTION: For a pair of switching elements 6a, 6b which constitute an inverter, their electrical continuity ratio is identical, their switching frequency is lower than that of the resonance frequency of the primary-winding input impedance of a step-up transformer 9 in a current-conduction state of a magnetron 12 with a resonance capacitor 8, and the frequency is higher than the resonance frequency of the primary-winding input impedance of the step-up transformer 9 in a non-current-conduction state of the magnetron 12 with the resonance capacitor 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter power supply for a high-frequency heating device for driving, for example, a magnetron of a microwave oven.

[0002]

2. Description of the Related Art Conventionally, a commercial power supply has been rectified by a rectifier bridge, and a primary winding of a step-up transformer and a resonance capacitor have been connected in parallel to an inverter power supply for a microwave oven. A voltage resonance type inverter having a switching element connected thereto has been put to practical use. Also, a half-bridge type inverter power supply has been put to practical use for an AC 200 V power supply.

FIG. 15 shows a conventional half-bridge type inverter power supply (Japanese Patent Application Laid-Open Nos. 5-242962 and 7-45361). FIG. 16 is an operation waveform diagram thereof. Rectifier bridge 102, choke coil 103, smoothing capacitor 104 for commercial power supply 101
Constitutes a DC power supply. Between the positive and negative output terminals of the DC power supply, a series connection of the first and second resonance capacitors 105 and 106 and the first and second switching elements 111 and 1 are connected.
Twelve series-connected bodies are connected in parallel. The first and second switching elements 111 and 112 have a third resonance capacitor 107 and a fourth resonance capacitor 10 respectively.
8 are connected in parallel. The primary winding of the step-up transformer 113 is connected between the connection point of the first and second resonance capacitors 105 and 106 and the connection point of the first and second switching elements 111 and 112. 109, 11
0 is a freewheel diode. Step-up transformer 1
A full-wave voltage doubler rectifier circuit 114 is connected to the secondary winding 13 to supply a high DC voltage to the magnetron 115. 1
Reference numeral 16 denotes a control unit. The operation of the inverter power supply will be described. The control unit 116 drives the step-up transformer 113 at a frequency higher than the primary winding input impedance and the resonance frequencies of the first and second resonance capacitors 105 and 106. That is, as shown in FIG. 16, the current of the switching element 109 is such that the switching element 109 interrupts the current when the magnetron 115 is energized. In the input control, the frequency is increased to change the phase of the current and the voltage to control.

[0004]

In the conventional inverter power supply, since the switching element interrupts a large current when the magnetron is energized, the switching loss of the switching element increases and the switching element (transistor) also increases in size. At the same time, the cooling structure such as the heat sink becomes large. In addition, the cooling structure is a structure that cools the magnetron and the inverter at the same time by the cooling fan provided in the machine room of the microwave oven, and a large and inefficient cooling structure such as wind circulation inside the machine room Had become. By the way, the volume ratio (effective volume of oven / outer volume) of a microwave oven generally used at present is about 30 to 35%. However, in recent years, the improvement of the volume ratio has been greatly desired.

[0005] The present invention has been made in view of the above,
First, the loss of the switching element can be reduced so that a small one can be used. Second, the effective volume of the high-frequency heating device can be improved to be small and large-capacity. Fourth, it is an object of the present invention to provide an inverter power supply for a high-frequency heating device capable of stabilizing cooking time and the like, fourthly being safe and highly reliable, and fifthly reducing noise. I do.

[0006]

According to a first aspect of the present invention, at least a pair of DC and DC output terminals is provided between a positive and a negative output terminals of a DC power supply obtained by rectifying an AC voltage from a commercial power supply. And a resonance capacitor in series with a primary winding of a step-up transformer for generating a magnetron driving voltage between a connection point of the pair of switching elements and one of output terminals of the DC power supply. A pair of switching elements having the same conduction ratio and a switching frequency of the primary winding input impedance of the step-up transformer and the resonance capacitor when the magnetron is energized. , The primary winding input impedance of the step-up transformer and the resonance capacitor when the magnetron is not energized. And summarized in that the a frequency higher than the resonance frequency. With this configuration, by making the conduction ratio of the pair of switching elements the same, the primary winding current of the step-up transformer is positive and negative and the peak current is small. In addition, the primary winding current of the step-up transformer has a larger peak current value and a shorter cycle when the magnetron is turned on than when the magnetron is not turned on, due to a change in the resonance frequency of the inverter when the magnetron is turned on and when the magnetron is turned off. That is, the frequency increases. The switching frequency of the pair of switching devices is set lower than the resonance frequency of the inverter when the magnetron is energized and higher than the resonance frequency of the inverter when the magnetron is not energized, so that the switching device is turned off when the primary winding current is small. And the loss of the switching element is reduced.

According to a second aspect of the present invention, at least one pair of switching elements is connected in series between positive and negative output terminals of a DC power supply obtained by rectifying an AC voltage from a commercial power supply, and the pair of switching elements is connected. A half-bridge type inverter in which a primary winding of a step-up transformer that generates a magnetron driving voltage and a resonance capacitor are connected in series between a connection point of the element and one of output terminals of the DC power supply, The inverter has the same conduction ratio of the pair of switching elements and the pair of switching elements when the magnetron is in a resonance state based on the primary winding input impedance of the step-up transformer and the resonance capacitor when the magnetron is not energized. The gist is to oscillate at an oscillating frequency at which each element is switched off. With this configuration, by setting the conduction ratio of the pair of switching elements to be the same, the primary winding current of the step-up transformer is positive and negative and the peak current is small as described above. In addition, by setting the oscillation frequency of the inverter to a frequency at which a pair of switching elements are switched off when the magnetron is de-energized, the switching elements can be switched off at a small primary winding current, and the switching can be performed. The element loss is reduced.

According to a third aspect of the present invention, in the inverter power supply for a high-frequency heating device according to the first or second aspect, a voltage doubler rectifier circuit is provided on a secondary side of the step-up transformer. With this configuration, a DC high voltage is supplied to the magnetron. At this time, 1
Since the secondary windings have the same positive and negative duty ratios, the voltage and current of the secondary side voltage doubler rectifier are well balanced. As a result, the secondary side double rectifying capacitor can be commonly used, and the size can be reduced.

