JPH0528716Y2 - - Google Patents

Description

[Detailed description of the invention] [Field of industrial application] This invention relates to the improvement of high-frequency heating devices for so-called dielectric heating such as microwave ovens. The present invention relates to an improvement in a high-frequency heating device configured to convert DC power into high-frequency power and boost the voltage using a step-up transformer to drive a magnetron.

[Prior Art] FIG. 4 is a circuit diagram showing an example of a conventional high-frequency heating device using an inverter circuit as a power supply device.
In the figure, the AC output voltage of a commercial power source 1 is given to a rectifier and smoothing circuit 100 via a switch 2,
Rectified and smoothed to DC voltage. Rectifier smoothing circuit 100
The output of is given to the inverter circuit 200. The inverter circuit 200 converts a DC voltage into a high frequency voltage of a desired frequency by switching a semiconductor switching element 201 such as a power transistor, and further boosts this high frequency voltage with a drive transformer 202. The secondary side of the drive transformer 202 is provided with a filament winding 202b, a high voltage winding 202c, and an auxiliary winding 202d. The output voltage of the filament winding 202b is applied to the cathode of the magnetron 400, that is, the filament 401,
This filament 401 is heated. Further, the output voltage of the high voltage winding 202c is boosted by a half-wave voltage doubler circuit 300 including a capacitor 301 and a diode 302, and then applied between the filament 401 and anode 402 of the magnetron 400. The control circuit 500 has an on-time width that corresponds to the set output of the output setting means 3, and the inverter circuit 20.
Outputs an on/off pulse signal synchronized with the 0 operation cycle. This on/off pulse signal is amplified by the drive circuit 5 and then applied to the switching element 201. Therefore, the control circuit 500 has 30~
Switching element 201 at a frequency of about 100kHz
Operate switching. Note that the control circuit 50
0 detects the operating cycle of the inverter circuit 200 via the auxiliary winding 202d of the drive transformer 202,
This controls the on/off timing of the switching element.

In the high frequency heating device shown in FIG. 4, the flyback energy generated in the high voltage winding 202c is stored in the capacitor 301 during the off period (flyback period) of the switching element 201, and this capacitor 301 is discharged during the on period of the switching element 201. , and is supplied to the magnetron 400 to drive it.

Further, in the high-frequency heating device shown in FIG. 4, the magnetic field applied to the magnetron 400 is fixed by a permanent magnet, and the output of the magnetron 400 is controlled only by the control circuit 500. That is, by the output setting means 3 setting the on-time width according to the amount of operation of the operating knob 4, the control circuit 400 changes the on-time width of the switching element 201, thereby controlling the output.

[Problems to be solved by the invention] In the case of a device that uses a permanent magnet to apply a magnetic field to the magnetron 400, such as the high-frequency heating device shown in FIG. , the anode voltage of magnetron 400 decreases and the output decreases (i.e., drive transformer 2
The load impedance of the inverter circuit 200 was significantly reduced), causing modulation in the operation of the inverter circuit 200. Further, problems caused by such load fluctuations also occur when the position and shape of the food to be heated changes. Furthermore, since the permanent magnet occupies a large space, there is also the problem that the high-frequency heating device becomes large-sized.

Furthermore, in the high-frequency heating device shown in FIG. 4, the secondary side of the drive transformer 202 is at a high voltage in order to drive the magnetron 400, but circuit components to which such a high voltage is applied are expensive due to safety concerns. From the viewpoint of However, in the high frequency heating device shown in FIG. 4, a high voltage is applied to the capacitor 301 and the diode 302, which poses problems in terms of safety and cost. Therefore, the fifth
As shown in the figure, it is also possible to apply the output of the high voltage winding 202c directly to the magnetron 400. However, in the high-frequency heating device shown in FIG. 5, the magnetron 400 is not conductive during the flyback period (the off period of the switching element 301), and the secondary side of the drive transformer 202 is in a no-load state. Therefore, during this flyback period, the L (inductance) component of the resonant circuit formed by the inverter circuit 200 is
The excitation inductance is only 2, and the resonance period, that is, the off period becomes long. Therefore, the duty ratio of the high frequency voltage generated by the inverter circuit 200 becomes low, creating a new problem that makes it difficult to increase the output.

This idea was devised to solve the various problems mentioned above; it always maintains a stable load condition during steady operation, is compact, has fewer high-voltage parts, and has high output. The purpose of the present invention is to provide a high-frequency heating device that can be

[Means for Solving the Problems] The high-frequency heating device according to this invention includes a DC power supply, an inverter, a magnetron, and a control circuit. An inverter includes a transformer whose primary winding receives the output of a DC power source, a switching element inserted between the primary winding and the DC power source, and a switching element that connects the primary winding to the DC power source when the switching element is off. It includes a resonant element that cooperates to form a resonant circuit and generate a resonant voltage across the primary winding, and the switching operation of this switching element and the resonant action of the resonant circuit cause a voltage to be applied to the secondary side of the transformer. Induces high frequency voltage. The above magnetron is
The secondary output of the transformer in the inverter is directly applied between its cathode and anode to drive it.
The control circuit detects the operating period of the inverter based on the secondary output of the transformer, and thereby turns on and off the switching elements in synchronization with the resonant period.
Control off. Furthermore, in this invention, the magnetic field generation coil installed in the magnetron and the secondary output of the transformer are rectified and smoothed and applied to the magnetic field generation coil. and circuit means for increasing/decreasing the energizing current of the magnetic field generating coil in response to fluctuations in the secondary output of the transformer caused by fluctuations in the resonant voltage.

[Function] In this invention, by controlling the applied magnetic field of the magnetron in accordance with the fluctuation of the secondary output of the transformer caused by the fluctuation of the resonant voltage of the resonant circuit due to the load fluctuation of the magnetron in the steady operating state, This is intended to stabilize variations in load impedance due to variations in anode voltage.

[Embodiment] FIG. 1 is a circuit diagram showing a high frequency heating device according to an embodiment of this invention. In the figure, like the conventional device in FIG. 4, this embodiment has a commercial power source 1,
switch 2, rectifier smoothing circuit 100, inverter circuit 200, magnetron 400, control circuit 500, output setting means 3, and operation knob 4
and a drive circuit 5. In addition, the half-wave voltage doubler circuit 3 to which a high voltage is applied in the conventional device shown in FIG.
00 is not provided in this embodiment.

The rectifier smoothing circuit 100 includes a rectifier bridge 101 that performs full-wave rectification of an AC voltage from a commercial power source 1, a chiyoke coil 102 and a smoothing capacitor 10 connected in series between both ends of the rectifier bridge 101.
Contains 3. The inverter circuit 200 includes a resonant capacitor 203 and a diode 204 connected in series to both ends of a smoothing capacitor 103, a switching element 201 such as a power transistor connected in parallel to the diode 204, and a drive transformer 20.
2. The drive transformer 202 has its primary winding 202a connected in parallel with the resonant capacitor 203. The secondary side of the drive transformer 202 is provided with a filament winding 202b, a high voltage winding 202c, and an auxiliary winding 202d, similar to the device of FIG. Both ends of the filament winding 202b are connected to the filament 401 (cathode) of the magnetron 400. Therefore, filament 4
01 is heated by the output of the filament winding 202b. One end of the high voltage winding 202c is connected to the filament 401 of the magnetron 400, and the other end is connected to one end of the coil 602 via the cathode and anode of the diode 601. The other end of the coil 602 is grounded via a capacitor 603 and connected to the magnetron 400.
It is grounded via the magnetic field generating coil 604 of. Further, the high voltage winding 202c is provided with an intermediate tap, and this intermediate tap is grounded to the same potential as the anode of the magnetron 400. Therefore, the magnetron 400 has its cathode 401
- A voltage induced in the upper (in FIG. 1) winding from the middle tap of the high voltage winding 202c is applied across the anode 402. Also, the diode 60
1, the coil 602, and the capacitor 603 constitute a circuit for rectifying and smoothing the AC output induced in the lower winding (in FIG. 1) from the middle tap of the high voltage winding 202c. Note that the magnetron 400 has, apart from the magnetic field generating coil 604,
Equipped with a permanent magnet that provides a minimal magnetic field.

One end of the auxiliary winding 202d is connected to the control circuit 500.
It is grounded via a resistor 501 inside. Further, the other end of the auxiliary winding 202d is connected to the input end of the timing circuit 502. This timing circuit 502 turns on/off the inverter circuit 200.
It outputs a timing signal synchronized with the off operation, and its output is given to the triangular wave generation circuit 503. The triangular wave generation circuit 503 generates a triangular wave in response to the falling pulse of the output of the timing circuit 502, and its output is sent to the comparator circuit 50.
This signal is applied to one input terminal of 4. The setting output of the output setting means 3 is applied to the other input terminal of the comparison circuit 504. The comparison circuit 504 compares the levels of the triangular wave output of the triangular wave generating circuit 503 using the set output of the output setting means 3 as a threshold value, and the comparison result output is given to the pulse generating circuit 505. The pulse generating circuit 505 outputs a pulse signal whose on/off time width is determined by the output of the comparator circuit 504, and its output is applied to the switching element 201 via the drive circuit 5 as a switching control voltage. In addition, the control circuit 500 uses a power transformer 506 and a power circuit 507 to convert the AC output of the commercial power supply 1 into a DC voltage (V _D = -12V).
This DC voltage is applied to the timing circuit 5.
02 is applied to the triangular wave generation circuit 503, comparison circuit 504, and pulse generation circuit 505 as a driving voltage.