According to a fourth aspect of the present invention, in the inverter power supply for a high frequency heating device according to the first or second aspect, when the high frequency heating device is started, the step-up transformer is operated when the magnetron is operated at the maximum output power. The gist is to heat the magnetron heater by driving the inverter at a frequency higher than the primary winding input impedance and the resonance frequency of the resonance capacitor. With this configuration, a high voltage is not applied to the magnetron at the time of starting the magnetron, and the maximum output of the magnetron is not exceeded when the magnetron is heated up and turned on.

According to a fifth aspect of the present invention, in the inverter power supply for a high frequency heating apparatus according to the first or second aspect, a voltage control oscillator is provided in a control circuit for driving the inverter, and the output of the high frequency heating apparatus is controlled. The gist is that the PWM output output from the computer is smoothed to obtain a voltage for determining the oscillation frequency of the voltage controlled oscillator. With this configuration, the inverter can be controlled by a PWM signal from the microcomputer.

According to a sixth aspect of the present invention, in the inverter power supply for a high frequency heating device according to the fifth aspect, the characteristics of the voltage controlled oscillator are such that when the oscillation frequency is low, the oscillation frequency is lower than when the oscillation frequency is high. The point is that the change is gradual. With this configuration, the input power-oscillation frequency characteristic is leveled over the input adjustment range by using a voltage-controlled oscillator having a characteristic opposite to the input power-oscillation frequency characteristic of the inverter.

According to a seventh aspect of the present invention, in the inverter power supply for a high-frequency heating device according to the fifth aspect, anode current detecting means for detecting an anode current of the magnetron is provided. The gist of the present invention is to control the oscillation frequency of the voltage-controlled oscillator so that the anode current becomes constant based on this. With this configuration, the input power of the high-frequency heating device is controlled to be constant by controlling the anode current of the magnetron to a constant value.

According to an eighth aspect of the present invention, in the inverter power supply for a high-frequency heating device according to the fifth aspect, there is provided an AC current detecting means for detecting an AC current from the commercial power supply, and the AC current detecting means detects the AC current. The gist is that the oscillation frequency of the voltage controlled oscillator is controlled based on the value so that the alternating current does not exceed a predetermined value. With this configuration, an overcurrent input from the commercial power supply is detected, and the maximum input power is controlled.

According to a ninth aspect of the present invention, in the inverter power supply for a high-frequency heating apparatus according to the eighth aspect, there is provided an AC current detecting means for detecting an AC current from the commercial power supply. The gist of the invention is to control the oscillation frequency of the voltage-controlled oscillator so that the alternating current has a predetermined value based on the value. With this configuration, the input power of the high-frequency heating device is controlled to be constant.

According to a tenth aspect of the present invention, in the inverter power supply for a high frequency heating device according to the first or second aspect,
A duct for passing cooling air through cooling fins of the magnetron is provided, and a board on which equipment and electronic components including the step-up transformer, switching element, and rectifying element are mounted is housed in the duct, and the magnetron in the duct is housed. The gist is to provide a cooling fan on the opposite side of the mounting portion. With this configuration, devices and electronic components that constitute the magnetron, the inverter, and the like are efficiently cooled. In addition, there is no need for wiring for connecting between the magnetron and electronic circuits constituting the inverter and the like.

[0016] The invention according to claim 11 is the invention according to claim 10.
In the inverter power supply for a high-frequency heating device described above, the gist is that the duct is formed of any one of a material having a radio wave shielding effect including a metal or a conductive resin. With this configuration, an electronic circuit constituting the inverter and the like is shielded, and unnecessary radiation noise is reduced.

According to the twelfth aspect of the present invention,
In the above-mentioned inverter power supply for a high-frequency heating device, the gist of the invention is that the suction port of the cooling fan has a duct communicating with the outside of the housing of the high-frequency heating device. With this configuration, the air flowing into the cooling fan becomes air outside the high-frequency heating device, thereby increasing the cooling efficiency.

The invention according to claim 13 is the invention according to claim 10.
In the inverter power supply for a high-frequency heating device according to the present invention, the cooling air from the cooling fan diverges a part of the cooling air into the housing of the high-frequency heating device including the machine room before being required for the magnetron. The gist is to become With this configuration, the electrical components and the like in the machine room are also cooled by the cooling air from the cooling fan.

The invention according to claim 14 is the invention according to claim 10.
In the inverter power supply for a high-frequency heating device, a DC motor is used to drive the cooling fan, and a predetermined number of turns of a fan motor is provided on the secondary side of the step-up transformer, and the output of the fan motor is output. The rectified DC power supply serves as a power supply for the DC motor.
With this configuration, a separate power supply for driving the fan motor is not required. Further, when the output of the high-frequency heating device is small, the output voltage of the DC power supply for the fan motor is also reduced, so that unnecessary cooling is not performed. Thus, a wide output control range can be obtained and noise can be reduced when the output is small.

The invention according to claim 15 is the invention according to claim 10.
The gist of the inverter power supply for a high-frequency heating device described above is that a cooling fan for sending cooling air is separately provided inside the housing of the high-frequency heating device including the machine room. With this configuration, the cooling fan of the magnetron and the cooling fan of the machine room are separated, so that the machine room can be cooled even when used in a microwave oven.

The invention of claim 16 provides the above-mentioned claim 10.
The inverter power supply for a high-frequency heating device according to claim 1, further comprising: a noise filter connected to an input unit from the commercial power supply on the substrate; and a shielding plate that shields the noise filter from magnetic flux leaking from the step-up transformer and the duct. It is the gist that is provided in. With this configuration, the magnetic coupling between the step-up transformer and the noise filter is eliminated, and the step-up transformer and the noise filter can be arranged on the same substrate, so that the size can be reduced.

According to the seventeenth aspect, the tenth aspect is provided.
In the above-mentioned inverter power supply for a high-frequency heating device, a connecting portion between the magnetron and the substrate has a structure in which a connection terminal of the magnetron is inserted and fixed to a connector provided on the substrate. With this configuration, the high-voltage portion does not float in the space, so that the size of the substrate can be reduced.

The invention according to claim 18 is the invention according to claim 10.
In the inverter power supply for a high-frequency heating device according to the present invention, the duct in which the magnetron and the substrate are arranged, and the cooling fan are arranged in a vertical direction, and the cooling air is sucked from a housing bottom side of the high-frequency heating device. The gist is to constitute. With this configuration, the suction port of the cooling fan is arranged on the bottom surface of the housing, and the suction port of the cooling fan is eliminated on the rear side of the housing. Therefore, even when the microwave oven is placed on the table, the sense of strangeness is eliminated.

[0024]

FIG. 1 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIGS. This embodiment is applied to a microwave oven.

First, the circuit configuration of the inverter power supply will be described with reference to FIG. The commercial power supply 1 is connected to a diode bridge rectifier circuit 2 via a noise filter 22.
The AC voltage from the commercial power supply 1 is full-wave rectified by the diode bridge rectifier circuit 2 and then smoothed by a smoothing circuit formed by the choke coil 3 and the smoothing capacitor 4 to obtain a DC voltage. That is, the commercial power supply 1, the diode bridge rectifier circuit 2, and the smoothing circuit constitute a DC power supply. The DC voltage from the DC power supply is supplied to a half-bridge type inverter. The half-bridge type inverter includes first and second switching elements 6a and 6b connected between positive and negative output terminals of a DC power supply.
And the first and second switching elements 6
a, a primary winding 9a of a high-frequency transformer (step-up transformer) 9 connected between the connection point of 6b and the negative output terminal of the DC power supply
And a series circuit of the resonance capacitor 8. In the example of the figure, the first and second switching elements 6a,
An IGBT is used as 6b, and a snubber circuit including a series circuit of a capacitor 48 and a resistor 49 is connected between the collector and the emitter of the second switching element 6b. 7a and 7b are freewheel diodes. A secondary winding 9b of the high-frequency transformer 9 has a magnetron 12
High-voltage capacitors 10a for supplying a DC high voltage to
A full wave voltage doubler rectifier circuit composed of 10b and diodes 11a and 11b is connected. 20 is magnetron 12
Is a resistance as an anode current detecting means for detecting the anode current of the above, and its detection signal is inputted to the microcomputer 13. The oscillation frequency of a voltage-controlled oscillator described later is controlled based on the detected value of the anode current so that the anode current becomes constant, and the input power of the inverter power supply is controlled to be constant. On the secondary side of the high-frequency transformer 9, a heater winding 9c connected to the filament of the magnetron 12 is provided. The resonance capacitor 8 connected to the primary winding 9a of the high-frequency transformer 9 includes a power stabilizing circuit 14 for controlling the voltage applied to the inverter to a constant value by comparing it with a reference value, and an anode / magnetron 12 of the magnetron 12 when the power is turned on. Until the conduction between the cathodes occurs, the inverter is driven at a frequency higher than the input impedance of the primary winding 9a of the high-frequency transformer 9 and the resonance frequency of the resonance capacitor 8 when the magnetron 12 is operating at the maximum output, and only the filament is driven. Heats the primary winding 9a of the high-frequency transformer 9
A warm-up circuit 15 is connected for setting the voltage applied to the motor to a fixed value or less so that the voltage of the secondary high-voltage circuit does not exceed the withstand voltage of the magnetron 12. An inverter output can be set by an output setting circuit 16 from the outputs of the circuits 14 and 15. The output setting circuit 16 receives an output setting signal from the microcomputer 13. A voltage controlled oscillator (VCO) 17 outputs a switching frequency signal to the switching elements 6a and 6b in proportion to the output voltage from the output setting circuit 16. The waveform shaping circuit 18 shapes the output of the voltage-controlled oscillator 17 into an accurate waveform that performs switching correctly. The output terminal of the waveform shaping circuit 18 is connected to the drive circuit 5. According to the frequency signal from the waveform shaping circuit 18, a signal for alternately turning on and off the switching elements 6a and 6b at the same duty ratio is output from the drive circuit 5 to each gate of the switching elements 6a and 6b. Between the commercial power supply 1 and the noise filter 22, a current transformer 21 is provided as an AC current detecting means for detecting an AC current from the commercial power supply 1. The detected alternating current is rectified and smoothed by the rectifying and smoothing circuit 19 and then input to the microcomputer 13. The oscillation frequency of the voltage controlled oscillator 17 is controlled based on the detected value of the alternating current so that the input alternating current does not exceed a predetermined value, and the maximum input power from the commercial power supply 1 is controlled. Further, when the oscillation frequency of the voltage controlled oscillator 17 is controlled so that the input AC current becomes a predetermined value based on the detected value of the AC current, the input power of the inverter power supply is controlled to be constant.

FIG. 2 shows the configuration of the noise filter 22 in detail. A coil winding 50 is provided on a toroidal core, and capacitors 51a and 51b are connected to both ends of the coil winding 50. The commercial power supply 1 is connected to both ends of the capacitor 51a.

Next, the circuit operation of the inverter power supply for the high-frequency heating device configured as described above in a steady state will be described together with the details of the circuit configuration of the voltage controlled oscillator 17. As shown in FIG. 1, the secondary side of the high-frequency transformer 9 is a full-wave voltage doubler rectifier. As described later in each cycle, the magnetron 12 has a conducting state and a non-conducting state.
The input impedance from the primary side of the high-frequency transformer 9 greatly differs depending on these two states. That is, the primary winding input impedance of the high-frequency transformer 9 and the resonance frequency of the resonance capacitor 8 have two resonance frequencies when the magnetron 12 is conductive and non-conductive. FIG.
Are the primary winding current of the high-frequency transformer (FIG. 10A), the magnetron current (FIG. 10B), and the first and second switching elements 6 when the magnetron 12 is conducting and not conducting.
The timing charts of ON and OFF (FIGS. (c) and (d)) of FIGS. Switching elements 6a, 6
When b turns on, current starts to flow through the magnetron. Then, due to the inductance of the primary winding 9a when the magnetron is conducting and the series resonance circuit of the resonance capacitor 8, the current in the primary winding increases, reaches a peak, and then decreases. As the current in the primary winding decreases, the magnetron becomes non-conductive (not shown, but the magnetron voltage drops). When the magnetron is turned off, the inductance seen from the primary winding of the high-frequency transformer becomes
It becomes larger than that during conduction and becomes a resonance frequency line when Mg is not conducted in FIG. During this time, the primary current is a smaller current than when Mg is conducted, and this magnetron 12
Is turned off when the switch is off. That is,
When the inverter oscillates at an oscillation frequency that turns off the switching elements 6a and 6b when the primary winding input impedance when the magnetron 12 is non-conductive and the resonance frequency of the resonance capacitor 8, the switching elements 6a and 6b are turned off.
b turns off when the primary winding current becomes very small. This is further changed to the switching elements 6a,
6B, the switching frequency is set to 1 when the magnetron is turned on, as shown in FIG.
By setting the frequency lower than the input impedance of the secondary winding and the resonance frequency of the resonance capacitor 8 and higher than the input impedance of the primary winding and the resonance frequency of the resonance capacitor 8 when the magnetron is not conducting (FIG. 4A) The cycle of the resonance frequency when the magnetron is turned on is shorter than the cycle of the resonance frequency when the magnetron is not turned on, and the resonance frequency when the magnetron is turned on is higher.
a and 6b are switched off when the primary winding current becomes very small. As described above, the switching elements 6a and 6b perform the switching operation in the non-conducting state of the magnetron 12 in which the primary winding current of the high-frequency transformer 9 is extremely small. Thus, it is possible to achieve a reduction in switching loss.