Furthermore, in the embodiment of FIG.
A current detection coil 104 that detects fluctuations in the output current of the magnetron 400 and a power detection circuit 508 that detects load fluctuations of the magnetron 400 based on the output of the current detection coil 104 are provided. The output is given to the output setting means 3 for setting value correction.

FIG. 2 is a diagram showing the structure of the magnetron 400 shown in FIG. 1. As shown in the figure, the magnetron 400 is provided with a magnetic field generating coil 604, and is configured to be able to adjust the applied magnetic field by changing the energizing current flowing through the magnetic field generating coil 604.

FIG. 3 is a diagram showing voltage waveforms at various parts of the high-frequency heating device shown in FIG. 1. The operation of the above embodiment will be explained below with reference to FIG.

First, steady state operation that does not take into account load fluctuations will be explained. The switching element 201 is turned on and off by on/off pulses from the control circuit 500. When the switching element 201 is on, the rectifier bridge 101 is turned on on the primary side of the drive transformer 202. An output current flows through the choke coil 102, the primary winding 202a, and the collector/emitter of the switching element 201. Conversely, when the switching element 201 is off, the primary side of the drive transformer 202 is 1
Due to the back electromotive force of the secondary winding 202a, a parallel resonant closed loop is formed between the primary winding 202a and the resonant capacitor 203, and a resonant current flows within this closed loop. Therefore, an alternating current flows through the primary winding 202a depending on whether the switching element 201 is turned on or off. On the other hand, drive transformer 2
On the secondary side of the high-voltage winding 202c, the voltage induced in the winding on the upper surface of the intermediate tap is applied to the magnetron 4.
00 is applied between the cathode 401 and the anode 402, and drives the magnetron 400 in the on mode of the inverter circuit 200. Regarding the magnetic field generating coil 604, when the inverter circuit 200 is in the off mode, current flows into the magnetic field generating coil 604 from the intermediate tap of the high voltage winding 202c via the ground, and this current flows through the coil 602 and the diode 601 to the high voltage coil 604. It flows to the other end of the masterpiece wire 202c. Also, the current flowing from the intermediate tap to ground charges capacitor 603 to the polarity shown in FIG. Conversely, when the inverter circuit 200 is in the on mode, the charge accumulated in the capacitor 603 flows into the magnetic field generating coil 604 via the ground, and energizes the magnetic field generating circuit 604. Therefore, magnetic field generating coil 604 is constantly energized regardless of whether inverter circuit 200 is in on mode or off mode.

The on/off control of the inverter circuit 200 is performed by the control circuit 500 based on the set output of the output setting means 3. That is, the control circuit 500
The timing circuit 502 turns on/off the inverter circuit 200 based on the voltage change (or current change) of the auxiliary winding 202d of the drive transformer 202.
Rising and falling pulses synchronized with the OFF operation are generated as shown in FIG. Triangular wave generation circuit 5
03 generates a triangular wave synchronized with the falling pulse from the timing circuit 502 as shown in FIG.
Further, the comparison circuit 504 compares the level of the triangular wave output of the triangular wave generating circuit 503 using the set output of the output setting means 3 as a threshold value, and compares the level of the triangular wave output of the triangular wave generating circuit 503.
05 controls the on/off time width of the pulse signal emitted. Therefore, this pulse generating circuit 5
05 outputs a pulse signal as shown in FIG. The on/off operation of the switching element 201, that is, the inverter circuit 200 is controlled by this pulse signal. Incidentally, by detecting the load fluctuation of the magnetron 400 by the power detection circuit 508 and appropriately correcting the set output of the output setting means 3, the load fluctuation of the magnetron 400 can be suppressed to some extent.