Next, a start-up operation when the power is turned on will be described. Since the magnetron is an electron tube, if the filament does not reach the required temperature at startup, electron emission will not occur,
Does not oscillate. Therefore, at this time, the constant voltage characteristic does not appear on the magnetron filament (between the anode and the cathode), and the filament is almost open. For this reason, the voltage applied to the primary winding of the high-frequency transformer is set to a certain value or less until the filament reaches the required temperature so that the voltage of the high-voltage circuit does not exceed the circuit withstand voltage. The warm-up circuit 15
By detecting the phase difference between the primary winding current and the output voltage of the inverter, it is detected that the magnetron 12 has been heated up, and the magnetron 12 is not heated.
Only the filament can be heated when is unable to conduct.
By providing the warm-up circuit 15, it is possible to prevent the switching elements 6a and 6b from being destroyed due to the generation of a large voltage on the primary side of the high-frequency transformer 9.

FIG. 3 shows the circuit configuration of the voltage controlled oscillator 17 in detail. When a PWM signal flows from the microcomputer 13 via the photodiode 23, the phototransistor 24 of the photocoupler is turned on, and a current flows from the power supply 25. This current is smoothed by the resistor 28 and the capacitor 29, and a stable DC voltage is supplied to the inverting input terminal of the comparator 32. Although PWM signal is transmitted from the microcomputer 13, the waveform between t ₂ hours against period t _1, and is adapted to energize the phototransistor 24. It is possible to change the voltage of the capacitor 29 by changing this time. on the other hand,
The current from the power supply 25 is charged in the capacitor 45 via the resistor 34 and the transistor 37. When there is no charge in the capacitor 45, the base current of the transistor 42 flows through the resistor 36, but when the charge of the capacitor 45 is large (when the voltage is high), the base current of the transistor 42 becomes small. Although the collector current flows according to the base current of the transistor 42, when the collector current is large, the base current of the NPN transistor 39 is small, that is, the base current of the PNP transistor 37 is small, and the collector current of the PNP transistor 37 is small. This means that the time constant for charging the capacitor 45 is large. As the capacitor 45 is charged, the base current of the PNP transistor 42 becomes smaller. Accordingly, the base current of NPN transistor 39 increases, and as a result, PNP transistor 37
, The base current and the collector current also increase. That is, it means that the charging time constant for the capacitor 45 has become smaller. FIG. 5A shows a charging voltage waveform of the capacitor 45. The charging voltage is not simply a CR time constant, but shows a waveform in which the charging time constant is large at first and gradually decreases. Comparator 31
Compares the voltage 0.8V generated in the resistor 41 with the charging voltage A of the capacitor 45, and the comparator 32 compares the charging voltage A with the point C voltage applied to the inverting input terminal. FIG. 5C shows the comparator 31 and the comparator 3
3 is a truth table of the flip-flop 33 connected to the output of No. 2; The capacitor 45 is charged via the current flowing through the transistor 39. Charge voltage A is resistance voltage 0.8V
If it is lower, the output of the comparator 31 becomes H, the output of the comparator 32 becomes L, and the output of the Q terminal (point B) of the flip-flop 33 becomes L from the truth table, so that the transistor 47 remains off and the capacitor 45 Is charged. When the charging voltage A exceeds the resistance voltage 0.8V, the output of the comparator 31 becomes L, the output of the comparator 32 becomes L, the output of the Q terminal of the flip-flop 33 remains L, and the capacitor 45 continues to be charged. However, if the potential of the charging voltage A exceeds the potential of the point C as a result of continuing charging, the comparator 31
Remains at L, but the output of the comparator 32 becomes H, so that the Q terminal output (point B) becomes H, the transistor 47 is turned on, and the charge of the capacitor 45 is transferred from the resistor 44 through the transistor 47. Will be discharged. FIG. 5A shows a capacitor 45 showing these.
5A (in FIG. 5A, the broken line is a conventional example), FIG.
(B) is an output waveform of the Q terminal (point B) of the flip-flop 33. The voltage controlled oscillator 17 oscillates in this manner. The oscillation frequency is determined by the charging voltage waveform of the capacitor 45 and the voltage at the point C determined by the voltage of the capacitor 29. Therefore, the relationship between the oscillation frequency and the VCO voltage (C point voltage) has a waveform as shown in FIG. 6B (the broken line in FIG. 6B is a conventional example). On the other hand, the relationship between the input power of the inverter and the oscillation frequency is as shown in FIG. That is, when the oscillating frequency is high, the amount of change is small, but when the oscillating frequency is low (when the output is large), the output largely fluctuates due to slight frequency fluctuation.

As described above, the characteristics of the voltage controlled oscillator 17 for controlling the oscillation frequency change greatly when the oscillation frequency is high and change slowly when the oscillation frequency is low. Will be. The microcomputer 13 detects the anode current flowing through the magnetron 12 with the resistor 20 and outputs a PWM signal. Since the voltage controlled oscillator 17 corrects the characteristics of the inverter, the microcomputer 13 particularly converts the PWM signal waveform which is a digital signal. Operation is stable when used.

In the above-described circuit configuration of the inverter power supply, the anode current detecting means for detecting the anode current of the magnetron 12 is not limited to the resistor 20, but may be a current transformer. Although the switching elements 6a and 6b have been described using IGBTs, the invention is not limited thereto, and other switching elements such as bipolar transistors may be used. Although the snubber circuit is connected between the collector and the emitter of the second switching element 6b, this may be omitted. Further, the circuit of the voltage controlled oscillator 17 in FIG. 3 is not limited to this configuration, and may be another circuit configuration as long as the circuit can correct the control characteristics of the inverter.

Next, the structure of a microwave oven including a cooling system and the like will be described. 7 is a partial perspective view of the microwave oven as viewed from the rear of the rear machine room, FIG. 8 is a side view as viewed from the machine room side, and FIG. 9 is a diagram of FIG. 8 as viewed from the lower surface side. In these figures, 60 is a microwave oven main body, and 61 is a front panel. Reference numeral 63 denotes an oven of a microwave oven. On the side of the machine room on the side of the oven, a hot air circulating device 64 provided for circulating the airflow in the oven 63 in the case of the airflow circulation type oven,
Magnetron 12 and magnetron 12 antenna 12
A waveguide 65 into which a is inserted is provided. Waveguide 6
5 has an excitation port 68 provided on the side surface of the oven 63, from which microwaves are radiated into the oven 63. Although not shown, the excitation port 68 is covered with an insulator, and has a structure in which the excitation port 68 cannot be touched from inside the oven 63. A duct 62 made of metal is provided on the rear side of the housing of the magnetron 12.
The second cooling fin 12f is attached to the cooling fin 12f in a direction in which the cooling air flows. The duct 62 has one side wall closed by an outer wall of the oven 63, a flange 62 a is provided, and the duct 62 is fixed to the oven cavity by a screw 73. A bell mouth 62c for the cooling fan 54 is formed on the rear side of the microwave oven housing of the duct 62, and is integrated with the duct 62 by welding with a flange 62d of the bell mouth 62c. The cooling fan 54 is fixed to the bell mouth 62c by a fan arm (not shown). Inside the duct 62, the substrate 69 is provided with the spacer 7.
The solder side of the board 69 is fixed to the duct 62 made of metal by means of 4 and screws 75. At this time, an insulating sheet may be arranged between the solder surface of the substrate 69 and the duct 62. The distal end of the duct 62 does not have a structure in which all cooling air is used as cooling air for the magnetron 12, but has a structure in which the cooling air slightly flows out of the ventilation hole 62e into the machine room. On the board 69, a heat sink 70 to which the high-frequency transformer 9, the switching element 6, and the diode bridge rectifier circuit 2 are fixed, and other electronic components are arranged. When the duct 62 is made of a metal having a radio wave shielding effect, an inverter circuit disposed in the duct 62 is shielded, and unnecessary radiation noise is reduced. From the vacuum tube section 12b of the magnetron 12, the choke coil 1
An external connection terminal is fixed via 2c and the feedthrough capacitor 12d. The external connection terminal and the board 69 are connected by a connector. Figure 1 shows this connector connection.
0 is shown. An L-shaped female type terminal 71 is soldered to a substrate 69. In the lower part of the duct 62, a see-through opening 62
First, the substrate 69 is fixed to the duct 62, and the female type terminal 71 provided on the substrate 69 integrated with the duct 62 and the external connection terminal 1 of the magnetron are provided.
It is configured to fit by inserting 2g together. The procedure of assembling the connector connection portion will be further described. First, the spacer 74 and the screw 75 are provided in the duct 62.
The substrate 69 is attached, and the cooling fan 54 is attached to the bell mouth 62c by a fan arm. Thereafter, the external connection terminal 1 of the magnetron is connected to the female type terminal 71.
Insert 2g together. After connecting the magnetron 12 and the electronic circuit on the substrate 69 in this manner, the duct 6
The second flange 62 a is fixed to the outer wall of the oven 63 with the screw 73. There is no inconvenience even if the see-through opening 62f is provided on the side surface of the duct 62. FIG. 11 shows another configuration example of the connector connection unit. A female connection terminal 76 is connected to the substrate 69 by a lead wire 72. The external connection terminal 12g of the magnetron is inserted and fitted into the female type connection terminal 76.

FIG. 12 is a top view of the substrate portion. Substrate 69
Components such as the high-frequency transformer 9 and the heat radiating plate 70 are arranged on the upper side. The coil winding 50 and the capacitor 51 are components of the noise filter 22 provided in the commercial power input unit. A slit 69a is provided on the substrate 69 around the noise filter 22, and a cut-and-raised portion 62f from the duct 62 is arranged corresponding to the slit 69a. Thereby, the leakage of the magnetic flux of the high-frequency transformer 9 is prevented from being coupled to the coil winding 50 of the noise filter 22,
Noise is prevented from coming out of the power supply unit to the outside.

When the microwave oven is driven, the inverter operates as described above. At the same time, the fan motor 54
a is driven. By the cooling fan 54, the cooling air flows as shown by the arrow A, and the equipment, electronic components and the magnetron 12 constituting the inverter are efficiently cooled. Then, the magnetron 12 and the like are cooled, and
Through the oven 63. On the other hand, a small amount of cooling air leaks into the machine room through the ventilation holes 62e as indicated by the arrow B. And it has a structure which cools other electrical components arranged in the machine room.

FIG. 13 shows another connection example of the secondary side of the high-frequency transformer 9. 9b, 9 as the secondary winding
In addition to c, it has a winding 9d for a fan motor. After a diode bridge 52 is connected to the fan motor winding 9d, a smoothing capacitor 53 is connected to the fan motor winding 9d, which serves as a DC power supply for a fan motor 54a that is a DC motor. The winding 9d for the fan motor is tightly wound with the primary winding 9a so that the winding voltage is greatly affected by the output power (specifically, the vicinity of the primary winding 9a). Rolled around). Therefore, if the output of the microwave oven changes, the supply voltage to the fan motor 54a changes. That is, when the output power is large, a high voltage is supplied to obtain a sufficient cooling capacity, and when the output power is small, the magnetron 1
Since the heat generation of No. 2 is small, the cooling capacity may be small, and a low voltage is supplied. This makes it possible to minimize the noise of the cooling fan 54 that causes noise.