Next, a description will be given of the operation when the anode voltage of the magnetron 400 changes due to a change in the magnetic properties of the permanent magnet due to a change in temperature or a change in the position and shape of the food to be heated, that is, when a load fluctuation occurs. In this case, magnetron 40
If the anode voltage of 0 changes, for example, in a decreasing direction, the load impedance decreases, so the effective power consumption on the load side (the secondary side of the drive transformer 202) decreases. On the other hand, on the power supply side (the primary side of drive transformer 202), the reactive power component in inverter circuit 202 increases to compensate for the decrease in active power on the load side. As a result, when the inverter circuit 200 is off, the primary winding 202a
The resonant current (reactive current) flowing through the resonant capacitor 203 and therefore the resonant voltage (flyback voltage) increase or rise. As the flyback voltage increases, on the secondary side of the drive transformer 202, the energizing current of the magnetic field generating coil 604, to which a voltage proportional to the flyback voltage is applied, increases. Therefore, the anode voltage of the magnetron 400 is pushed up, which acts to increase the load impedance. Conversely, when the anode voltage of the magnetron 400 changes in the direction of increasing, the flyback voltage decreases and the magnetic field generating coil 6
04 energizing current decreases. Therefore, the anode voltage of the magnetron 400 decreases, and the load impedance decreases. Due to the feedback effect described above, the magnetron 40
The anode voltage of 0 converges to a constant value, and fluctuations in the load impedance of the inverter circuit 200 are stabilized. Therefore, operational modulation of the inverter circuit 200 due to variations in load impedance is eliminated.

In the embodiment shown in FIG. 1, the inverter circuit 2
L of the resonant circuit formed when 00 is off
The (inductance) component includes not only the excitation inductance of the drive transformer 200, but also its leakage inductance, the magnetic field generating coil 604, and the coil 6.
02 inductance will also be included. Therefore, the resonance period, ie, the flyback period, of the resonant circuit is shorter than that in the conventional device shown in FIG. 5, which does not include a magnetic field generating coil.
As a result, large output becomes possible.

[Effects of the invention] As described above, according to this invention, since a coil is used to apply a magnetic field to the magnetron, the magnetic properties change due to temperature changes compared to the conventional method using permanent magnets. There are few changes, the load condition of the magnetron can be kept stable, and the device can be made smaller. Also,
Even if a magnetron load fluctuation occurs, the load fluctuation is detected on the primary side of the inverter transformer and fed back to the secondary side to increase or decrease the energizing current of the magnetic field generating coil. The load condition can be kept more stable. Furthermore, when the inverter is off, the magnetic field generating coil is connected to the secondary side of the transformer as a load, so the inverter's resonant period, or flyback period, can be shortened compared to conventional devices, and higher output can be achieved. I can do it.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of this invention. FIG. 2 is a diagram showing the structure of the magnetron 400 shown in FIG. 1. FIG. 3 is a diagram showing voltage waveforms at various parts of the high-frequency heating device shown in FIG. 1. FIG. 4 is a circuit diagram showing an example of a conventional high frequency heating device. FIG. 5 is a circuit diagram showing another example of a conventional high-frequency heating device. In the figure, 1 is a commercial power supply, 3 is an output setting means, 100 is a rectifier and smoothing circuit, 200 is an inverter circuit, 201 is a switching element, 202 is a drive transformer, 203 is a resonant capacitor, 400 is a magnetron, 500 is a control circuit, 601, 602 and 603 are rectifying and smoothing diodes, respectively;
Coil and capacitor 604 indicates a magnetic field generating coil.

Claims (1)

[Claims for Utility Model Registration] A DC power supply, a transformer whose primary winding receives the output of the DC power supply, a switching element interposed between the primary winding and the DC power supply, and a switching element inserted between the primary winding and the DC power supply. a resonant element that cooperates with the primary winding to form a resonant circuit and generates a resonant voltage across the primary winding when the element is turned off; an inverter that induces a high-frequency voltage on the secondary side of the transformer by resonance; a magnetron that is driven by applying the secondary output of the transformer directly between the cathode and the anode; and a secondary output of the transformer. A high-frequency heating device comprising a control circuit that detects the operating cycle of the inverter based on the operating cycle and thereby controls on/off of the switching element in synchronization with the operating cycle, the magnetic field provided in the magnetron. a generating coil, and a secondary output of the transformer is rectified and smoothed and applied to the magnetic field generating coil, and is generated due to fluctuations in the resonant voltage of the resonant circuit due to load fluctuations of the magnetron in a steady operating state. 2 of said transformer
A high-frequency heating device characterized by comprising circuit means for increasing or decreasing the energizing current of the magnetic field generating coil according to fluctuations in the next output.