FIG. 14 shows a vertically arranged combination of the magnetron 12, the inverter circuit and the cooling fan 54. By arranging the cooling fan vertically, the cooling port of the cooling fan 54 can be arranged on the bottom surface, so that there is no cooling hole at the rear. It was very unsightly to place the microwave oven where the back could be seen, such as in a face-to-face kitchen, but by arranging it this way, the rear can be finished finely. Further, since the hot air circulation device 64 can be arranged vertically, the intake port 64a and the exhaust port 64b can be arranged vertically long as shown. Thus, when performing oven cooking, it is possible to maintain a sufficiently uniform temperature distribution even when the shelf plate has two stages, although not shown.

In the above configuration, when the power is turned on, the driving of the inverter starts. At the same time, the cooling fan 54 starts rotating. The cooling fan 54 takes in air outside the microwave oven and sends it to the substrate 69. First, after cooling the high-frequency transformer 9 and the heat radiating plate 70 on which the switching element 6 which is a heat-generating component on the substrate 69 is cooled, it is put into the magnetron 12 (arrow A). At the same time, a small amount of cooling air is supplied to the ventilation holes 6
It is guided into the machine room through 2e (arrow B). Arrow A
After cooling the magnetron 12, the cooling air of
From the oven 63 to exhaust the steam inside the oven 63. On the other hand, the cooling air indicated by the arrow B cools other components provided on the front panel 61 and the like. The shield of the magnetic circuit of FIG. 12 cuts and raises the duct 62 to form a cut and raise 62f. The core of the high-frequency transformer 9 slightly leaks magnetic flux. As the size of the substrate 69 is reduced, the high-frequency transformer 9 and the noise filter 22 are arranged closer. However, since the leakage magnetic flux from the high frequency transformer 9 is shielded by the cut-and-raised metal (shielding plate) 62f, the magnetic flux of the high frequency transformer 9 is not coupled to the toroidal core of the noise filter 22, and the high frequency current leaks to the outside. Can be prevented.

In the above-described microwave oven structure, the electronic components on the substrate 69 can be further reduced in size by using chip components, and a greater effect can be obtained. Is also good. Although the structure is such that the magnetron 12 and the machine room are cooled by one cooling fan 54, a cooling fan for sending cooling air to the inside of the housing of the microwave oven including the machine room is separately provided, and two cooling fans are provided. May be used. With such a configuration, the machine room can be cooled even when used in a microwave oven. Although the flow of the cool air from the cooling fan 54 is all linear, the structure may be such that it flows from the side surface, the upper surface, or the like of the substrate 69 depending on the space inside the machine room. In FIG. 13, the DC power supply circuit is configured only by the diode bridge 52 and the smoothing capacitor 53, but a low-voltage power supply or a voltage limiter may be provided. Although the duct 62 is made of metal, it may be made of a material having a radio wave shielding effect including a conductive resin. The noise filter 22
Are arranged on the same substrate 69, but the arrangement of the components is not limited to this, such as disposing the noise filter 22 separately.

[0039]

As described above, according to the first aspect of the present invention, the pair of switching elements constituting the inverter have the same conduction ratio, and the switching frequency is one of the step-up transformers when the magnetron is energized. Because the input impedance of the secondary winding and the resonance frequency of the resonance capacitor are lower than the primary winding input impedance of the step-up transformer and the resonance frequency of the resonance capacitor when the magnetron is not energized, the primary impedance of the step-up transformer is reduced. The winding current is positive and negative, and the peak current is small, so that the core can be downsized. The number of windings can also be reduced, and the window area for storing the winding can be reduced. Also a small one
The switching element can be turned off at the time of the next winding current, the loss of the switching element is reduced, a small switching element can be used, and the heat radiation plate and the like can be reduced. Therefore, the inverter power supply can be reduced in size, and the effective volume of the high-frequency heating device can be improved.

According to the second aspect of the invention, in the inverter, the conduction ratio of the pair of switching elements is the same, and the input impedance of the primary winding of the step-up transformer and the resonance frequency of the resonance capacitor when the magnetron is not energized. In this case, the pair of switching elements are caused to oscillate at an oscillating frequency for switching off, so that the primary winding current of the step-up transformer is positive and negative, and the peak current becomes small, as described above. The size of the core can be reduced, the number of windings can be reduced, and the window area for storing the windings can be reduced. In addition, the switching element can be turned off when the primary winding current is small, the loss of the switching element is reduced, and a small switching element can be used. Can be. Therefore, the inverter power supply can be reduced in size, and the effective volume of the high-frequency heating device can be improved.

According to the third aspect of the present invention, since the voltage doubler rectifier circuit is provided on the secondary side of the step-up transformer, the primary windings of the step-up transformer have the same positive and negative duty ratios. The voltage and current of the voltage doubler rectifier circuit for supplying a high DC voltage are well balanced, and the voltage tolerance of the high-voltage capacitor constituting the voltage doubler rectifier circuit is reduced, so that the size can be reduced.

According to the fourth aspect of the present invention, when the high-frequency heating apparatus is started, the primary winding input impedance of the step-up transformer and the resonance frequency of the resonance capacitor when the magnetron is operating at the maximum output power. Since the heater of the magnetron is heated by driving the inverter at a high frequency, a high voltage is not applied to the magnetron at the time of starting the magnetron,
Further, when the magnetron is heated up and turned on, the maximum output of the magnetron is not exceeded, and safety can be achieved.

According to the fifth aspect of the present invention, the voltage control oscillator is provided in the control circuit for driving the inverter, and the PWM control output from the microcomputer for controlling the output of the high-frequency heating device is smoothed to perform the voltage control. Since the voltage for determining the oscillation frequency of the oscillator is obtained, the inverter can be controlled by the PWM signal from the microcomputer. Therefore, the number of connection lines between the microcomparator and the inverter is reduced, and the reliability of the circuit board is reduced while the size is reduced. Can be improved.

According to the sixth aspect of the invention, the characteristics of the voltage controlled oscillator are such that when the oscillation frequency is low, the change in the oscillation frequency is more gradual than when the oscillation frequency is high. By using a voltage controlled oscillator having the opposite characteristic to the power-oscillation frequency characteristic,
The input power-oscillation frequency characteristic is leveled over the input adjustment range, and stable input control can be performed.

According to the seventh aspect of the present invention, there is provided an anode current detecting means for detecting an anode current of the magnetron,
Since the oscillation frequency of the voltage-controlled oscillator is controlled so that the anode current becomes constant based on the detection value of the anode current detection means, the input power of the high-frequency heating device is controlled to be constant, and the cooking time is reduced. Stabilization can be achieved.

According to the present invention, there is provided an AC current detecting means for detecting an AC current from the commercial power supply,
Since the oscillation frequency of the voltage controlled oscillator is controlled based on the detection value of the AC current detection means so that the AC current does not exceed a predetermined value, the overcurrent input from the commercial power supply is detected and the maximum input power is detected. Is controlled, and even if the power supply voltage fluctuates, the breaker in the home of the power supply does not fall off, and safety and high reliability can be achieved.

According to the ninth aspect of the present invention, there is provided an AC current detecting means for detecting an AC current from the commercial power supply,
Since the oscillation frequency of the voltage controlled oscillator is controlled so that the AC current becomes a predetermined value based on the detection value of the AC current detection means, the input power of the high-frequency heating device is controlled to be constant, and the cooking time is controlled. Can be further stabilized.

According to the tenth aspect of the present invention, a duct for passing cooling air through cooling fins of the magnetron is provided, and equipment and electronic components including the step-up transformer, the switching element, and the rectifying element in the duct. Since the cooling fan is provided on the side of the duct opposite to the magnetron mounting portion, the magnetron and the devices and electronic components constituting the inverter and the like can be efficiently cooled. This eliminates the need for wiring for connecting the electronic circuits constituting the inverter and the like, thereby further reducing the size.

According to the eleventh aspect of the present invention, since the duct is formed of any one of materials having a radio wave shielding effect including a metal or a conductive resin, an electronic circuit constituting an inverter or the like is shielded and unnecessary. Radiation noise is reduced, and safe operation can be achieved.

According to the twelfth aspect of the present invention, since the cooling fan has a suction port having a duct communicating with the outside of the housing of the high-frequency heating device, the air flowing into the cooling fan is prevented from flowing into the high-frequency heating device. As the air becomes external, the cooling efficiency increases, and the operation can be further stabilized.

According to the thirteenth aspect, before the cooling air from the cooling fan is requested from the magnetron, part of the cooling air is diverted into the housing of the high-frequency heating device including the machine room. With such a structure, electrical components and the like in the machine room can be cooled by the cooling air from the cooling fan, so that the structure can be simplified and the size can be reduced.

According to the fourteenth aspect of the present invention, a DC motor is used to drive the cooling fan, and a predetermined number of windings for the fan motor are provided on the secondary side of the step-up transformer. Since a DC power supply in which the output of the wire is rectified is used as the power supply for the DC motor, a separate power supply for driving the fan motor is not required, and the size can be reduced. In addition, when the output of the high-frequency heating device is small, the output voltage of the DC power supply for the fan motor is also reduced, so that unnecessary cooling is not performed, so that a wide output control range can be obtained and noise can be reduced when the output is small. it can.

According to the fifteenth aspect of the present invention, since a cooling fan for sending cooling air is separately provided inside the housing of the high-frequency heating device including the machine room, a cooling fan for the magnetron and a cooling fan for the machine room are provided. By separating, it is possible to cool the machine room even when using in a microwave oven,
The operation can be stabilized over a wide range of applications.

According to the sixteenth aspect of the present invention, a noise filter connected to an input section from the commercial power supply is provided on the substrate, and a shielding plate for shielding the noise filter against magnetic flux leaking from the step-up transformer is provided. Since it is provided integrally with the duct, magnetic coupling between the step-up transformer and the noise filter is eliminated, and the step-up transformer and the noise filter can be arranged on the same substrate, so that the size can be further reduced.

According to the seventeenth aspect, the connecting portion between the magnetron and the board has a structure in which the connecting terminal of the magnetron is inserted and fixed to a connector provided on the board, so that the high-voltage section is located in the space. Safety can be improved without floating.

According to the eighteenth aspect of the present invention, the duct in which the magnetron and the substrate are arranged, and the cooling fan are arranged in a vertical direction, and the cooling air is taken in from the bottom side of the housing of the high-frequency heating device. As a result, the suction port of the cooling fan is arranged on the bottom surface of the housing, and there is no suction port for the cooling fan on the rear side of the housing, so that the appearance can be finished finely. In addition, since the hot air circulation device and the like are also arranged vertically long, even when performing oven cooking, a sufficiently uniform temperature distribution can be maintained and stable cooking can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram of an inverter power supply for a high-frequency heating device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing details of a noise filter in FIG. 1;

FIG. 3 is a circuit diagram showing details of a voltage controlled oscillator in FIG. 1;

FIG. 4 is a timing chart for explaining the operation of the embodiment.

FIG. 5 is a diagram showing waveforms and the like for explaining the operation of the voltage controlled oscillator in the embodiment.

FIG. 6 is a diagram showing input power characteristics and oscillation frequency characteristics of a voltage controlled oscillator according to the embodiment.

FIG. 7 is a partial perspective view of the microwave oven to which the above-described embodiment is applied, as viewed from the rear of a rear machine room.

FIG. 8 is a side view of the microwave oven to which the above embodiment is applied, as viewed from the machine room side.

FIG. 9 is a bottom view of FIG.

FIG. 10 is a side view showing a connector connection portion between an external connection terminal of the magnetron and a board-side terminal in the embodiment.

FIG. 11 is a side view showing another configuration example of the connector connection section in the embodiment.

FIG. 12 is a plan view of a substrate portion in a microwave oven to which the above embodiment is applied.

FIG. 13 shows the high-frequency transformer 2 in the above embodiment.
It is a circuit diagram showing an example of connection on the next side.

FIG. 14 is a side view showing a modification in which a combination of a magnetron, an inverter circuit, a cooling fan, and the like is vertically arranged in the microwave oven to which the above-described embodiment is applied.

FIG. 15 is a circuit diagram of a conventional inverter power supply.

FIG. 16 is a diagram showing operation waveforms of the conventional example.

[Description of Signs] 1 Commercial power supply 2 Diode bridge rectifier circuit 6a, 6b Switching element 8 Resonant capacitor 9 High frequency transformer (step-up transformer) 9d Winding for fan motor 10a, 10b High voltage capacitor 11a, 11b Full-wave double voltage rectification together with high voltage capacitor Diodes Constituting Circuit 12 Magnetron 12g External Connection Terminal 13 Microcomputer 15 Warm-up Circuit 17 Voltage Controlled Oscillator 20 Resistance (Anode Current Detecting Means) 21 Current Transformer (AC Current Detecting Means) 22 Noise Filter 54 Cooling Fan 54a Fan Motor 62 Duct 62f Cutting and raising metal (shielding plate) 69 Substrate

Continued on the front page (72) Inventor Toshiaki Muranaka 1-1-1, Shibaura, Minato-ku, Tokyo Inside Toshiba Corporation Head Office (72) Inventor Toshio Kakizawa 1-1-1, Shibaura, Minato-ku, Tokyo Co., Ltd. F-term (reference) in Toshiba head office

Claims (18)

[Claims] At least one pair of switching elements is connected in series between positive and negative output terminals of a DC power supply obtained by rectifying an AC voltage from a commercial power supply, and a connection point of the pair of switching elements and the connection point. A half-bridge type inverter in which a primary winding of a step-up transformer for generating a magnetron driving voltage and a resonance capacitor are connected in series between any one output terminal of the DC power supply, , The conduction ratio is the same, and the switching frequency is lower than the input impedance of the primary winding of the step-up transformer when the magnetron is energized and the resonance frequency of the resonance capacitor, and the primary winding of the step-up transformer when the magnetron is not energized. A high frequency heating device having a frequency higher than a line input impedance and a resonance frequency of the resonance capacitor. Inverter power supply. 2. A DC power supply obtained by rectifying an AC voltage from a commercial power supply, at least one pair of switching elements is connected in series between positive and negative output terminals, and a connection point of the pair of switching elements is connected to the connection point. A half-bridge type inverter in which a primary winding of a step-up transformer for generating a magnetron driving voltage and a resonance capacitor are connected in series between any one output terminal of the DC power supply, The duty ratio is the same, and the inverter switches off the pair of switching elements when the magnetron is in a resonance state based on the primary winding input impedance of the step-up transformer and the resonance capacitor when the magnetron is not energized. An inverter power supply for a high-frequency heating device, which oscillates at a high oscillation frequency. 3. The inverter power supply for a high frequency heating device according to claim 1, wherein a voltage doubler rectifier circuit is provided on a secondary side of the step-up transformer. 4. When the high-frequency heating device is started, the inverter is driven at a frequency higher than the primary winding input impedance of the step-up transformer and the resonance frequency of the resonance capacitor when the magnetron is operating at the maximum output power. The inverter power supply for a high-frequency heating device according to claim 1 or 2, wherein the heater of the magnetron is heated by doing so. 5. A voltage-controlled oscillator is provided in a control circuit for driving the inverter, and a PWM output outputted by a microcomputer for controlling an output of the high-frequency heating device is smoothed to generate an oscillation frequency determining voltage of the voltage-controlled oscillator. The inverter power supply for a high-frequency heating device according to claim 1, wherein the inverter power supply is configured to obtain the configuration. 6. The characteristic of the voltage-controlled oscillator is such that when the oscillation frequency is low, the change in the oscillation frequency is more gradual than when the oscillation frequency is high.
An inverter power supply for the high-frequency heating device according to the above. 7. An anode current detecting means for detecting an anode current of the magnetron, wherein an oscillation frequency of the voltage controlled oscillator is controlled based on a detection value of the anode current detecting means so that the anode current becomes constant. The inverter power supply for a high-frequency heating device according to claim 5, wherein 8. An AC current detecting means for detecting an AC current from the commercial power supply, and based on a detection value of the AC current detecting means, the voltage controlled oscillator is controlled so that the AC current does not exceed a predetermined value. The inverter power supply for a high-frequency heating device according to claim 5, wherein the oscillation frequency is controlled. 9. An AC current detecting means for detecting an AC current from the commercial power supply, wherein the oscillation of the voltage controlled oscillator is controlled so that the AC current becomes a predetermined value based on a detection value of the AC current detecting means. 9. The inverter power supply for a high-frequency heating device according to claim 8, wherein the frequency is controlled. 10. A duct for passing cooling air through cooling fins of the magnetron, in which a board including the booster transformer, the switching device, the device including the rectifying device, and the electronic component are mounted. 3. The inverter power supply for a high-frequency heating device according to claim 1, wherein a cooling fan is provided on the side of the duct opposite to the magnetron mounting portion. 11. The inverter power supply for a high-frequency heating device according to claim 10, wherein the duct is formed of any one of a material having a radio wave shielding effect including a metal or a conductive resin. 12. The inverter power supply for a high-frequency heating device according to claim 10, wherein the suction port of the cooling fan has a duct communicating with the outside of the housing of the high-frequency heating device. 13. The cooling air from the cooling fan has a structure in which a part of the cooling air is diverted into a housing of a high-frequency heating device including a machine room before being required for the magnetron. The inverter power supply for a high-frequency heating device according to claim 10. 14. A DC power supply using a DC motor for driving the cooling fan, applying a predetermined number of windings for the fan motor to the secondary side of the step-up transformer, and rectifying the output of the winding for the fan motor. 11. An inverter power supply for a high-frequency heating device according to claim 10, wherein the power supply serves as a power supply for the DC motor. 15. The inverter power supply for a high-frequency heating device according to claim 10, wherein a cooling fan for sending cooling air is separately provided inside the housing of the high-frequency heating device including the machine room. 16. A noise filter connected to an input from the commercial power supply is provided on the board, and a shield plate for shielding the noise filter against magnetic flux leaking from the step-up transformer is provided integrally with the duct. The inverter power supply for a high-frequency heating device according to claim 10, characterized in that: 17. The high frequency heating apparatus according to claim 10, wherein a connection portion between the magnetron and the substrate has a structure in which a connection terminal of the magnetron is inserted and fixed to a connector provided on the substrate. Inverter power supply. 18. A duct in which the magnetron and the substrate are arranged, and the cooling fan are vertically arranged,
The inverter power supply for a high-frequency heating device according to claim 10, wherein the cooling air is taken in from a bottom surface side of a housing of the high-frequency heating device.